Genetic diversity and new genotyping scheme for fish pathogenic Streptococcus agalactiae

This study aimed to assess the genetic diversity of fish isolates of Streptococcus agalactiae by capsular serotyping, MLST and the pattern of selected virulence genes. Forty-six isolates from Nile tilapia and Amazon catfish were screened by PCR for the twelve virulence genes. The molecular capsular type and sequence type (ST) were determined. Two capsular types (Ia and Ib) and four STs (103, 260, 552 and 553) were identified. The ST-552 and ST-553 represent new allelic combinations. Variable results were found for the genes gbs2018-6, lmb, hylB and cylE. The combined evaluation of serotype, sequence type and pattern of the presence or absence of cylE and hylB allowed the classification of isolates into nine genetic profiles (I-IX). The proposed scheme showed higher discriminatory power and was able to detect evolutionary events missed by MLST analysis. This study provides new information about the genetic diversity of fish pathogenic Strep. agalactiae, and the proposed scheme was shown to be an improved approach to genotyping these strains.

This study showed that critical genetic events in Streptococcus agalactiae isolates pathogenic for fish have been missed by serotyping and multilocus sequence typing (MLST). A proposed genotyping scheme based on the evaluation of concatenated data from serotyping, MLST, and the presence/absence of virulence genes was created, and this was able to detect old and recent evolutionary events. It provided a better understanding of the genetic diversity of Strep. agalactiae populations from fish and will contribute to future studies of the molecular epidemiology, pathogenesis and evolutionary aspects of this pathogen.