Genetic diversity, virulence genes, antimicrobial resistance genes, and antimicrobial susceptibility of group B Streptococcus (GBS) associated with mass mortalities of cultured Nile tilapia in Brazil

Streptococcus agalactiae, group B Streptococcus (GBS), stands as the primary bacterial pathogen affecting cultured Nile tilapia (Oreochromis niloticus) globally, leading to significant mortalities throughout the farming cycle. This study investigated the genetic diversity, virulence and antimicrobial resistance (AMR) genes presence, and antimicrobial susceptibility of 72 GBS strains associated with mass mortalities of Nile tilapia in Brazil. Isolate identity was confirmed by morphological, biochemical and molecular analyses. Capsular serotype, multi-locus sequence typing (MLST) allelic profiles and putative pathogenic factors were determined through polymerase chain reaction (PCR), gel electrophoresis, DNA sequencing and molecular analyses. The presence of AMR genes and antimicrobial susceptibility to florfenicol (FFC), oxytetracycline (OTC), thiamphenicol (TAP) and their combination were evaluated by PCR, followed by gel electrophoresis, and broth microdilution antimicrobial susceptibility testing, respectively. All clinical isolates studied were confirmed to be GBS, one from serotype III (IA2201) and 71 from serotype Ib, suggesting that serotype Ib was the most prevalent strain between 2011 and 2016 in the southern region of Brazil. Eight different allelic profiles were identified for the first time, with adhP-52, pheS-2, atr-31, glnA-4, sdhA-2, tkt-19 being the most predominant. Between one (glcK) and three (adhP and glnA) alleles were present at each locus. All strains, except IA2201, were negative for the glcK gene. Hyaluronate lyase (hlyB) and the GBS immunogenic bacterial adhesin A (bibA) were detected in all strains, except for 18P, which was negative for hlyB. On the other hand, α and β antigens of the C protein were only detected in IA2201. All antimicrobials showed high minimum inhibitory concentration (MIC ≥16 μg/mL) values against several strains with negative results for resistance genes. The combination involving OTC and TAP or FFC is a likely candidate for improving the treatment of streptococcosis caused by GBS using combination therapy, even for strains showing phenotypic and genotypic resistance to OTC. This study provides important data on pathogenic GBS genetic diversity, the presence of virulence and antimicrobial resistance genes and antimicrobial susceptibility, which may be useful in the development of effective vaccines and therapeutic strategies for the prevention and control of streptococcosis in aquaculture farms.