Distribution of Streptococcus agalactiae in Nile tilapia (Oreochromis niloticus) after oral inoculation

Abstract

Streptococcus agalactiae is an important pathogen that causes severe mortality in Nile tilapia within a short period of time. The objective of this study was to determine the distribution of S. agalactiae in fish organs at various intervals of time after oral inoculation in order to understand the systemic dissemination of S. agalactiae after inducing the fish with the disease. Thirty Nile tilapia with an average weight of 14 g were used in this study. Nile tilapia (N=27) were each orally infected with 0.3 ml of bacterial concentration of 5.92x108 CFU/ml and were maintained out of water for 5 min. Three fish were consecutively and randomly sampled within 30 min to 48 h. The control fish (N=3) were uninfected with S. agalactiae and sampled prior to S. agalactiae inoculation. All samples were investigated for S. agalactiae by bacteriological and polymerase chain reaction (PCR) techniques. S. agalactiae was recovered from the stomach, intestine, spleen, liver, kidneys and eyes after 30 min post inoculation but for the heart and brain, where were recovered S. agalactiae at 1 h post inoculation. All samples consistently recovered S. agalactiae by the end of the experiment, except the intestine and the eyes following 48 h of a period. PCR was performed on all sampled fish organs throughout the experiment. The results in this study demonstrated that the gastrointestinal tract successfully produced S. agalactiae infection that was then differently disseminated throughout important organs in the Nile tilapia