Why aren’t all E. coli resistant to antibiotics ?

Abstract

We don’t know why drug-resistant strains and susceptible strains often coexist and what determines the level of resistance in a pathogen population (e.g., E. coli, S. pneumonia, HIV). However, there is a lot we do know: we know that resistance levels are often stable over years, we know that resistance levels depend on drug usage at the country level, and we know that resistance is often due to many independent acquisition events (whether mutational or through horizontal gene transfer). Here I present a simple population-genetic model, akin to mutation-selection balance which can explain all these observations.