Trends in Microbiology
OpinionThe role of microbiota in infectious disease
Introduction
Most bacterial pathogens infect their hosts via mucosal surfaces of the respiratory, urogenital or gastrointestinal tracts. Mucosal surfaces are protected against infection by several mechanical and immunological barriers, which have recently been reviewed elsewhere 1, 2. Here, we focus on an additional protective mechanism – colonization resistance – which is characteristic of the heavily colonized intestinal mucosa (Box 1). Colonization resistance describes the failure of most pathogenic bacteria to colonize the normal gut and cause enteric disease (Box 2). It results from the presence of a dense (1012 organisms per ml) microbial community called the microbiota (see Glossary). In the normal gut the relationship between the microbiota and the host is mutually beneficial. The microbiota is provided with steady growth conditions and a (somewhat limited) nutrient supply. In return, the microbiota contributes to the host's nutrition, immune system development, angiogenesis and fat storage 1, 3, 4, 5, 6, 7, 8, 9. This complex network of interactions is thought to stabilize the population structure of the microbiota and to prohibit colonization by intruding pathogens. The molecular basis of colonization resistance is still poorly understood.
In spite of colonization resistance and numerous other defenses, some pathogens are still capable of infecting the gut. The mechanisms that pathogens use to overcome these barriers, to compete against the intrinsic microbiota and to guarantee successful infection also remain elusive. One such mechanism has recently been shown in studies on the enteropathogenic bacteria Citrobacter rodentium and Salmonella enterica spp. I serovar Typhimurium (S. Typhimurium) 10, 11. Remarkably, both enteropathogens were shown to rely on the inflammatory host response, which they evoke in the gut: inflammation changed the composition of the commensal gut microbiota and concurrently fostered pathogen growth. Similar observations were made in patients and animal models for inflammatory bowel diseases (IBD). In this case, the gut inflammation also coincided with altered population structure of the microbiota 11, 12, 13, 14. These findings identify a shared mechanism of gut ecosystem intrusion by enteropathogens: that is, triggering the host's inflammatory response to overcome colonization resistance. The molecular basis of this strategy is still unclear and might involve different molecular mechanisms. The inflamed gut might offer altered conditions such as changes in the available nutrients and adhesion sites that can be exploited by the pathogen but not by the microbiota (the ‘food hypothesis’). Alternatively, changes in antimicrobial compounds such as lectins and defensins released by the inflamed tissue might be detrimental for the microbiota but not for the pathogen (the ‘differential killing hypothesis’).
In this review, we describe the ‘classical’ observations linking a disturbed gut microbiota to increased susceptibility to gut infections. We then discuss how inflammation might alleviate colonization resistance and how these basic findings can be extended to help elucidate the interactions between bacteria and the gut mucosa in health and disease.
Section snippets
Classical observations linking the microbiota to colonization resistance
It has long been recognized that disruption of the normal microbiota by antibiotics increases the risk for gut infections. The classic example is pseudomembranous colitis, a frequent form of infectious diarrhea caused by Clostridium difficile in hospitalized patients [15]. Pseudomembranous colitis occurs after broad-spectrum antibiotic treatment (e.g. ampicillin, cephalosporins and clindamycin). Similar observations have been made for several other pathogens [16]. Several animal models for
Disrupting colonization resistance by triggering mucosal inflammation
Recent work has shown that colonization resistance can be disrupted by gut inflammation. This strategy is used by at least two enteropathogenic bacteria, Citrobacter rodentium (a close relative of enteropathogenic E. coli) and Salmonella enterica spp. I serovar Typhimurium (S. Typhimurium) 10, 11. In mouse infection models, pre-existing or pathogen-induced inflammatory conditions in the large intestine drastically boosted colonization by the pathogen. Conversely, both pathogens failed to
Possible mechanisms of disruption
What is the causal link between gut inflammation and disruption of colonization resistance? There are several plausible hypotheses:
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The inflamed mucosa might release antibacterial factors that could kill or retard growth of certain members of the microbiota that would normally inhibit enteropathogen growth under steady-state conditions. However, the pathogen would be able to resist these factors (see ‘differential killing hypothesis’, following section).
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There could be a loss of key species (i.e.
The food hypothesis
The large intestine represents an anaerobic bioreactor synthesizing essential amino acids, vitamins and short chain fatty acids (SCFA) while breaking down a variety of proteins and otherwise indigestible polysaccharides, including plant-derived pectin, cellulose, hemicelluloses and resistant starches [21]. This ‘bioreactor’ is fuelled only by those parts of the diet that cannot be processed or resorbed by the small intestine and by glycoconjugates, proteins and cellular debris released by the
The differential killing hypothesis
The differential susceptibility towards host defenses might provide an alternative explanation for the loss of colonization resistance in the inflamed gut. At the cellular level inflammatory responses are well understood. Immune effector cells such as macrophages, dendritic cells and neutrophils are attracted to the site of bacterial intrusion. They produce lysozyme, acidic hydrolases, nitric oxide, cationic antimicrobial peptides, iron-scavenging lactoferrin and the respiratory burst
Altered microbiota composition in inflammatory bowel disease
Altered population structures of the gut microbiota are also observed in inflammatory bowel diseases. In this case, the inflammation is triggered by exaggerated immune defenses directed against members of the commensal microbiota and not by pathogen insult. It is assumed that similar antibacterial defenses are induced as in the case of an acute enteropathogenic infection (Table 1). Interestingly, the fraction of γ-proteobacteria was increased in the microbiota of IBD patients 12, 13, 14.
Concluding remarks and future directions
Is the triggering of gut inflammation a common strategy used by enteropathogenic bacteria to invade the intestinal ecosystem? In most cases this question has not been addressed directly. However, there are multiple reports providing circumstantial evidence to support this concept: mutations attenuating mucosal inflammation often result in reduced gut colonization levels. These examples include Salmonella infections in the calf model 31, 32, S. flexneri infections [33], V. cholerae infections
Update
It was demonstrated recently that S. Typhimurium can benefit form sugars (i.e. galactose) released as a component of the mucosal defence in the inflamed gut. Stecher B. et al. (2008) Motility allows S. Typhimurium to benefit from the mucosal defence. Cellular Microbiology (in press).
Acknowledgements
This work was funded by a grant from the Swiss National Foundation (SNF) 310000–113623/1 and a grant from the European Union ‘SavinMucoPath’ FP6–2004-INCO-DEV-3, both to Wolf-Dietrich Hardt.
Glossary
- Axenic or germfree
- animals that have been raised in a sterile environment without microbiota. Axenic mice differ from colonized mice in many ways. They have an underdeveloped immune system, no colonization resistance, and require higher caloric intake than normal mice to maintain body weight.
- Commensal
- Originating from the Latin meaning ‘sharing the same table’; an alternative term for microbiota.
- Gnotobiotic
- animals colonized with a defined microbiota.
- Inflammation
- host response following extraneous
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