Abstract
The population dynamics of
Helicobacter pylori
during colonization in an infected animal host provide a quantifiable experimental model of
in vivo
microbial phenotype evolution. Phenotype variability in
H. pylori
populations can be typed as polymorphic expression of Lewis antigens on their cell surfaces. The high mutational frequency of
H. pylori
for Lewis expression provides substrate for differential selection by the host. Experimental challenge and successful colonization of mice and gerbils allows tracking of
H. pylori
phenotype variability from the initial inoculation to the ultimate establishment of a quasispecies. Colonization data provide a quantitative experimental model of phenotype evolution in a relatively large population (>10
4
individuals) over a relatively long evolutionary time scale (>10
3
generations). A mathematical model is developed to interpret the data in terms of the dynamic processes occuring during colonization. The mathematical model distinguishes the roles of selection and mutation; quantifies the effects of initial phenotype diversity, mutational frequency, and selective advantage; and applies generally to phenotype evolution in biological populations.