Survival of Streptococcus agalactiae from frozen fish following natural and experimental infections

Abstract

We evaluated the survival of Streptococcus agalactiae from naturally infected wild mullet ( Liza klunzingeri) and experimentally infected Nile tilapia ( Oreochromis niloticus) frozen at −20 and −70 °C, respectively, for an extended period of time. The brain, eye, head kidney, and intestine of individually frozen wild mullet ( N=22), culture positive for S. agalactiae from fresh tissues following an S. agalactiae epizootic in Kuwait Bay in 2001, were re-sampled after 9 months. Nares, eye, head kidney and intestines, not previously sampled, were also evaluated. Tilapia were inoculated with either 5.6×10 2 colony forming units CFU/fish or 4.5×10 6 CFU/fish and S. agalactiae survival assessed from frozen tissues after 7, 14, 30, or 180 days. S. agalactiae was recovered from nare, brain, eye, and head kidney of 100% of the frozen mullet after 9 months post-freezing. The nare, brain, and head kidney of 100% of the experimentally infected tilapia were culture positive at 7, 14, 30, and 180 days post-freezing. The use of frozen fish may prove to be a useful alternative to fresh fish for recovering pathogenic streptococci in instances when fresh fish diagnostic analysis are unavailable or impractical. Furthermore, archived frozen fish can be used for retrospective microbiological analyses of streptococcal infection from multiple or different tissues not originally sampled.