Research Program 1: The Starvation-Stress Response of Salmonella enterica serovar Typhimurium
William J. Kenyon (University of West Georgia) and Michael P. Spector (University of South Alabama)
In response to starvation for a source of carbon and energy, nitrogen, or phosphorus Salmonella enterica serovar Typhimurium (S. Typhimurium) undergoes a dramatic change in global gene expression and physiology. These changes are collectively referred to as the starvation-stress response (SSR). The SSR ultimately results in a distinct type of bacterial cell which shows remarkable tolerance toward a wide variety of other stresses. Although not true spores, carbon/energy-starved (C-starved) cells of S. Typhimurium cells have many of the characteristics of spores and other types of dormant, stress-resistant microbial cells. C-starved cells are much more resistant to extremes in temperature, pH, and osmolarity compared to actively growing cells. They also exhibit increased tolerance toward many antimicrobial agents including oxidizing compounds like hydrogen peroxide and antimicrobial peptides such as polymyxin B. Furthermore, cells which have transitioned from exponential growth to C-starvation are able to survive periods of prolonged nutrient deprivation for weeks or even months. S. Typhimurium is likely to encounter starvation for various nutrients in both host and non-host habitats. Therefore, study of the SSR is important in order to understand how salmonellae survive within the human body and to understand how they are able to persist in the external environment as a reservoir of foodborne infection.
Our research group is interested in S. Typhimurium genes which are upregulated in response to starvation stress (i.e., starvation-inducible genes). Genetic studies have revealed that the level of transcription of hundreds of S. Typhimurium genes more than doubles in response to C-starvation. These genes are scattered throughout the Salmonella chromosome and are also found on the large Salmonella virulence plasmid and are involved in a myriad of physiological functions including stress protection, metabolism, gene regulation, virulence, etc. We are especially interested in how the cell envelope of S. Typhimurium (e.g., inner-membrane, periplasm, cell wall, and outer-membrane) changes during the adaptation to starvation stress. Previously, we discovered that the alternative sigma factor RpoE is one of the master regulators of the SSR. This regulatory protein is known as an extracytoplasmic function (ECF) sigma factor responsible for directing the expression of a group of genes whose products help to alleviate envelope stress. Our research continues to focus on the RpoE-regulated branch of the SSR with the goal of identifying individual genetic loci that contribute to long-term starvation survival and starvation-induced cross-resistance to other stresses. Another aim of ours is to investigate the molecular mechanism of RpoE activation in response to starvation and other forms of nutritional stress (e.g., carbon source shifts).
Research Program 2: Biology and Biotechnology of the Genus Cellulomonas
William J. Kenyon (University of West Georgia)
Our lab is also interested in a group of Gram-positive bacteria in the genus Cellulomonas. Cellulomonas species have attracted a great deal of research interest because of their ability to efficiently convert cellulose into soluble sugars and other potentially useful products. Cellulomonas flavigena strain KU was isolated by Dr. Clarence Buller and co-workers at The University of Kansas and produces a unique type of gel-forming extracellular polysaccharide known as curdlan. We have shown that C. flavigena KU produces the curdlan exopolysaccharide when growing on microcrystalline cellulose. In fact, we discovered that curdlan is the primary constituent of an extracellular matrix that enables C. flavigena KU to grow as a biofilm on the surface of cellulose fibers. These observations have led to the hypothesis that Cellulomonas spp. can be categorized into two groups based on differing strategies for cellulose degradation. Members of the first group, including the well studied species C. fimi, possess flagella, are motile, do not produce curdlan, do not adhere to cellulose, secrete soluble degradative enzymes, and release soluble sugars from the breakdown of cellulose. Members of the second group, represented by the species C. flavigena and C. uda, are non-motile, produce a curdlan capsule, adhere to cellulose particles, produce cell-bound enzymes in the form of large protein complexes called cellulosomes, and release very little soluble sugar into the surrounding environment. We are currently in the process of studying these surprisingly different strategies for cellulose degradation within this bacterial genus.
We have also found that C. flavigena KU produces two other glucose-storage carbohydrates in addition to curdlan. These carbohydrates are the disaccharide trehalose and a glycogen-type polysaccharide, both of which are accumulated intracellulary. Graduate students and undergraduate researchers working in our lab have recently shown that all three of these carbohydrates (curdlan, trehalose, and glycogen) accumulate under conditions of carbon and energy excess and that all three are degraded when cells are starved of an exogenous source of carbon and energy. These characteristics suggest physiological roles as carbon and energy reserve compounds. Trehalose also appears to serve as a compatible solute protecting C. flavigena KU against hyperosmotic stress.
These findings may prove to be important for future biotechnological applications involving the use of Cellulomonas spp. As mentioned above, the biofilm mode of growth on cellulose fibers does not release free sugars into the growth medium. Instead, most of the glucose residues from cellulose degradation are trapped by the adherent Cellulomonas biofilm and directly converted into one of the three reserve compounds. This may actually be an advantage for some industrial applications, and further research in our lab will be focused on the potential for C. flavigena biofilms to produce biofuels such as ethanol from cellulosic substrates under anaerobic conditions.
- B.S., Life Sciences, University of Missouri - Rolla, 1991
- Ph.D., Microbiology, The University of Kansas, 1996
Courses and Sections
- BIOL-1107 (Principles of Biology I)
- BIOL-1107L (Principles of Biology I Lab)
- BIOL-3134 (Cell and Molecular Biology)
- BIOL-3310 (Microbiology)
- BIOL-4315 (Microbial Physiology & Genetic)
- BIOL-4325 (Advanced Medical Microbiology)
- BIOL-4728 (Bacterial Pathogenesis)
- BIOL-4985 (Bacterial Pathogenesis)
- BIOL-5315 (Microbial Physiol & Genetics)
- BIOL-5325 (Advanced Medical Microbiology)
- BIOL-5728 (Bacterial Pathogenesis)
- BIOL-6985 (Bacterial Pathogenesis)
Fall 2013 Sections