The Shift of an Intestinal "Microbiome" to a "Pathobiome" Governs the Course and Outcome of Sepsis Following Surgical Injury

The intestinal microenvironment in sepsis

The gastrointestinal tract has long been hypothesized to function as "the motor" of multiple organ dysfunction syndrome. The gastrointestinal microenvironment is comprised of a single cell layer epithelia, a local immune system, and the microbiome. These three components of the intestine together play a crucial role in maintaining homeostasis during times of health. However, the gastrointestinal microenvironment is perturbed during sepsis, resulting in pathologic changes that drive both local and distant injury. In this review, we seek to characterize the relationship between the epithelium, gastrointestinal lymphocytes, and commensal bacteria during basal and pathologic conditions and how the intestinal microenvironment may be targeted for therapeutic gain in septic patients.

The role of NIGMS P50 sponsored team science in our understanding of multiple organ failure

The history of the National Institute of General Medical Sciences (NIGMS) Research Centers in Peri-operative Sciences (RCIPS) is the history of clinical, translational, and basic science research into the etiology and treatment of posttraumatic multiple organ failure (MOF). Born out of the activism of trauma and burn surgeons after the Viet Nam War, the P50 trauma research centers have been a nidus of research advances in the field and the training of future academic physician-scientists in the fields of trauma, burns, sepsis, and critical illness. For over 40 years, research conducted under the aegis of this funding program has led to numerous contributions at both the bedside and at the bench. In fact, it has been this requirement for team science with a clinician-scientist working closely with basic scientists from multiple disciplines that has led the RCIPS to its unrivaled success in the field. This review will briefly highlight some of the major accomplishments of the RCIPS program since its inception, how they have both led and evolved as the field moved steadily forward, and how they are responsible for much of our current understanding of the etiology and pathology of MOF. This review is not intended to be all encompassing nor a historical reference. Rather, it serves as recognition to the foresight and support of many past and present individuals at the NIGMS and at academic institutions who have understood the cost of critical illness and MOF to the individual and to society.

The evolutionary logic of sepsis

The recently proposed Microbiome Mutiny Hypothesis posits that members of the human microbiome obtain information about the host individuals' health status and, when host survival is compromised, switch to an intensive exploitation strategy to maximize residual transmission. In animals and humans, sepsis is an acute systemic reaction to microbes invading the normally sterile body compartments. When induced by formerly mutualistic or neutral microbes, possibly in response to declining host health, sepsis appears to fit the 'microbiome mutiny' scenario except for its apparent failure to enhance transmission of the causative organisms. We propose that the ability of certain species of the microbiome to induce sepsis is not a fortuitous side effect of within-host replication, but rather it might, in some cases, be the result of their adaptive evolution. Whenever host health declines, inducing sepsis can be adaptive for those members of the healthy human microbiome that are capable of colonizing the future cadaver and spread by cadaver-borne transmission. We hypothesize that such microbes might exhibit switches along the 'mutualist - lethal pathogen - decomposer - mutualist again' scenario, implicating a previously unsuspected, surprising level of phenotypic plasticity. This hypothesis predicts that those species of the healthy microbiome that are recurring causative agents of sepsis can participate in the decomposition of cadavers, and can be transmitted as soil-borne or water-borne infections. Furthermore, in individual sepsis cases, the same microbial clones that dominate the systemic infection that precipitates sepsis, should also be present in high concentration during decomposition following death: this prediction is testable by molecular fingerprinting in experimentally induced animal models. Sepsis is a leading cause of human death worldwide. If further research confirms that some cases of sepsis indeed involve the 'mutiny' (facultative phenotypic switching) of normal members of the microbiome, then new strategies could be devised to prevent or treat sepsis by interfering with this process.

Myosin light chain kinase knockout improves gut barrier function and confers a survival advantage in polymicrobial sepsis

Sepsis-induced intestinal hyperpermeability is mediated by disruption of the epithelial tight junction, which is closely associated with the peri-junctional actin-myosin ring. Myosin light chain kinase (MLCK) phosphorylates the myosin regulatory light chain, resulting in increased permeability. The purpose of this study was to determine whether genetic deletion of MLCK would alter gut barrier function and survival from sepsis. MLCK^-/- and wild type (WT) mice were subjected to cecal ligation and puncture and assayed for both survival and mechanistic studies. Survival was significantly increased in MLCK^-/- mice (95% vs. 24%, p<0.0001). Intestinal permeability increased in septic WT mice compared to unmanipulated mice. In contrast, permeability in septic MLCK^-/- mice was similar to that seen in unmanipulated animals. Improved gut barrier function in MLCK^-/- mice was associated with increases in the tight junction mediators ZO-1 and claudin 15 without alterations in claudin 1, 2, 3, 4, 5, 7, 8, 13, occludin or JAM-A. Other components of intestinal integrity (apoptosis, proliferation and villus length) were unaffected by MLCK deletion as were local peritoneal inflammation and distant lung injury. Systemic IL-10 was decreased greater than 10-fold in MLCK^-/- mice; however, survival was similar between septic MLCK^-/- mice given exogenous IL-10 or vehicle. These data demonstrate that deletion of MLCK improves survival following sepsis, associated with normalization of intestinal permeability and selected tight junction proteins.

Aggressive Antimicrobial Initiation for Suspected Intensive Care Unit-Acquired Infection Is Associated with Decreased Long-Term Survival after Critical Illness

BACKGROUND

The long-term significance of early and prolonged antibiotic use in critically ill patients is yet to be described. Several studies suggest that antimicrobial exposure may have as yet unrecognized long-term effects on clinically meaningful outcomes. Our group previously conducted a quasi-experimental, before and after observational cohort study of hemodynamically stable surgical patients suspected of having an intensive care unit-acquired infection. This study demonstrated that aggressive initiation of antimicrobial therapy was associated with increased 30-day deaths. In a follow-up survival analysis, we hypothesized that aggressive antimicrobial treatment would not be a significant predictor of long-term death.

METHODS

Survival data for the 201 patients included in the initial study were obtained from our clinical data repository. Univariable analysis, Kaplan-Meier, and Cox proportional hazards models were used. Survival was evaluated at one and four years. Age, gender, Acute Physiology and Chronic Health Evaluation (APACHE) II score, and co-morbidities were chosen a priori for potential inclusion in the model. Variables that met the model assumptions after testing were included in the final model.

RESULTS

Follow-up data were available for 190 patients (95 in each group) representing 94.5% of the initial cohort. Twenty-four (25.3%) patients in the aggressive group had initial APACHE II scores of less than 15 compared with 13 (13.7%) patients in the conservative group (p = 0.04). There was a trend toward higher deaths in the aggressive group at four years (41.1% vs. 30.5%; p = 0.13). Kaplan-Meier analysis demonstrated a difference in survival at one year but not at four years. The Cox proportional hazards model showed a higher long-term death for patients in the aggressive antimicrobial group at both one and four years (hazard rate: 2.26 and 1.70, respectively).

CONCLUSION

Aggressive initiation of antimicrobial therapy is independently associated with decreased long-term survival after critical illness. While further work is needed to confirm these findings, waiting for evidence of infection before initiation of antibiotic agents may be beneficial.

The gut microbiome and the mechanism of surgical infection

BACKGROUND

Since the very early days of surgical practice, surgeons have recognized the importance of considering that intestinal microbes might have a profound influence on recovery from surgical diseases such as appendicitis and peritonitis. Although the pathogenesis of surgical diseases such as cholelithiasis, diverticulosis, peptic ulcer disease and cancer have been viewed as disorders of host biology, they are emerging as diseases highly influenced by their surrounding microbiota.

METHODS

This is a review of evolving concepts in microbiome sciences across a variety of surgical diseases and disorders, with a focus on disease aetiology and treatment options.

RESULTS

The discovery that peptic ulcer disease and, in some instances, gastric cancer can now be considered as infectious diseases means that to advance surgical practice humans need to be viewed as superorganisms, consisting of both host and microbial genes. Applying this line of reasoning to the ever-ageing population of patients demands a more complete understanding of the effects of modern-day stressors on both the host metabolome and microbiome.

CONCLUSION

Despite major advances in perioperative care, surgeons today are witnessing rising infection-related complications following elective surgery. Many of these infections are caused by resistant and virulent micro-organisms that have emerged as a result of human progress, including global travel, antibiotic exposure, crowded urban conditions, and the application of invasive and prolonged medical and surgical treatment. A more complete understanding of the role of the microbiome in surgical disease is warranted to inform the path forward for prevention.

New insights into the gut as the driver of critical illness and organ failure

PURPOSE OF REVIEW

The gut has long been hypothesized to be the 'motor' of multiple organ dysfunction syndrome. This review serves as an update on new data elucidating the role of the gut as the propagator of organ failure in critical illness.

RECENT FINDINGS

Under basal conditions, the gut absorbs nutrients and serves as a barrier that prevents approximately 40 trillion intraluminal microbes and their products from causing host injury. However, in critical illness, gut integrity is disrupted with hyperpermeability and increased epithelial apoptosis, allowing contamination of extraluminal sites that are ordinarily sterile. These alterations in gut integrity are further exacerbated in the setting of preexisting comorbidities. The normally commensal microflora is also altered in critical illness, with increases in microbial virulence and decreases in diversity, which leads to further pathologic responses within the host.

SUMMARY

All components of the gut are adversely impacted by critical illness. Gut injury can not only propagate local damage, but can also cause distant injury and organ failure. Understanding how the multifaceted components of the gut interact and how these are perturbed in critical illness may play an important role in turning off the 'motor' of multiple organ dysfunction syndrome in the future.

The Influence of Host Stress on the Mechanism of Infection: Lost Microbiomes, Emergent Pathobiomes, and the Role of Interkingdom Signaling

Mammals constantly face stressful situations, be it extended periods of starvation, sleep deprivation from fear of predation, changing environmental conditions, or loss of habitat. Today, mammals are increasingly exposed to xenobiotics such as pesticides, pollutants, and antibiotics. Crowding conditions such as those created for the purposes of meat production from animals or those imposed upon humans living in urban environments or during world travel create new levels of physiologic stress. As such, human progress has led to an unprecedented exposure of both animals and humans to accidental pathogens (i.e., those that have not co-evolved with their hosts). Strikingly missing in models of infection pathogenesis are the various elements of these conditions, in particular host physiologic stress. The compensatory factors released in the gut during host stress have profound and direct effects on the metabolism and virulence of the colonizing microbiota and the emerging pathobiota. Here, we address unanswered questions to highlight the relevance and importance of incorporating host stress to the field of microbial pathogenesis.

Influence of nutrition therapy on the intestinal microbiome

PURPOSE OF REVIEW

This review describes the relationship between nutritional therapies and the intestinal microbiome of critically ill patients.

RECENT FINDINGS

The intestinal microbiome of the critically ill displays a near complete loss of health-promoting microbiota with overgrowth of virulent healthcare-associated pathogens. Early enteral nutrition within 24 h of admission to the ICU has been advocated in medical and surgical patients to avoid derangements of the intestinal epithelium and the microbiome associated with starvation. Contrary to previous dogma, permissive enteral underfeeding has recently been shown to have similar outcomes to full feeding in the critically ill, whereas overfeeding has been shown to be deleterious in those patients who are not malnourished at baseline. Randomized clinical trials suggest that peripheral nutrition can be used safely either as the sole or supplemental source of nutrition even during the early phases of critical care. The use of probiotics has been associated with a significant reduction in infectious complications in the critically ill without a notable mortality benefit.

SUMMARY

Focus of research is shifting toward strategies that augment the intestinal environment to facilitate growth of beneficial microorganisms, strengthen colonization resistance, and maintain immune homeostasis.

The role of the gut microbiota in sepsis

For decades, the gut was thought to play an important role in sepsis pathogenesis. Sepsis is defined as life-threatening organ dysfunction caused by a dysregulated host response to infection. Organ failure assessment for sepsis focuses on respiratory, cardiovascular, hepatic, renal, neurological, and haematological systems. Unfortunately, symptoms of gut failure are non-specific and are therefore not assessed. The composition of the intestinal microbiome, however, is affected by sepsis, and might contribute to the development of organ failure. Experimental work underscores the role of the microbiota in maintaining gut-barrier function, and modulation of the innate and adaptive immune system. Translation of these preclinical findings into functional characterisations will be essential to understand how disruption of commensals affects susceptibility and outcome of sepsis. In this Review, we identify knowledge gaps which, if addressed, will help researchers understand the role of the microbiota in sepsis, and provide microbiota-targeted tools to improve sepsis management.

The role of the microbiota in surgical recovery

PURPOSE OF REVIEW

The purpose of this review is to highlight new research findings in the complex bidirectional crosstalk that occurs between the intestinal microbiome and the host immune system in the context of surgical recovery and outcomes.

RECENT FINDINGS

Significant evidence has been generated emphasizing the central role of the intestinal microbiome on surgical outcomes such as wound healing, surgical site infections and anastomotic leak. Current preventive strategies, including the use of some parenteral antibiotics, may actually exacerbate the problem by selecting for drug-resistant pathogens.

SUMMARY

A delicate balance exists between the human host and its microbial counterparts that is directly related to postsurgical healing. This balance can be easily altered in favor of the pathogen through perioperative and surgical interventions leading to intestinal dysbiosis and loss of colonization resistance. Current strategies to prevent infectious complications with the escalating use of broader and more powerful antibiotics are not an evolutionarily stable strategy. A more complete understanding of the ecological and molecular interactions of the host with its microbiome is necessary to uncover new therapeutic strategies that preserve the composition and function of the intestinal microbiome and constrain virulent pathogens through the course of surgical injury.

Gastrointestinal dysmotility in the critically ill: a role for nutrition

PURPOSE OF REVIEW

The role of enteral nutrition on gastrointestinal dysmotility in the critically ill remains controversial.

RECENT FINDINGS

The mechanisms of gastrointestinal dysmotility during critical illness remain poorly investigated. Low amounts of enteral feeding stimulate motility and have trophic effects. Therefore, enteral feeding is feasible even during gastrointestinal dysmotility as seen in the hemodynamically compromised patient. Rapid 'ramp-up' of administration rate of tube feeding bears the risk of overload and even detrimental ischemic bowel necrosis. The recent American Society for Parenteral and Enteral Nutrition guidelines do not recommend the measurement of gastric residual volume. The use of concentrated enteral solutions with 1.5 kcal/ml may result in greater calorie delivery. Biomarkers like plasma citrulline and plasma or urine intestinal fatty-acid-binding protein reflect the functional integrity of the bowel and may potentially support monitoring.

SUMMARY

To improve enteral nutrition protocols, the definitions of gastrointestinal dysfunction, gastric dysmotility, and feeding intolerance should be clearly defined in the future. In the concept of integrity of the gut, enteral nutrition should not be stopped completely during gastrointestinal dysfunction but restricted to a 'minimal' trophic feeding rate. In malnourished and high-risk patients intolerant to enteral feeding supplemental parenteral nutrition should be started on day 4 or earlier.