OBJECTIVE:In this study, the pathogenicity of two genetically distinct Lactococcus petauri strains, one isolated from catfish Ictalurus spp. in Alabama, USA, belonging to sequence type 142, and one from Rainbow Trout Oncorhynchus mykiss in California, USA, belonging to sequence type 145, were evaluated in experimental challenges using a Nile Tilapia Oreochromis niloticus fingerling infection model. METHODS:Since tilapia can be cultured at a wide range of temperatures in vivo, experiments were conducted at 25 ± 2°C and 30 ± 2°C. The fish were challenged via intracoelomic injection of ∼107 CFU/fish or by immersion in 5 L of water containing ∼107 CFU/mL for 1 h. Morbidity and mortality were monitored twice daily for 3 weeks. Water samples from each tank were collected at 6 h and at 1, 9, and 15 d postchallenge to evaluate L. petauri DNA load using a species-specific quantitative PCR. RESULTS:No significant morbidity or mortality was observed in any of the treatment groups during the challenge. At the end of the 3 weeks, all of the fish were euthanized and brain samples from 10 fish per group were cultured on nutrient agar to evaluate the bacterial persistence in the survivors. None of the surviving fish yielded positive isolation of L. petauri. Additionally, no significant histopathological changes were observed between the control and challenged tilapia. Bacterial DNA in the water was detected only at 6 h and at 1 d postchallenge in some treatments. CONCLUSIONS:These findings suggest that under the conditions that were tested, the two strains of L. petauri that were evaluated appear of low virulence to Nile Tilapia.
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