Identification and MLST typing of Aeromonas hydrophila isolates

To investigate the prevalence and classification of Aeromonas hydrophila from freshwater fish in Zhenjiang City, this study isolated and purified A. hydrophila from 74 pathogenic bacteria strains collected from several aquaculture farms in the city. Morphological observations, gyrB gene sequence analysis, and multilocus sequence typing（MLST）were conducted. The results showed that A. hydrophila appeared as light yellow, round, smooth, and neatly edged colonies on RS agar plates and red, rod-shaped bacteria with blunt ends by microscopic examination after Gram staining. After conducting sequence alignment analysis on the gyrB gene of isolated strains, 40 strains of Aeromonas were identified, of which 12 were preliminarily determined to be A. hydrophila. A phylogenetic tree of Aeromonas isolates based on the gyrB gene was further constructed, finally identifying eight isolates as A. hydrophila. PCR amplification and sequence analysis were performed on the housekeeping genes gyrB, groL, gltA, metG, ppsA, and recA of A. hydrophila. Two strains of A. hydrophila type ST251, three strains of type ST378, and one strain each of types ST469, ST361, and ST1075 were identified. The research results indicate that there is a potential risk of the prevalence of highly pathogenic A. hydrophila type ST251 in this region, posing a threat to the green and high-quality development of the local aquaculture industry.