Comparison of antibiotic susceptibility and pathogenicity of wild and serially passaged Streptococcus agalactiae to Nile tilapia (Oreochromis niloticus L.)

Abstract

The specific objectives of this study were to develop a spontaneously mutated S. agalactiae by serial passage in agar (T1 = control or wild, T2 = 10 days’ serial passage, T3 = 20 days’ serial passage, T4 = 30 days’ serial passage, T5 = 40 days’ serial passage and T6 = 50 days’ serial passage) and to compare the antibiotic susceptibility and pathogenicity of wild and serially passaged S. agalactiae to Nile tilapia (Oreochromis niloticus L.). S. agalactiae in T4 has resulted to increase zone of inhibition (ZOI) to penicillin and ampicillin when compared to T1. As the bacterial passage was increased to 50 days (T6), it resulted to significant decrease on the ZOIs to penicillin and ampicillin. For tetracycline, 40 days’ serial passage (T5) was the most consistent in terms of decreased ZOIs as compared to T1; meanwhile, 30 days’ serial passage (T4) had the most numbered increase in ZOIs. Therefore, T4 and T5 bacterial passages were needed in order to significantly change the ZOIs of S. agalactiae to the antibiotics used in this experiment. In the pathogenicity experiment, except for T4 (20.00%), the wild S. agalactiae (T1 = 30.00%) had lower percent mortality as compared to serially passaged bacterium (T2 = 50.00%, T3 = 50.00%, T5 = 40.00%, T6 = 30.00%). Based on the onset of first mortality, it appeared that 40 (T5) and 50 days (T6) serial passages were less pathogenic as compared to T1. All of the infected fish share the same signs of bacterial infection.