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Chapter 9 : Evolution of Bacterial Opportunistic Pathogens
Opportunistic pathogens are particularly important at hospitals, as they are significantly responsible for nosocomial infections. In trying to understand the underlying reasons for the acquisition and evolution of virulence of opportunistic pathogens, this chapter addresses two topics. The first one is to know whether the environment where they grow during infection is changing through time and thus forcing their evolution. In other words, during infection, the infected patient is both the habitat and the food source of the infecting bacteria. The second topic addressed is the mechanisms of evolution, which allow bacteria that usually do not infect humans to produce a disease in sick people. The study of Pseudomonas aeruginosa isolates from chronic and acute infections has demonstrated that transition to chronicity involves an increase in the ability to form biofilms and a concomitant reduction in the expression of the type III secretion system. This transition does not consist of phenotypic changes, but consists of the accumulation of adaptive mutations during the in-host evolution of this bacterial opportunistic pathogen. Concerning bacterial-protozoa interactions as forces shaping the evolution of bacterial opportunistic pathogens, it is important to recall that protozoa are major grazers of bacteria in natural environments. If natural nonclinical environments are relevant for the evolution of bacterial opportunistic pathogens, modifications of these environments can also produce changes in bacterial populations.