ABSTRACT
Streptococcus agalactiae (Lancefield group B; GBS) is a pathogen that causes meningoencephalitis in fish, mastitis in cows, and neonatal sepsis in humans. The objective of this study was to characterize S. agalactiae isolated from fish (n=27), cows (n=9), and humans (n=10) using pulsed-field gel electrophoresis (PFGE) and to investigate the virulence of the identified strains in Nile tilapia (Oreochromis niloticus). The PFGE types were determined by dendogram analyses and the in vivo virulence was evaluated by experimental infection (using i.p. and immersion routes) of Nile tilapia. Among the fish strains, 5 different PFGE patterns were observed and 21 strains showed the same genetic pattern. In some farms two or three profiles occurred simultaneously. The bovine and human strains exhibited high genetic diversity and few relationships were established among S. agalactiae strains from the three host origins analyzed. Eight S. agalactiae strains from fish caused high mortality of Nile tilapia. Three bovine strains infected Nile tilapia (by i.p. route) and two of those strains caused clinical signs of meningoencephalitis. All human strains (n=5) infected Nile tilapia (by i.p. route) and meningoencephalitis was induced by one strain (by both i.p. and immersion routes). In conclusion, the analyzed strains from the three natural hosts did not show genetic relatedness, yet some of the bovine and human strains were able to infect fish and cause meningoencephalitis. We suggest that genetic linkage is not a prerequisite for S. agalactiae to cross the host-specific barrier.
Subject(s)
Cichlids , Fish Diseases/microbiology , Streptococcal Infections/microbiology , Streptococcus agalactiae/genetics , Streptococcus agalactiae/pathogenicity , Virulence/physiology , Animals , Bacterial Typing Techniques , Cattle , Electrophoresis, Gel, Pulsed-Field , Female , Fish Diseases/mortality , Fish Diseases/pathology , Humans , Molecular Sequence Data , Streptococcal Infections/mortality , Streptococcal Infections/pathology , Streptococcus agalactiae/isolation & purificationABSTRACT
Streptococcus agalactiae is an emerging pathogen in Nile tilapia (Oreochromis niloticus) worldwide. To investigate aspects of the epidemiology, transmission and virulence of S. agalactiae infections, nine outbreaks of meningoencephalitis and septicemia in Nile tilapia farms in Brazil were analyzed. Records from the outbreaks revealed large variation in the weight of fish affected, high mortality, and disease occurrence at water temperatures above 26 degrees C. S. agalactiae was isolated from diseased fish from all farms, and 29 strains were identified by phenotypic tests and 16S rRNA gene sequencing. Five strains from different geographic origins were selected to determine the 50% lethal dose (LD(50)). All strains were highly virulent; for example, strain SA 20-06 had an LD(50) of 90 bacteria. To investigate S. agalactiae transmission, we conducted cohabitation assays with diseased and healthy fish and fish challenges using an immersion bath or gill inoculation. Strain SA 20-06 was used in all assays. The disease was reproduced with characteristic clinical signs and S. agalactiae was reisolated in all trials. The infection route studies were identified as by direct contact or through the water. In conclusion, S. agalactiae, a major pathogen of Nile tilapia in Brazil, exhibited high virulence, regardless of the geographic origin of the isolated strains.
