Chapter 2 : Collective Traits in Pathogenic Bacteria

Abstract:

This chapter focuses on how selection at the microbial level may lead to changes at the infection level and examines some predictions one can draw from evolutionary theory. The major motivation for this review stems from the fact that natural selection among microbes can potentially have two opposite outcomes on pathogenesis. Pathogenic bacteria use a chemical communication system, called quorum sensing, to regulate within- host density. The chapter reviews the effects of within-host selection; as predicted by theoretical models and as empirically observed. There are three reasons why an allele favored within a host may nevertheless not go to complete fixation in the population at large. The first reason is that a locally favored variant may have partially, or completely, lost the ability to disperse from its host. This may be a direct result of selection on dispersal: evolutionary ecology shows that dispersal tends to be locally counter selected. Second, a locally favored variant, V, may have lost (completely or partially) the ability to survive and/or replicate in environments other than in the focal host itself. The third mechanism is that a pathogen variant, V, may increase in frequency within a focal host but not fix globally because the traits favored by within host selective pressures are not those that maximize the total infective output.