Combination of virulence and antibiotic resistance: a successful bacterial strategy to survive under hostile environments

Bacterial pathogens have coevolved with the human host for millions of years. During the course of evolution, the bacterial pathogens have adapted to escape the immunological defense responses of the host and to sustain and survive the action of various antibiotics. The widespread use of antibiotics and the host defense system creates a hostile environment for bacteria during their infection and proliferation stages. Bacteria that are either virulent or multidrug-resistant can fight with one of the two onslaughts, either the host defense or the treatment with antibiotics. The development of antibiotic resistance in bacterial species often results in a fitness cost and mutations associated with it may disrupt essential biological functions. Microbes have developed ways to mitigate this hostile situation in unconventional ways via combining the virulence and resistance mechanisms overcoming the evolutionary trade-offs and compensating for the fitness cost. Bacteria employ mechanisms such as hypermutation, compensatory mutations, cross coselection, antibiotic tolerance and persistence in maintaining antibiotic resistance by offsetting the fitness costs. Such strains are widespread among human infections resulting in huge direct medical costs, morbidity, and causing social distress. In this chapter, the authors discuss the interplay between virulence and resistance with respect to the presented mechanisms in providing an added advantage to bacterial pathogens to adapt in different environments, particularly the infective environment. The biological factors and mechanisms that influence the successful association of resistance and virulence mechanisms are described herein, with accompanying examples for specific virulence and resistance factors in several pathogens. Systematic understanding of mechanisms in bacterial pathogens related to their survival strategies will help direct research toward better treatment and prevention strategies.