Background: Group B Streptococci (GBS, Streptococcus agalactiae) are prominent pathogens in newborns and infants invasive infections but they also affect adults, particularly elderly people. Our study aimed to determine the serotype distribution, antibiotic resistance and molecular epidemiology of invasive GBS strains isolated from adults in Italy during the years 2015- 2019.
Methods: all strains were analyzed for serotype by both serological and molecular typing methods. The genes encoding for pili and for the hypervirulent ST-17 GBS adhesin (HvgA) were investigated by PCR. Erythromycin and clindamicin resistance was phenotipically and genotipically assayed.
Results: A total of 103 S. agalactiae isolates were characterized. The most common serotypes were type III (n=42) followed by type Ia (n=21) and type V (n=19). Pili islands (PI) 1+2a, PI PI-2a alone were represented in different serotypes. On the contrary, PI1+2b was associated to only ST17 isolates (hvgA and tetM positive) and PI-2b alone was found in only MDR-ST17 isolates (hvgA, ermB and tetO positive) and few serotypes IV (hvgA positive). Tetracycline resistance rate did not changed significantly during the period of study but an increasing temporal trend in the rate of resistance to clindamycin was noted (21%, in total).
Conclusions: The presence of HvgA gene in serotype IV suggested a capsular switching from the ST-17 serotype III.
The increase of resistance to clindamycin was also due to the presence of a MDR-CC17 clone not circulating in the years 2015-2016. This highlights the need for continue molecular epidemiology studies of GBS responsible for invasive disease in adults.