Zoonotic Approach to Streptococcus agalactiae: Integrated Analysis of Sympatric Dairy Cattle and Human Isolates

Streptococcus agalactiae (Group B Streptococcus, GBS) is one of the most important pathogens causing bovine mastitis which also can colonise humans and cause severe diseases. To investigate the potential hazard of interspecific transmission, we selected 215 isolates (150 human + 65 bovine ones) obtained in a same period and geographical area. Comparative analysis of these sympatric isolates was carried out in relation to serotype, virulence and antimicrobial resistance (AMR) genes, and a selection of them, studied by MLST. Six serotypes were detected: Ia and III predominated in humans, III and II, in cattle. Virulence genes cpsA, cylE and hylB were detected in all isolates, bac, hvgA, lmb, PI-1, PI-2a and scpB, only in human isolates; bca, rib, spb1 and PI-2b, with variable frequencies between host isolates. Pilus typing detected PI-1 and PI-2a only in human isolates. All bovine isolates were PI-2b. Clustering analysis showed 59 virulence profiles, without shared virulence profiles between human and bovine isolates. In relation to AMR-encoding genes, ermB, tetM, tetO and aphA3 were detected in both origins; linB, only in bovine isolates; aad6, only in human ones. Genes tetO and aphA3 were significantly associated with bovine isolates; tetM, with human ones. Selected isolates were sequenced in order to assign CC and ST. Two human infective isolates showed ST10 (CC12) and ST17 (CC17), one colonising isolate, ST147 (CC17) and two new STs were detected in bovine genomes. The genetic characteristics among the GBS isolates demonstrate differences between the two populations, questioning the GBS zoonotic status.