Serotype replacement in Streptococcus pneumoniae after conjugate vaccine introduction: impact, doubts and perspective for new vaccines

Variation in Streptococcus pneumoniae susceptibility to human antimicrobial peptides may mediate intraspecific competition

Streptococcus pneumoniae is a facultative pathogen inhabiting the nasopharynx of humans where it is exposed to a range of antimicrobial peptides (AMPs) of the innate immune response. It is possible therefore that the susceptibility of strains to AMPs plays a role in determining their ability to colonize, and furthermore, that AMPs could mediate competitive interactions between co-colonizing genotypes. However, little is known about patterns of natural variation in AMP susceptibility of S. pneumoniae, and it is unclear whether the susceptibilities of an isolate to multiple human AMPs are correlated. We tested this by characterizing the susceptibility of 31 S. pneumoniae natural isolates to human neutrophil peptide (HNP-1) (α-defensin) and LL-37 (cathelicidin). We observed significant variation in susceptibility between isolates to both AMPs, and in the majority of isolates, susceptibilities to HNP-1 and LL-37 were uncorrelated. Clinical isolates were more susceptible to AMPs than were carriage isolates. The polysaccharide capsule of S. pneumoniae is thought to protect cells against AMPs. However, serotype alone could not explain the observed variation in susceptibility suggesting that genetic background plays an equally important role. We tested directly whether AMPs could mediate competition between isolates using competition experiments in the presence and absence of AMPs. These experiments demonstrated that AMPs could indeed reverse the outcome of competition between selected isolates. AMP-mediated competition could therefore contribute to the maintenance of intraspecific genetic diversity in S. pneumoniae.

Increasing incidence of Streptococcus pneumoniae serotype 19A and emergence of two vaccine escape recombinant ST695 strains in Liguria, Italy, 7 years after implementation of the 7-valent conjugated vaccine

Two serotype 19A (ST695) Streptococcus pneumoniae vaccine escape recombinant strains attributable to capsular switching events were detected by a laboratory surveillance system that is an integral part of a vaccination program begun in Liguria, Italy, in May 2003, an Italian administrative region with long-lasting high coverage, an unusual occurrence in Europe. To our knowledge, this is the first detection of an occurrence of capsular switching outside the United States.