SUMMARY Although Escherichia coli can be an innocuous resident of the gastrointestinal tract, it also has the pathogenic capacity to cause significant diarrheal and extraintestinal diseases. Pathogenic variants of E. coli (pathovars or pathotypes) cause much morbidity and mortality worldwide. Consequently, pathogenic E. coli is widely studied in humans, animals, food, and the environment. While there are many common features that these pathotypes employ to colonize the intestinal mucosa and cause disease, the course, onset, and complications vary significantly. Outbreaks are common in developed and developing countries, and they sometimes have fatal consequences. Many of these pathotypes are a major public health concern as they have low infectious doses and are transmitted through ubiquitous mediums, including food and water. The seriousness of pathogenic E. coli is exemplified by dedicated national and international surveillance programs that monitor and track outbreaks; unfortunately, this surveillance is often lacking in developing countries. While not all pathotypes carry the same public health profile, they all carry an enormous potential to cause disease and continue to present challenges to human health. This comprehensive review highlights recent advances in our understanding of the intestinal pathotypes of E. coli .

Citrobacter rodentium is a mucosal pathogen of mice that shares several pathogenic mechanisms with enteropathogenic Escherichia coli (EPEC) and enterohaemorrhagic E. coli (EHEC), which are two clinically important human gastrointestinal pathogens. Thus, C. rodentium has long been used as a model to understand the molecular basis of EPEC and EHEC infection in vivo. In this Review, we discuss recent studies in which C. rodentium has been used to study mucosal immunology, including the deregulation of intestinal inflammatory responses during bacteria-induced colitis and the role of the intestinal microbiota in mediating resistance to colonization by enteric pathogens. These insights should help to elucidate the roles of mucosal inflammatory responses and the microbiota in the virulence of enteric pathogens.

Antibiotics are often used in the clinic to treat bacterial infections, but the effects of these drugs on microbiota composition and on intestinal immunity are poorly understood. Citrobacter rodentium was used as a model enteric pathogen to investigate the effect of microbial perturbation on intestinal barriers and susceptibility to colitis. Streptomycin and metronidazole were used to induce alterations in the composition of the microbiota prior to infection with C. rodentium. Metronidazole pretreatment increased susceptibility to C. rodentium-induced colitis over that of untreated and streptomycin-pretreated mice, 6 days postinfection. Both antibiotic treatments altered microbial composition, without affecting total numbers, but metronidazole treatment resulted in a more dramatic change, including a reduced population of Porphyromonadaceae and increased numbers of lactobacilli. Disruption of the microbiota with metronidazole, but not streptomycin treatment, resulted in an increased inflammatory tone of the intestine characterized by increased bacterial stimulation of the epithelium, altered goblet cell function, and thinning of the inner mucus layer, suggesting a weakened mucosal barrier. This reduction in mucus thickness correlates with increased attachment of C. rodentium to the intestinal epithelium, contributing to the exacerbated severity of C. rodentium-induced colitis in metronidazole-pretreated mice. These results suggest that antibiotic perturbation of the microbiota can disrupt intestinal homeostasis and the integrity of intestinal defenses, which protect against invading pathogens and intestinal inflammation.The intestinal microbiota can be thought of as an organ system, essential for nutrient acquisition, metabolism of indigestible compounds, defense against colonization by pathogens, and the development of intestinal architecture and the immune system (40,43). Compositional changes in the intestinal microbiota can lead to severe dysregulation of the physiological and immunological intestinal homeostasis, with serious adverse consequences for the host (43). A well-known case of this is antibiotic treatment, and previous studies have shown that antibiotic treatment can predispose the host to enteric infections (48). A recent investigation by Sekirov et al. showed that various doses of antibiotic treatments predispose mice to increased colonization by Salmonella enterica serovar Typhimurium and intestinal pathology (48). Additionally, Brandl et al. showed that administration of a broad-spectrum combination of metronidazole (Met), neomycin, and vancomycin promotes infection by vancomycin-resistant enterococci (9). Further, there are significant differences in microbiota compositions of inflammatory bowel disease (IBD) patients and healthy individuals, further implicating microbial factors in the initiation and perpetuation of colitis (17,19,32,46). It is unknown if such changes precede and contribute to the onset of IBD or are simply a result of IBD. Moreover,

Although antibiotics have significantly improved human health and life expectancy, their disruption of the existing microbiota has been linked to significant side effects such as antibiotic-associated diarrhea, pseudomembranous colitis, and increased susceptibility to subsequent disease. By using antibiotics to break colonization resistance against Clostridium, Salmonella, and Citrobacter species, researchers are now exploring mechanisms for microbiota-mediated modulation against pathogenic infection, revealing potential roles for different phyla and family members as well as microbiota-liberated sugars, hormones, and short-chain fatty acids in regulating pathogenicity. Furthermore, connections are now being made between microbiota dysbiosis and a variety of different diseases such as rheumatoid arthritis, inflammatory bowel disease, type 1 diabetes, atopy, and obesity. Future advances in the rapidly developing field of microbial bioinformatics will enable researchers to further characterize the mechanisms of microbiota modulation of disease and potentially identify novel therapeutics against disease.

SummaryThe bacterial pathogen Listeria monocytogenes replicates within the cytosol of mammalian cells. Mechanisms by which the bacterium exploits the host cytosolic environment for essential nutrients are poorly defined. L. monocytogenes is a lipoate auxotroph and must scavenge this critical cofactor, using lipoate ligases to facilitate attachment of the lipoyl moiety to metabolic enzyme complexes. Although the L. monocytogenes genome encodes two putative lipoate ligases, LplA1 and LplA2, intracellular replication and virulence require only LplA1. Here we show that LplA1 enables utilization of host-derived lipoyl peptides by L. monocytogenes. LplA1 is dispensable for growth in the presence of free lipoate, but necessary for growth on low concentrations of mammalian lipoyl peptides. Furthermore, we demonstrate that the intracellular growth defect of the DlplA1 mutant is rescued by addition of exogenous lipoic acid to host cells, suggesting that L. monocytogenes dependence on LplA1 is dictated by limiting concentrations of available host lipoyl substrates. Thus, the ability of L. monocytogenes and other intracellular pathogens to efficiently use host lipoyl peptides as a source of lipoate may be a requisite adaptation for life within the mammalian cell.

Residing within the intestine is a large community of commensal organisms collectively termed the microbiota. This community generates a complex nutrient environment by breaking down indigestible food products into metabolites that are used by both the host and the microbiota. Both the invading intestinal pathogen and the microbiota compete for these metabolites, which can shape both the composition of the flora, as well as susceptibility to infection. After infection is established, pathogen mediated inflammation alters the composition of the microbiota, which further shifts the makeup of metabolites in the gastrointestinal tract. A greater understanding of the interplay between the microbiota, the metabolites they generate, and susceptibility to enteric disease will enable the discovery of novel therapies against infectious disease.

Glucose is a major source of energy and carbon in promastigotes of Leishmania mexicana, and its uptake is mediated by three glucose transporters whose genes are encoded within a single cluster. A null mutant in which the glucose transporter gene cluster was deleted by homologous gene replacement was generated previously and shown to grow more slowly than wild type promastigotes but not to be viable as amastigotes in primary tissue culture macrophages or in axenic culture. Further phenotypic characterization demonstrates that the null mutant is unable to import glucose, mannose, fructose, or galactose and that each of the three glucose transporter isoforms, LmGT1, LmGT2, and LmGT3, is capable of transporting each of these hexoses. Complementation of the null mutant with each isoform is able to restore growth in each of the four hexoses to wild type levels. Null mutant promastigotes are reduced in size to about 2/3 the volume of wild type parasites. In addition, the null mutants are significantly more sensitive to oxidative stress than their wild type counterparts. These results underscore the importance of glucose transporters in the parasite life cycle and suggest reasons for their non-viability in the disease-causing amastigote stage.

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