Identification and characterization of Streptococcus agalactiae recovered from farmed tilapia in Chiang Mai

Abstract

Streptococcus agalactiae is a bacterial pathogen affecting farmed tilapia losses. In this study, five bacterial samples were recovered from farmed tilapia in Chiang Mai. They were identified and characterized by standard conventional methods, Lancefield serogrouping, biochemical assay with API 20 STREP, confirmed S. agalactiae and capsular typing by PCR. Bacterial growth and antibiotic susceptibility were also investigated. The results showed that all isolates were Gram-positive cocci, mostly in long chains, white colonies measuring 1-2 mm and grow from 28 to 37 o C at 24-48 h. They appeared β-hemolysis, non-motile, catalase and oxidase negative, CAMP test positive but no grow at higher than 2% NaCl and serogroup B. The biochemical profiles showed some differences in reactions while all the PCR samples were confirmed as S. agalactiae serotype III. The bacterial growth were similar including lag phase at 0-3 h, log phase at 3-12 h, stationary phase at 12-54 h and death phase after 54 h. In addition, the isolates were susceptible to oxytetracycline, sulphamethoxazole/ trimethoprim, enrofloxacin, amoxycillin, penicillin and chloramphenicol.