Gut barrier dysfunction and multidrug-resistant bacterial translocation in adult critical illness: Mechanistic insights from a systematic review

Abstract

BACKGROUND

The gastrointestinal tract plays an important role in host defence during critical illness. Disruption of epithelial integrity, microbiome imbalance, and immune dysregulation have all been linked to the translocation of multidrug-resistant (MDR) organisms from intestinal colonization to invasive infection. However, whether these associations reflect true causal mechanisms remains uncertain, and available human evidence has not been comprehensively synthesized using current methodological standards.

AIM

To systematically evaluate human evidence examining the relationship between intestinal barrier dysfunction, microbial colonization, and subsequent MDR infection in adult critical illness, with particular attention to study quality, heterogeneity, and potential confounding factors.

METHODS

This systematic review was conducted in accordance with PRISMA guidelines. A structured literature search was performed in PubMed, EMBASE, and the Cochrane Library (2000-2025) using predefined Boolean combinations and Medical Subject Headings. Prospective and retrospective cohort studies involving intensive care units (ICU) adults were included if they evaluated intestinal colonization, biomarkers of barrier dysfunction (citrulline and intestinal fatty acid-binding protein), microbiome alterations, or endotoxemia. Study selection and data extraction were undertaken independently by two reviewers, with disagreements resolved through discussion. Risk of bias was assessed using the Newcastle-Ottawa Scale and ROBINS-I tool. Owing to methodological and clinical heterogeneity, findings were synthesized using a structured narrative approach rather than meta-analysis.

RESULTS

Across the included studies, intestinal colonization with carbapenem-resistant Enterobacteriaceae, carbapenem-resistant Klebsiella pneumoniae, Acinetobacter baumannii, and vancomycin-resistant Enterococcus was consistently associated with an increased risk of subsequent bloodstream infection. However, progression rates varied considerably across cohorts, likely reflecting differences in patient characteristics, antimicrobial exposure, and ICU practices rather than a consistent effect size. Biomarker studies showed reduced citrulline levels and elevated intestinal fatty acid-binding protein concentrations in patients with gastrointestinal dysfunction; however, these markers indicate enterocyte injury rather than directly measuring intestinal permeability or bacterial translocation. Microbiome analyses demonstrated reduced diversity and impaired colonization resistance, although the extent and timing of these changes were not uniform across studies. Taken together, the evidence supports a biologically plausible link between epithelial injury, dysbiosis, and infection risk, but does not establish a direct causal relationship, largely due to the observational design of available studies and the influence of confounding factors such as illness severity, antimicrobial exposure, and ICU environment.

CONCLUSION

Gut barrier dysfunction appears to contribute to the pathogenesis of MDR infection in critically ill adults; however, current evidence supports association rather than causation. Early recognition of intestinal colonization and strategies aimed at preserving mucosal integrity may offer potential clinical benefit, although their effectiveness requires confirmation in well-designed prospective and interventional studies.

Key Words: Gut barrier dysfunction; Tight junctions; Multidrug-resistant bacteria; Carbapenem-resistant Enterobacteriaceae; Bacterial translocation; Critical illness; Sepsis; Gut microbiome dysbiosis

Core Tip: Intestinal colonization with multidrug-resistant organisms often precedes bloodstream infection in critically ill adults. Findings from clinical cohorts, microbiome analyses, and biomarker studies indicate that disruption of epithelial barrier integrity and loss of microbiome-mediated colonization resistance may increase the risk of systemic infection. However, most available evidence is observational and remains susceptible to confounding, particularly from antibiotic exposure, illness severity, and intensive care units-related factors. This PRISMA-compliant systematic review brings together mechanistic and clinical evidence, with careful appraisal of study quality, and identifies gut barrier dysfunction as a plausible - though not definitively causal - contributor to antimicrobial resistance-associated infections, highlighting important directions for future research.

INTRODUCTION

The gastrointestinal tract is increasingly recognized as an active contributor to systemic inflammation and organ dysfunction during critical illness. The long-standing concept of the “motor of multiple organ failure” suggests that disruption of the intestinal barrier may promote sepsis progression through translocation of microbes and endotoxins[1]. Both experimental and clinical studies indicate that factors such as hypoperfusion, pro-inflammatory cytokines - including tumor necrosis factor-alpha and interleukin-6 - and oxidative stress can impair epithelial tight junction integrity during shock states[2,3]. With the introduction of Sepsis-3, sepsis is now understood as life-threatening organ dysfunction arising from a dysregulated host response to infection[4]. Within this context, the gut may function not only as a target of systemic inflammation but also as a potential source of ongoing immune activation. However, the independent contribution of intestinal barrier dysfunction to systemic infection in humans remains uncertain, particularly given the influence of confounding factors such as illness severity, antimicrobial exposure, and intensive care units (ICU) environmental conditions.

The intestinal barrier is a complex, multi-layered system comprising mechanical (tight junctions), immunological (gut-associated lymphoid tissue and secretory immunoglobulin A), microbiological (commensal microbiota), and mucus-related components that together regulate permeability and host defence. Tight junctions - formed by proteins such as claudins, occludin, and zonula occludens (zonula occludens-1 and zonula occludens-2) - play a central role in controlling paracellular transport[5]. Inflammatory mediators, including tumor necrosis factor-alpha and interleukin-6, can disrupt this architecture by altering claudin expression and promoting junctional disassembly, thereby increasing permeability[6]. In human inflammatory conditions, reduced expression of zonula occludens-1 has been associated with impaired barrier function[7].

Critical illness is also associated with structural and functional alterations of the intestinal epithelium, including villous atrophy, reduced enterocyte mass, and increased epithelial apoptosis[8]. Plasma citrulline is widely used as a surrogate marker of enterocyte functional mass and is typically reduced in severe sepsis[9]. Conversely, intestinal fatty acid-binding protein (I-FABP), released during enterocyte injury, is elevated in the setting of mucosal damage[10]. It is important to note, however, that these biomarkers reflect epithelial injury or loss of functional mass rather than providing a direct measure of intestinal permeability or bacterial translocation, and should therefore be interpreted with appropriate caution.

Under normal physiological conditions, the gut microbiome provides colonization resistance against pathogenic organisms. In critically ill patients, however, exposure to broad-spectrum antibiotics is frequently associated with rapid and sometimes profound disruption of the microbiome, although the timing and extent of these changes vary across clinical settings[11]. This disruption is often characterized by reduced microbial diversity and relative dominance of Proteobacteria and other opportunistic pathogens[12].

Recent microbiome studies in ICU populations have linked reduced diversity with increased acquisition of multidrug-resistant (MDR) organisms and subsequent infection[13,14]. Loss of commensal anaerobes may diminish competitive inhibition, thereby facilitating expansion of organisms such as carbapenem-resistant Enterobacteriaceae (CRE)[15]. In addition to antibiotic-driven effects, ICU-specific factors - including cross-transmission and selective pressure from infection control practices - may further shape microbial dynamics.

Intestinal colonization with MDR bacteria is now recognized as an important predictor of bloodstream infection. Prospective studies of rectal CRE carriers, for example, have demonstrated a substantially higher risk of subsequent bacteremia compared with non-colonized patients[16-18]. Similar associations have been reported for carbapenem-resistant Klebsiella pneumoniae (CRKP)[19], Acinetobacter baumannii[20], and vancomycin-resistant Enterococcus (VRE)[21].

Genotypic analyses have shown concordance between rectal and bloodstream isolates[22], supporting the possibility of an endogenous source of infection. Findings from microbiome sequencing studies further suggest persistence of strains from the gut to the bloodstream[23]. Nevertheless, these observations are derived largely from observational data and do not establish causality. The transition from colonization to infection is influenced by multiple host and environmental factors, including illness severity, antimicrobial exposure, invasive procedures, and ICU ecology. Importantly, colonization does not invariably lead to infection. A key unresolved question is whether epithelial barrier dysfunction represents a necessary mechanistic link in this process or simply a parallel phenomenon in critically ill patients.

Lipopolysaccharide, a component of Gram-negative bacteria, activates Toll-like receptor 4-mediated inflammatory pathways and is strongly associated with septic shock[24]. Elevated endotoxin activity has been correlated with the severity of organ failure[25]. Although endotoxin-targeted therapies remain a subject of ongoing debate, endotoxemia illustrates the potential systemic consequences of barrier disruption. At the same time, circulating endotoxin levels are non-specific and do not definitively identify the intestine as the primary source of translocation.

Rationale for this review

Although accumulating evidence links intestinal colonization with MDR infection in critically ill patients, much of the existing literature examines epithelial barrier function, microbiome dynamics, biomarker data, and colonization cohorts separately. As a result, these interconnected domains are rarely evaluated within a unified analytical framework, and critical aspects such as study quality, heterogeneity, and confounding remain inconsistently addressed.

Insights from related fields - including hepatology and gastrointestinal diseases such as cirrhosis and inflammatory bowel disease - have underscored the importance of barrier dysfunction and microbial translocation in infection risk. However, these concepts have not been systematically contextualized within critically ill populations, where antimicrobial exposure, invasive interventions, and ICU-specific factors may substantially modify these relationships.

This systematic review therefore aims to integrate mechanistic and clinical evidence using a structured methodological approach, with specific focus on MDR organisms. In doing so, it seeks to distinguish association from causation, clarify the role of intestinal barrier dysfunction in relation to colonization, and identify key gaps to inform future research.

MATERIALS AND METHODS

Study design

This systematic review was conducted in accordance with the PRISMA guidelines[26]. The review was not prospectively registered; however, a predefined protocol was followed to guide the search strategy, eligibility criteria, and methods for data synthesis. The objective was to evaluate human evidence examining the relationship between intestinal barrier dysfunction, microbial colonization, and subsequent invasive infection caused by MDR bacteria in adult critical illness. As this study involved analysis of previously published data, ethical approval was not required.

Eligibility criteria

Studies were eligible if they included adult critically ill patients (≥ 18 years) admitted to ICUs and evaluated at least one of the following exposures: Intestinal colonization with MDR bacteria (e.g., CRE, CRKP, carbapenem-resistant Acinetobacter baumannii, or VRE); biomarkers of intestinal barrier dysfunction (including citrulline and I-FABP); microbiome dysbiosis; or evidence of endotoxemia or bacterial translocation. Given the breadth of these domains, heterogeneity across studies was anticipated. This was addressed through structured narrative synthesis rather than quantitative pooling, enabling integration of mechanistic and clinical findings without implying spurious precision.

Comparator and outcomes

Comparators included non-colonized patients, individuals with lower biomarker levels, preserved microbiome diversity, or absence of invasive infection. Primary outcomes were development of bloodstream infection, ICU-acquired infection, and mortality. Secondary outcomes included severity of organ dysfunction, gastrointestinal dysfunction scores, microbiome diversity, and molecular concordance between colonizing and infecting strains.

Inclusion and exclusion criteria

Eligible studies were human studies published from 2000 to 2025, including prospective or retrospective cohort studies and case-control studies with at least 10 participants, conducted in adult ICU populations. Studies were required to report either progression from colonization to infection or associations with intestinal barrier dysfunction biomarkers.

Exclusion criteria included animal studies, in vitro mechanistic studies without human data, single case reports, paediatric-only cohorts, and narrative reviews (used only for contextual background). Non-English studies and grey literature were excluded due to feasibility constraints; this has been acknowledged as a potential source of selection bias.

Information sources and search strategy

A comprehensive search of PubMed (MEDLINE), EMBASE, and the Cochrane Library was conducted for studies published from January 2000 to December 2025. The search strategy combined Medical Subject Headings and free-text terms using Boolean operators. A representative PubMed strategy was as follows: (“intestinal barrier” OR “tight junction” OR claudin OR occludin) AND (“critical illness” OR ICU OR sepsis) AND (“bacterial translocation” OR endotoxemia) AND (“multidrug-resistant” OR CRE OR CRKP OR VRE OR “Acinetobacter baumannii”) AND (microbiome OR dysbiosis).

Equivalent strategies were adapted for EMBASE and Cochrane databases. The complete database-specific search strategies, including Boolean operators, Medical Subject Headings terms, search combinations, and applied filters, are provided in Supplementary Table 1. Reference lists of included articles and relevant reviews were also screened to identify additional studies. Search results were exported in .nbib format and managed using reference management software, with duplicate records removed prior to screening.

Study selection

Two reviewers independently screened titles and abstracts for eligibility. Full-text articles were subsequently assessed against predefined inclusion criteria. Discrepancies were resolved through discussion, with involvement of a third reviewer where necessary.

No automation tools were used during the selection process. A total of 24 primary clinical studies met the inclusion criteria and were included in the final synthesis. The study selection process is illustrated in the PRISMA flow diagram (Figure 1). Supporting mechanistic and epidemiological studies were included to contextualize the findings.

Figure 1 PRISMA flow diagram illustrating the study identification and selection process for the systematic review. ¹Records were identified through a systematic search of the PubMed (MEDLINE), EMBASE, Cochrane Library database (2000-2025) using predefined keywords and manual screening of reference lists of relevant articles. ²No automation tools were used; all records were screened manually by the authors. ICU: Intensive care units. Adapted from Page et al[26].

Data extraction

Data extraction was performed independently by two reviewers using a standardized data collection form. Extracted variables included author, year, country, study design, ICU population characteristics, colonizing organisms, progression to bloodstream infection, mortality outcomes, biomarker data (citrulline and I-FABP), microbiome findings, and evidence of genotypic concordance. Discrepancies were resolved through discussion and consensus.

Risk of bias assessment

Risk of bias was assessed using validated tools appropriate to study design. Cohort and case-control studies were evaluated using the Newcastle-Ottawa Scale (NOS), while non-randomized interventional studies, where applicable, were assessed using the ROBINS-I tool.

Assessment domains included selection bias, comparability of study groups, outcome assessment, and adequacy of follow-up. Studies were categorized as low, moderate, or high risk of bias. Detailed study-level NOS and ROBINS-I assessments are presented in Supplementary Table 2. Overall, most included studies were of moderate methodological quality, with common limitations including residual confounding, variability in colonization screening practices, and differences in microbiological methods.

Data synthesis

Given substantial heterogeneity in study design, microbiological definitions, and outcome measures, a formal meta-analysis was not performed. Instead, findings were synthesized using a structured narrative approach, incorporating qualitative comparison of study outcomes, thematic integration of mechanistic and clinical data, and assessment of biological plausibility.

Conceptual framework

The synthesis was guided by a conceptual framework in which critical illness is associated with hypoperfusion and systemic inflammation, leading to disruption of epithelial tight junction integrity and increased intestinal permeability. These changes may occur alongside microbiome alterations, resulting in loss of colonization resistance and expansion of MDR organisms within the gut. In a subset of patients, these processes may contribute to bacterial translocation and subsequent systemic infection. This framework was used to support interpretation of findings while explicitly acknowledging that the available evidence is observational and does not establish a definitive causal pathway.

RESULTS

Study selection

A systematic search of three electronic databases (PubMed, EMBASE, and the Cochrane Library) identified records relevant to the predefined study domains. After removal of duplicate entries and exclusion of ineligible studies - including animal studies, paediatric-only cohorts, narrative reviews, and small case reports - 24 primary clinical studies met the inclusion criteria and were included in the final synthesis.

Study selection was carried out independently by two reviewers. The process is summarized in the PRISMA flow diagram (Figure 1), which details the number of records identified, screened, excluded, and included at each stage, along with reasons for exclusion.

The 24 included studies comprised cohort and observational studies evaluating progression from colonization to infection, biomarker-based investigations assessing intestinal injury markers, and microbiome studies examining dysbiosis and colonization resistance. Considerable heterogeneity was observed across studies in terms of design, patient populations, microbiological definitions, and outcome measures, which precluded formal quantitative pooling. The key characteristics of the included studies are presented in Table 1.

Table 1 Characteristics of included clinical studies evaluating gut barrier dysfunction and multidrug-resistant bacterial translocation in critically ill adults.

Ref.	Country	Study design	ICU population	Organism/focus	Key findings
Thom et al[22], 2010	United States	Observational cohort	ICU patients	Acinetobacter baumannii	Identical strains detected in GI colonization and bloodstream infection
Jung et al[20], 2010	South Korea	Cohort	ICU patients	MDR Acinetobacter baumannii	Colonization associated with increased risk of bacteraemia
Giannella et al[17], 2014	Italy	Prospective multicentre cohort	Rectal carriers	CRKP	Rectal colonization strongly predicted subsequent bacteraemia
Giacobbe et al[19], 2017	Italy	Multicentre retrospective cohort	Colonized patients	CRKP	Prior infections predicted CRKP bacteraemia
Freedberg et al[11], 2018	United States	Prospective observational study	ICU patients	Gastrointestinal microbiome colonization	GI pathogen colonization at ICU admission associated with subsequent infection and mortality
Amberpet et al[21], 2018	India	Prospective study	Pediatric ICU patients	Vancomycin-resistant enterococci	Intestinal colonization identified as a major risk factor for MDR transmission
Zaborin et al[12], 2014	United States	Microbiome observational study	Critically ill patients	Gut pathogen communities	Ultra-low-diversity gut microbiota associated with pathogen domination during critical illness
Silago et al[29], 2020	Tanzania	Observational cohort	Critical care patients	MDR Gram-negative bacteria	Colonization and environmental contamination contributed to bacteraemia
Padar et al[31], 2021	Estonia	Prospective ICU study	ICU patients	Citrulline/I-FABP	Biomarkers reflected intestinal injury in critical illness
Reintam Blaser et al[33], 2021	Multicentre Europe	Prospective observational study	Critically ill patients	GI dysfunction	Developed gastrointestinal dysfunction scoring system
Chu et al[16], 2022	China	Prospective cohort	ICU patients	CRE	Rectal colonization significantly associated with bloodstream infection
Garcia et al[13], 2022	Spain	Prospective cohort	ICU patients	MDR bacteria/microbiome	Gut microbiome disruption associated with MDR colonization
Mu et al[23], 2022	China	Molecular cohort study	Septic patients	Gut–blood pathogen concordance	Bloodstream pathogens genetically matched gut strains
Falcone et al[41], 2022	Italy	Observational cohort	Severe COVID-19 ICU patients	MDR Klebsiella pneumoniae	Hypervirulent MDR strains spread in ICU population
Baek et al[14], 2023	South Korea	Microbiome analysis	ICU patients	CRE	Dysbiosis associated with CRE colonization
Casale et al[40], 2023	Italy	Cohort study	Hospitalized COVID-19 ICU patients	Carbapenem-resistant organisms	Colonization increased mortality and infection risk
Onuk et al[34], 2023	Turkey	Prospective cohort	ICU patients	Citrulline/I-FABP	Biomarkers correlated with gastrointestinal dysfunction
Tyszko et al[35], 2023	Poland	Observational study	Septic ICU patients	Citrulline/I-FABP	Biomarkers predicted gastrointestinal failure
Molinari et al[25], 2025	International multicentre	Prospective observational study	Septic shock patients	Endotoxin activity	Higher endotoxin activity associated with organ failure and mortality
Yang et al[15], 2025	China	Microbiome study	ICU patients	CRKP	Microbiome diversity associated with colonization resistance
Favier et al[18], 2026	Argentina	Prospective surveillance study	ICU patients	Carbapenem-resistant Enterobacterales	Rectal and extra-rectal colonization predicted subsequent infection
Fang et al[28], 2025	China	Molecular epidemiological study	Hospitalized patients	CRKP	Colonizing and bloodstream isolates showed plasmid and genotypic concordance
Nguyen et al[37], 2025	France	Prospective clinical study	Septic/peritonitis patients	Endotoxin burden/HDL	HDL levels associated with reduced endotoxin burden in sepsis
Lyu et al[27], 2026	China	Prospective cohort	ICU patients	CRE	Genotypic concordance between colonizing and infecting strains

ICU: Intensive care units; ESBL: Extended-spectrum beta-lactamase; GI: Gastrointestinal; MDR: Multidrug-resistant; CRKP: Carbapenem-resistant Klebsiella pneumoniae; I-FABP: Intestinal fatty acid-binding protein; HDL: High-density lipoprotein; CRE: Carbapenem-resistant Enterobacteriaceae; COVID-19: Coronavirus disease 2019.

Characteristics of included studies

Geographic distribution: The included studies spanned multiple geographic regions, including Europe, Asia, Africa, and the Americas. Differences in antimicrobial resistance patterns, healthcare infrastructure, and ICU practices across these regions may limit direct comparability of findings.

Colonization and progression to bloodstream infection: Across the reviewed studies, intestinal colonization with MDR organisms - such as CRKP, CRE, Acinetobacter baumannii, and VRE - was consistently associated with an increased risk of subsequent bloodstream infection[16-21,27-32]. However, the strength of this association varied between studies and appeared to be influenced by factors including patient characteristics, antimicrobial exposure, colonization burden, and ICU-specific practices.

Molecular and genotypic analyses demonstrated concordance between colonizing and bloodstream isolates[22,30], supporting the possibility of an endogenous source of infection. At the same time, evidence of cross-transmission and environmental contamination indicates that multiple pathways may contribute to infection in critically ill patients. Importantly, progression from colonization to infection was not observed in all cases. This variability suggests that host-related factors, severity of illness, invasive procedures, and antimicrobial pressure play a substantial role in determining clinical outcomes. Key studies examining the relationship between intestinal colonization and subsequent bloodstream infection are summarized in Table 2.

Table 2 Clinical studies evaluating the association between intestinal colonization and subsequent bloodstream infection in critically ill patients.

Ref.	Country	Study design			Population		Pathogen		Colonization site		Progression to infection		Key finding
Thom et al[22], 2010	United States	Observational study			ICU patients		Acinetobacter baumannii		GI tract		Identical strains detected		GI colonization matched bloodstream isolates
Jung et al[20], 2010	South Korea	Cohort			ICU patients		MDR Acinetobacter baumannii		Gastrointestinal tract		Increased risk of bacteraemia		Colonization independently predicted bloodstream infection
Giannella et al[17], 2014	Italy	Prospective multicentre cohort			ICU patients with rectal colonization		CRKP		Rectal		Approximately 20%-40% bacteraemia		Rectal carriage strongly predicted CRKP bloodstream infection
Giacobbe et al[19], 2017	Italy	Multicentre retrospective cohort			Colonized patients		CRKP		Rectal		Increased bacteraemia risk		Previous infections predicted CRKP bacteraemia
Freedberg et al[11], 2018	United States	Prospective observational study			ICU patients		MDR gut pathogens		Gastrointestinal microbiome		Increased infection and mortality risk		GI pathogen colonization at ICU admission predicted adverse outcomes
Amberpet et al[21], 2018	India	Prospective study			Pediatric ICU patients		Vancomycin-resistant enterococci		Intestinal tract		Increased colonization risk		Intestinal colonization facilitated MDR persistence and transmission
Silago et al[29], 2020	Tanzania	Observational cohort			Critical care patients		MDR Gram-negative bacteria		GI/environmental		Transmission linked to bacteraemia		Colonization and cross-transmission contributed to infection
Chu et al[16], 2022	China	Prospective cohort			ICU patients		CRE		Rectal		Approximately 15%-30% progression		Colonization associated with subsequent infection
Mu et al[23], 2022	China	Molecular cohort			Septic ICU patients		Various pathogens		Gut microbiome		High strain concordance		Bloodstream pathogens genetically matched gut strains
Casale et al[40], 2023	Italy	Cohort study			COVID-19 ICU patients		Carbapenem-resistant organisms		Gastrointestinal/respiratory		Increased mortality and infection risk		Colonization associated with superinfection and adverse outcomes
Fang et al[28], 2025	China	Molecular epidemiological study			Hospitalized patients		CRKP		Gastrointestinal tract		Genotypic concordance observed		Colonizing and infecting strains shared homologous resistance plasmids
Lyu et al[27], 2026	China		Prospective cohort	ICU patients		CRE		Rectal		Significant progression observed		Colonizing and infecting isolates showed genotypic concordance
Favier et al[18], 2026	Argentina		Prospective surveillance study	ICU patients		Carbapenem-resistant Enterobacterales		Rectal and extra-rectal		Increased infection risk		Surveillance cultures predicted subsequent CRE infection

ICU: Intensive care units; COVID-19: Coronavirus disease 2019; GI: Gastrointestinal; MDR: Multidrug-resistant; CRKP: Carbapenem-resistant Klebsiella pneumoniae; CRE: Carbapenem-resistant Enterobacteriaceae.

Microbiome alterations in critical illness: A number of studies have examined microbiome dynamics in ICU patients, consistently showing that reduced alpha diversity is linked to increased acquisition of MDR pathogens[13,14]. In this setting, critically ill patients frequently demonstrate a shift toward dominance of Proteobacteria, particularly following exposure to broad-spectrum antibiotics[12]. At the same time, depletion of obligate anaerobes has been associated with colonization by carbapenem-resistant organisms[15]. However, both the timing and magnitude of these microbiome changes vary across studies, and the frequently described early “collapse” of microbial diversity is not consistently observed.

Multicentre analyses have further reported associations between microbiome disruption and higher risks of infection and mortality[14,33]. These observations, however, require cautious interpretation. Changes in the microbiome may reflect underlying disease severity, antimicrobial exposure, and ICU-related environmental influences, rather than serving as independent drivers of infection.

Biomarkers of intestinal barrier dysfunction

Plasma citrulline: Reduced plasma citrulline levels have been consistently reported in critically ill patients with gastrointestinal dysfunction[9]. Lower concentrations are associated with greater severity of organ failure and increased mortality[34]. Importantly, citrulline reflects enterocyte functional mass rather than directly measuring intestinal permeability or bacterial translocation.

I-FABP: I-FABP concentrations are elevated in the presence of enterocyte injury[10,35], and higher levels have been linked to increased acute gastrointestinal injury scores[36,37]. Similar to citrulline, I-FABP serves as an indirect indicator of mucosal injury and does not directly quantify barrier integrity or translocation events.

Endotoxemia and organ dysfunction: Elevated endotoxin activity has been linked to greater severity of organ dysfunction in septic shock[25,38], with higher endotoxin burden also correlating with mortality risk[39,40]. However, circulating endotoxin levels are non-specific and cannot reliably distinguish intestinal translocation from other potential sources of infection.

Integrated mechanistic-clinical synthesis

Taken together, the available evidence supports a biologically plausible sequence in which critical illness - through hypoperfusion and systemic inflammatory responses - may contribute to disruption of epithelial tight junction integrity and altered intestinal permeability[2,5-7]. These changes are often accompanied by enterocyte injury, reflected by reduced citrulline levels and elevated I-FABP concentrations[9,10].

At the same time, antibiotic exposure and physiological stress are associated with microbiome alterations, including reduced diversity and relative dominance of MDR organisms[11-15]. In a subset of patients, intestinal colonization has been linked to subsequent bloodstream infection, and molecular studies demonstrating genotypic concordance between colonizing and infecting isolates support the possibility of an endogenous source[16-20,22,30].

These observations should, however, be interpreted with caution. Progression from colonization to infection is influenced by multiple host and environmental factors, and the available evidence remains largely observational. Accordingly, this sequence is best viewed as a conceptual framework derived from convergent findings rather than a definitive causal pathway.

Candida colonization

Although the present review focuses on bacterial MDR pathogens, fungal translocation has also been described in critically ill patients. In particular, Candida colonization may precede candidemia, especially in the context of mucosal injury and broad-spectrum antibiotic exposure. However, this aspect was not systematically evaluated in the current review and is included here for contextual relevance only. Biomarker and microbiome studies supporting intestinal barrier dysfunction in critically ill patients are summarized in Table 3.

Table 3 Biomarker and microbiome evidence associated with intestinal barrier dysfunction in critically ill patients.

Ref.	Country	Study design	Population	Biomarker/method	Key findings	Clinical implication
Wijnands et al[9], 2015	Netherlands	Review/clinical synthesis	Sepsis patients	Citrulline metabolism	Reduced citrulline associated with enterocyte dysfunction	Marker of intestinal barrier injury
Blaser et al[10], 2019	Europe	Observational clinical study	Critically ill patients	Citrulline, I-FABP	Low citrulline and elevated I-FABP associated with GI dysfunction	Biomarkers of mucosal injury
Padar et al[31], 2021	Estonia	Prospective ICU study	ICU patients	Citrulline, I-FABP	Dynamic changes correlated with enteral nutrition and gut injury	Monitoring intestinal function
Reintam Blaser et al[33], 2021	Multicentre Europe	Prospective multicentre study	Critically ill patients	GI Dysfunction Score	Developed standardized scoring for GI dysfunction	Clinical assessment tool
Garcia et al[13], 2022	Spain	Prospective cohort	ICU patients	Gut microbiome sequencing	Reduced diversity associated with MDR colonization and infection	Dysbiosis associated with increased infection risk
Onuk et al[34], 2023	Turkey	Prospective cohort	ICU patients	Citrulline, I-FABP, gastric ultrasound	Biomarkers correlated with gastrointestinal injury	Early detection of GI dysfunction
Tyszko et al[35], 2023	Poland	Observational cohort	Septic ICU patients	Citrulline, I-FABP	Biomarkers associated with severity of gastrointestinal injury	Prognostic markers
Baek et al[14], 2023	South Korea	Microbiome analysis	ICU patients	Microbiome sequencing	CRE colonization associated with microbiome disruption	Loss of colonization resistance
Yang et al[15], 2025	China	Microbiome study	ICU patients	Microbiome diversity analysis	Preserved microbiome diversity associated with reduced CRKP colonization	Microbiome-mediated resistance

I-FABP: Intestinal fatty acid-binding protein; GI: Gastrointestinal; ICU: Intensive care units; MDR: Multidrug-resistant; CRE: Carbapenem-resistant Enterobacteriaceae; CRKP: Carbapenem-resistant Klebsiella pneumoniae.

Summary of quantitative trends

Given the substantial heterogeneity across included studies, formal quantitative pooling was not undertaken. Instead, overall patterns were identified through structured narrative synthesis. Reported progression rates from colonization to infection ranged approximately from 15% to 40%, although these estimates varied considerably depending on study design, patient characteristics, and clinical setting. Molecular analyses frequently demonstrated genotypic concordance between gut and bloodstream isolates, supporting the possibility of an endogenous source of infection.

Biomarker data consistently indicated reduced citrulline levels and elevated I-FABP in association with gastrointestinal dysfunction and adverse outcomes. In parallel, reduced microbiome diversity was linked to an increased risk of colonization and infection with MDR organisms. These findings should be interpreted with caution. Differences in study design, definitions, and measurement approaches, along with residual confounding, limit the ability to draw definitive conclusions regarding the strength and direction of these associations.

Risk of bias in included studies

The methodological quality of included studies was assessed using the NOS for cohort and case-control studies, and the ROBINS-I tool for non-randomized studies. Overall, most studies were of moderate quality. Common limitations included potential selection bias, variability in colonization screening practices, inconsistent microbiological definitions, and limited adjustment for confounding factors such as illness severity and antimicrobial exposure.

DISCUSSION

Principal findings

This systematic synthesis of 24 primary human studies indicates that intestinal colonization with MDR bacteria in critically ill adults is consistently associated with subsequent invasive infection. Reported progression rates varied widely and should be interpreted with caution, as they likely reflect differences in study design, patient populations, antimicrobial exposure, and ICU practices rather than a uniform effect size.

Molecular concordance analyses in several studies demonstrated identical strains in rectal and bloodstream isolates[22,30], supporting the possibility of an endogenous source of infection. At the same time, alternative pathways - including cross-transmission and environmental acquisition within ICU settings - remain plausible and cannot be excluded.

When considered alongside biomarker and microbiome data, a biologically plausible framework emerges linking epithelial injury, dysbiosis, and immune dysregulation with MDR bacteraemia. However, this interpretation should remain cautious, as the available evidence is largely observational and does not establish a causal relationship.

Mechanistic integration: Conceptual links between barrier dysfunction and MDR infection

The following synthesis integrates mechanistic and clinical observations and is intended as a hypothesis-generating framework rather than a definitive causal sequence.

Epithelial disruption: Tight junction integrity may be compromised during systemic inflammation and shock, with cytokine-mediated alterations in claudin and occludin expression contributing to increased paracellular permeability[5-7]. Reduced plasma citrulline reflects diminished enterocyte mass and functional impairment[9,34], whereas elevated I-FABP indicates mucosal injury[10,35-37]. These biomarkers provide indirect evidence of epithelial injury rather than direct measurement of intestinal permeability or bacterial translocation. Accordingly, barrier dysfunction is likely to represent one component of a multifactorial process rather than a singular causal determinant of infection.

Microbiome dysbiosis and loss of colonization resistance: Critical illness is frequently accompanied by microbiome disruption, characterized by reduced diversity and relative dominance of opportunistic Gram-negative organisms[11-15]. Loss of obligate anaerobes may impair colonization resistance mechanisms that normally limit pathogen overgrowth.

However, the extent and timing of these changes vary considerably across studies and are influenced by factors such as antibiotic exposure, nutritional status, and ICU environmental conditions. Some microbiome analyses suggest that preserved diversity may protect against CRKP acquisition[33], underscoring the functional importance of microbial ecology. Nevertheless, microbiome alterations may also reflect underlying disease severity rather than acting as independent drivers of infection.

Colonization burden and strain persistence: The density and duration of colonization may influence progression to invasive infection, although this relationship is not consistently observed across all studies. Evidence suggests that a higher intestinal burden of CRE or CRKP is associated with increased infection risk[16-19].

Genetic analyses demonstrating shared resistance elements (e.g., blaKPC-2) between colonizing and infecting isolates support clonal persistence[30]. However, because progression from colonization to infection is not universal, additional host and environmental factors are likely required for transition to invasive disease.

Endotoxemia as a systemic amplifier: Endotoxin activity has been correlated with the severity of organ dysfunction in septic shock[25,38,39], reflecting broader systemic inflammatory activation. Although endotoxin-targeted therapies remain under investigation[40], endotoxemia highlights the potential systemic consequences of barrier disruption.

At the same time, circulating endotoxin levels are non-specific and do not definitively identify the intestine as the primary source of translocation. Thus, the gut may contribute to systemic inflammatory amplification in selected ICU patients, but its relative contribution compared with other infection sources remains uncertain.

Overall, the proposed pathway linking epithelial injury, microbiome dysbiosis, and MDR colonization with systemic infection should be interpreted as a conceptual framework supported by convergent observational evidence rather than a proven causal sequence. Taken together, these findings suggest a biologically plausible model in which gut barrier dysfunction may contribute to infection risk in critically ill patients, while acknowledging the complexity and multifactorial nature of this process. A conceptual overview of this framework is illustrated in Figure 2.

Figure 2 Conceptual model of gut barrier dysfunction and multidrug-resistant bacterial translocation in critical illness. Critical illness is associated with intestinal epithelial injury characterized by tight junction disruption, mucosal inflammation, and altered intestinal permeability. Antibiotic exposure and physiological stress are associated with microbiome dysbiosis and reduced colonization resistance, which may permit expansion of multidrug-resistant organisms such as carbapenem-resistant Enterobacteriaceae, carbapenem-resistant Klebsiella pneumoniae, and vancomycin-resistant Enterococcus. These processes are hypothesized to contribute to bacterial translocation and endotoxin release into the bloodstream, potentially leading to systemic infection and sepsis. This model represents a conceptual integration of mechanistic and clinical evidence derived from observational studies and should not be interpreted as establishing a definitive causal path. MDR: Multidrug-resistant; CRE: Carbapenem-resistant Enterobacteriaceae; CRKP: Carbapenem-resistant Klebsiella pneumoniae; VRE: Vancomycin-resistant Enterococcus; I-FABP: Intestinal fatty acid-binding protein.

Clinical implications

From a clinical perspective, identification of intestinal colonization with MDR organisms, including CRE and CRKP, may assist in risk stratification for subsequent bloodstream infection in critically ill patients. Measures aimed at preserving gut barrier integrity, such as early enteral nutrition, avoidance of unnecessary broad-spectrum antibiotic exposure, and optimization of hemodynamic status, may help support mucosal function. However, the direct impact of these interventions on MDR bacterial translocation and clinical outcomes remains incompletely defined.

Microbiome-directed approaches, including probiotics and faecal microbiota transplantation, have generated increasing interest in recent years. Nevertheless, current evidence in critically ill populations remains limited, heterogeneous, and largely observational. These strategies should therefore presently be regarded as investigational and hypothesis-generating rather than established therapeutic interventions. Similarly, risk-adapted antimicrobial stewardship approaches in colonized patients may help guide empiric therapy during septic episodes, although prospective validation studies are still required before routine implementation can be recommended.

Biomarkers such as citrulline and I-FABP may provide adjunctive information regarding gastrointestinal dysfunction and epithelial injury in critical illness. It is important to recognize, however, that these biomarkers represent indirect indicators of enterocyte injury rather than direct measures of intestinal permeability or bacterial translocation. Their interpretation should therefore be made cautiously and within the broader clinical context.

Comparison with prior concepts

The concept of the “gut as the motor of multiple organ failure” has been widely proposed[1], yet its direct clinical linkage to MDR bacteraemia in critically ill patients has not been clearly established. Insights from related disciplines - including hepatology (e.g., cirrhosis-associated bacterial translocation) and inflammatory bowel disease - have highlighted the role of barrier dysfunction and microbial translocation in infection risk, providing important biological context for the present findings. However, these areas have largely been investigated separately from the epidemiology of MDR infections in critical care settings.

The present review builds on these foundations by integrating epithelial barrier biology, microbiome ecology, colonization cohort data, and molecular strain concordance within a single PRISMA-compliant framework. By focusing specifically on MDR organisms and critically evaluating heterogeneous human evidence, this study offers a structured synthesis that distinguishes association from causation and identifies key mechanistic and clinical gaps for future research.

Limitations

Several limitations should be considered when interpreting the findings of this review. First, the majority of included studies were observational in design, which restricts the ability to draw causal inferences. Substantial heterogeneity was present across studies with respect to colonization screening methods, microbiological definitions, and outcome measures, limiting comparability and precluding formal quantitative synthesis. In addition, standardized assessment of intestinal permeability was limited, and direct permeability testing (e.g., lactulose-mannitol assays) was rarely performed.

Key confounding factors - including illness severity, antimicrobial exposure, and ICU-specific practices - may have influenced the observed associations. Furthermore, the exclusion of non-English studies and grey literature introduces the possibility of selection bias. Variability in microbiological and molecular diagnostic techniques across studies also limits direct comparison of findings. In light of these considerations, the results should be interpreted with caution, and the conclusions are best viewed as associative rather than definitive.

Reporting bias assessment

Formal assessment of reporting bias, including publication bias, was not conducted due to substantial heterogeneity across studies and the use of narrative synthesis rather than meta-analysis. However, efforts were made to minimize bias through comprehensive database searching and screening of reference lists.

Future directions

Future research should prioritize well-designed prospective and longitudinal studies that integrate intestinal permeability biomarkers with colonization surveillance, enabling clearer delineation of temporal relationships. Standardized approaches to microbiome profiling in ICU populations are also needed to enhance comparability across studies and to better define the role of dysbiosis in infection risk.

Interventional trials aimed at preserving gut barrier integrity - including nutritional optimization and hemodynamic support - as well as microbiome-targeted strategies, warrant systematic evaluation. In parallel, incorporation of genomic host-response phenotyping[41] may offer further insight into individual susceptibility and underlying disease heterogeneity. Ultimately, robust mechanistic and interventional studies will be essential to determine whether targeted modulation of gut barrier function can meaningfully reduce the risk of MDR infections in critically ill patients.

CONCLUSION

In critically ill adults, intestinal colonization with MDR organisms frequently precedes bloodstream infection. The available evidence, derived predominantly from observational and mechanistic studies, supports a clinically relevant association between colonization and subsequent infection, although a definitive causal relationship cannot currently be established.

Findings from epithelial biology, microbiome research, biomarker investigations, and molecular strain-concordance studies collectively suggest a biologically plausible framework in which gut barrier dysfunction may contribute to endogenous infection pathways. That said, this relationship is complex and likely influenced by multiple interacting factors, including illness severity, antimicrobial exposure, invasive device utilization, and ICU environmental dynamics.

Recognition of the gut as a potential reservoir and mediator of antimicrobial resistance may improve risk stratification and guide future translational research. However, microbiome-directed and barrier-preserving interventions should presently be regarded as hypothesis-generating approaches rather than established therapeutic strategies. Their clinical utility will require validation through well-designed prospective, mechanistic, and interventional studies before routine implementation can be recommended.

ACKNOWLEDGEMENTS

Authors wish to thank Dr. Arunkumar Rao, Professor, Head, Orthopedics, MIMSR Medical College, Latur and Prof. Amrapali Ganjewar, (M.A. English Literature), Smt. Janakibai Rama Salvi College of Arts, Commerce & Science, Thane for proofreading of manuscript for proper English language, grammar, punctuation, spelling, and overall style. Authors are also thankful to Dr. D V Tandle, Assistant Professor in Statistics, MIMSR Medical College, Latur. Thanks are also due to Mr. Vinod Jogdand and Mr. Deepak Badane for technical support.