Evolution and epidemiology of antibiotic-resistant pneumococci

Streptococcus pneumoniae is still an important human bacterial pathogen. The past two decades have witnessed the global spread of resistance to major groups of antipneumococcal drugs and there are no countries free of multidrug-resistant strains. In this naturally transformable organism resistance to antibiotics can arise by both inter-and intra-species recombination events, enabling resistance acquired by horizontal gene transfer or from point mutation to spread throughout the population. However, there is also strong evidence for clonal expansion by the international spread of multidrug-resistant strains. Among such successful clones, the Spain23F-1, Spain6B-2, and Spain9V-3 in particular have reached the status of pandemic clones. All three have emerged as being penicillin-non-susceptible (PNSP) and often resistant to tetracyclines, macrolides, chloramphenicol, or co-trimoxazole. The presence of the most successful, the Spain23F-1 clone, and its other capsular variants, has been documented in 42 countries all over the world. The susceptibility-testing results survey done for this study showed that prevalence of PNSP was above 40% in 25 out of 96 countries and below 5% in only 8 of them. This chapter offers insight into the mechanisms of the antibiotic resistance acquisition in pneumococci, their evolution, and the epidemiology of multidrug-resistant strains.