Virulence Evaluation Of Different Streptococcus Agalactiae Serotypes In Tilapia (Oreochromis SP.)

Abstract

Background: Fish farming is always challenging due to serious Streptococcus agalactiae (group B streptococcus; GBS) infections. In detail, there are three serotypes Ia, Ib and III which are the main cause of fish disease by the GBS strain. Objective: In this study, the objective is to identify pathogenic bacteria and determine the LD50 value result in GBS serotypes Ia (GBS-Ia), GBS serotypes Ib (GBS-Ib), and GBS serotypes III (GBS-III). Methods: The experiment was designed to identify GBS serotypes causing diseases by virulent test. In dead fish samples, pathogenic bacteria can be distinguished when going through the culture process and isolation, to verify the accuracy by PCR. Next, the Reed & Muench(1938) method was applied to determine LD50 values which caused the 50% mortality by injecting GBS-Ia, GBS Ib, and GBS-III to tilapia (Oreochromis niloticus) . Results: The virulent tests resulted in 100% mortality caused by GBS-Ib and GBS III, while the results of GBS-Ia just reached 20%. The determined LD50 of serotypes Ib and serotypes III were 105.7 CFU/ml and 105.04 CFU/ml respectively. That means the GBS-Ib had higher virulence than GBS-III based on the LD50 value and timeline of infection. Infectious fish demonstrated erratic swimming, loss of appetite, depigmentation on the body, operculum areas, caudal fin erosion, and hemorrhages eyes in the external sign of fish. In internal signs, most fish organs were hemorrhages. To confirm the bacterial isolation, multiplex PCR was used to identify the correct serotypes. Conclusion: Three serotypes of Streptococcus agalactiae (group B streptococcus; GBS) are different in biochemical characteristics and in some cases LD50 value may not calculate. The findings expand the information on GBS-specific serotypes and support previous literature on different serotypes for a deeper explanation of these serotypes in detail.