Serotype and clonal evolution of penicillin-nonsusceptible invasive Streptococcus pneumoniae in the 7-valent pneumococcal conjugate vaccine era in Italy - Gherardi, G; D'Ambrosio, F; Visaggio, D; Dicuonzo, G; Del Grosso, M; Pantosti, A (Antimicrob Agents Chemother, 2012)