Thomas S. Whittam, Ph.D.

whittam@msu.edu

Research

also see the Microbiology Research Unit 

• Genetics of Bacterial Populations
  Bacteria are a difficult and challenging problem for population geneticists. The difficulty arises because in nature bacteria reproduce asexually and recombine only occasionally through mechanisms of gene transfer. As a consequence, different species exhibit a range of population structures from clone mixtures to freely recombining populations similar to biological species of higher organisms. Often the amount of recombination in nature is intermediate - too much for a purely phylogenetic approach and too little to assume "random assortment." One of my main research interests has been the study of the genetic structure of natural populations of bacteria using molecular polymorphisms and the development of statistical methods for assessing recombination.

  Evolution of pathogenic forms of E. coli
  Although E. coli is normally a harmless organism in the human gut, certain strains are pathogens that have caused serious outbreaks of infectious disease. A major research effort in my laboratory has been the study of the evolution of pathogenic forms of E. coli associated with intestinal and extra-intestinal infections. Through the analysis of molecular polymorphisms, we are testing evolutionary hypotheses regarding the major genetic events leading to the origin of new pathogens. In the course of this work, we have elucidated the ancestry of a new type of food-borne pathogen, Escherichia coli O157:H7, which causes hemorrhagic colitis. In addition, we have investigated, in collaboration with microbiologists from other countries, the global distribution of bacterial clones and the dispersion of specific virulence genes in human populations and animal reservoirs.

  Experimental evolution of pathogen virulence and host resistance
  Recently developed evolutionary theory shows that natural selection can favor intermediate levels of parasite virulence depending on the relationship between transmissibility and the parasite's effect on host mortality. We are studying the evolution of virulence experimentally using a microbial host-parasite system. The parasite is Legionella pneumophila, the bacterium that causes Legionnaires disease. In nature, Legionella invade and multiply intracellularly in amoeba and other protozoan hosts. By propagating host-parasite genotypes for hundreds of generations we are measuring the direction and rate of evolution in virulence and changes in host resistance from the ancestral (original) conditions.