Adaptive Mechanisms of Streptococcus pneumoniae in the Postvaccine Era

Streptococcus pneumoniae is a vaccine-preventable bacterial pathogen but remains the leading cause of community-acquired pneumonia even after the worldwide administration of pneumococcal vaccines. In the era of routine immunization with pneumococcal conjugate vaccines (PCVs) and antibiotic pressure, S. pneumoniae uses specific adaptive mechanisms for vaccine escape and fast-growing mutations not recognized by the PCVs. Pneumococcus may evolve to adapt by mainly two mechanisms affecting their capsule antigens (the vaccine targets): capsule switching and capsule replacement. Vaccination affects the capsular antigens and resulted in a serotype replacement with an increased proportion of pneumococci with nonvaccine serotypes in asymptomatic carriers and symptomatic cases, indicating fast adaptation of this pathogen to the selective pressure of the PCVs. The ability of pneumococci to acquire and incorporate exogenous DNA is an important evolutionary mechanism for adaptation to antibiotic therapy and vaccination. Examining the changing epidemiology due to shifting serotypes and the adaptive mechanisms that allow the emergence of successful pneumococcal isolates is the key step to understanding the adaptive behavior of S. pneumoniae and the severe manifestations of pneumococcal diseases.