- Microbiology and Immunology Professor
The human microbiome or indigenous microflora inhabits every mucosal surface of the human body and outnumbers the human cells by more than 10 to 1. This normal bacterial flora has a mutualistic association with the host which influences many of the host's physiological, nutritional, and immunological activities. The beneficial relationship is maintained by physical barriers and immunological processes that keep the microbial populations in check. However, disruption of these defense mechanisms leads to opportunistic infections, which can have a serious impact on human health. So how does our indigenous microflora cause disease? In order to address this question we study intra-abdominal abscesses which are formed when the peritoneal cavity is contaminated with indigenous intestinal bacteria following perforation of the bowel (appendicitis, diverticulitis, carcinoma, surgery). These are polymicrobic infections that result in a multifactorial host response designed to wall off and contain the invading microbes. Bacteroides fragilis is the predominant anaerobe associated with intra-abdominal infections and overall is the most frequently isolated anaerobe from clinical samples.
We hypothesize that both resistance to oxidative stress and nutritional factors give B. fragilis a survival advantage during the development of these infections. In order to test these hypotheses we use a variety of approaches such as genomics, transcriptomics, and proteomics to measure gene expression and protein profiles of organisms with specific genetic mutations during the course of infection in a rat abscess model.
Oxidative stress. Relative to the colon, the peritoneal cavity is an oxygenated environment. This and the recruitment of PMNs to the site of infection will result in substantial exposure of B. fragilis to reactive oxygen species during colonization of the peritoneum. Our work has documented that B. fragilis induces an oxidative stress response that encompasses about 25% of the genome. This response includes an acute oxidative stress response which is designed to minimize the immediate effects of oxygen radicals. We have shown that this rapid response is mediated by the LysR-family regulator OxyR which is required for abscess formation. Following the acute phase, there is a novel widespread induction of genes associated with metabolism which occurs when there is prolonged exposure to oxidative stress. We are currently working to understand the genetic mechanisms that regulate this global response to prolonged oxidative stress and how this response enhances persistence in the abscess milieu of necrotic cell debris, viable PMNs, and host serum factors.
Nutritional factors. B. fragilis is saccharolytic and in the colon it derives energy by catabolism of polysaccharides that escape the human digestive system. The transition from growth in the colon to the peritoneal cavity requires rapid adaption to a new nutrient supply. Gene expression studies with cells grown in an artificial abscess in vivo suggest that B. fragilis quickly induces a novel set of polysaccharide utilization operons that reflect the availability of new nutrient sources that are rich in glycoproteins. These enzymes allow B. fragilis to harvest N-linked glycans from a range of glycoproteins that are abundant in serum. We ultimately wish to learn more about the regulatory mechanisms that control how the organism remodels its catabolic metabolism to survive for extended periods in the abscess.