Serotype and clonal evolution of penicillin-non susceptible invasive Streptococcus pneumoniae in the PCV7 era in Italy

The percentage of invasive penicillin-non susceptible pneumococci (PNSSP) isolated in Italy in the seven-valent pneumococcal conjugate vaccine (PCV7) era, moderately increased in comparison to the pre-PCV7 era. Increase of non-vaccine serotypes was observed among PNSSP. The most frequent PNSSP clones were the same as those identified in the pre-PCV7 era, although in different proportions. Clonal expansion, emergence of new clones and acquisition of penicillin resistance by established clones contributed to the maintenance of penicillin resistance.