Identification of Haemophilus influenzae Clones Associated with Invasive Disease a Decade after Introduction of H. influenzae Serotype b Vaccination in Italy

The introduction ofHaemophilus influenzae serotype b (Hib) conjugate vaccines has changed the epidemiology of invasiveH. influenzae disease, with a shift in the predominant serotype from Hib to nonencapsulatedH. influenzae (ncHi). The objective of this study was to identify the genotypes/clones associated with invasiveH. influenzae disease in Italy. Eighty-sevenH. influenzae strains isolated in the years 2009 to 2011 within the National Surveillance of Invasive Bacterial Disease program were analyzed. Strains were characterized by serotyping, antimicrobial susceptibility testing, and multilocus sequence typing (MLST). Genetic polymorphisms in thebla TEM gene promoter region as well as the occurrence of both adhesin genes (hmwA andhia ) and the IgA1 protease-encoding gene (igaB ) were also investigated. Of 87 strains, 67 were ncHi and 20 were encapsulated. Eleven strains were β-lactamase positive, harboring thebla TEM gene. Mostbla TEM genes (10/11) were associated with a Pdel promoter region exhibiting a 135-bp deletion; the remaining strain possessed the Pa/Pb overlapping promoter. MLST analysis showed that encapsulated isolates were clonal, with each serotype sharing a few related sequence types (STs). Forty-six different STs were identified among the 67 ncHi strains. Despite this heterogeneity, a group of closely related STs (ST103, ST139, and ST145) encompassed almost 25% of all ncHi strains and 45.5% of the β-lactamase producers carrying the Pdel promoter. These major ST clones were found to be associated with thehmwA gene but not with theigaB gene. To conclude, although the heterogeneity of the ncHi population was confirmed, diffusion of major successful ST clones was documented.