| Summary: | Vita. A streptococcal disease affecting hybrid tilapia (Tilapia nilotica x T aurea) at a commercial aquaculture facility in central Texas was investigated. The causative agent was identified as a biotype of Streptococcus iniae, based both on phenotypic characteristics and RRNA sequencing. The 0-hemolytic bacterium grew better at 37'C than at 10, 25 or 40'C, and its growth was inhibited at pH 9.6, in 6.5 % NaCl, and in 40 % bile. The pathogen was resistant to ampicillin and furazolidone but was susceptible to several antibiotics including tetracycline and sulfadimethoxineormetoprim (5: 1). After suspension of the bacteria in distilled water, viable cocci were recovered for up to 5 days at 5'C and < 24 h at 25'C. Gross signs of disease in tilapia included loss of orientation, exophthalmia, corneal opacity, petechiae around the mouth, vent and pectoral fin margins, and enlargement of the liver, spleen and kidney. Histology revealed cellular infiltration and numerous cocci in the eye, brain, liver, kidney and spleen. Cocci were also present in the blood. Monitoring of hematological parameters showed changes in the hematocrit, erythrocyte count, erythrocyte integrity and blood hemoglobin with disease progress. The 96-h and 168-h LD50 values in 30-90 g fish injected intraperitoneally were measured to be 4.9 x 10' and 3.18 x 10' cells, respectively. Artificial infection was achieved by dipping fish in water containing the bacteria and by oral administration. Greater rates of mortality were observed in artificially infected tilapia maintained at 20 than at 15, 25, 30 or 35'C; and at pH 9 compared to more acidic water. Hybrid striped bass were found susceptible whilst red drum and channel catfish were resistant to artificial infection. Size exclusion gel separation of cell-free media (Todd Hewitt broth, pH 7, supplemented with 10% fetal calf serum) produced three hemolytic fractions corresponding to molecular weights of 21,700, 2,200 and 125 daltons with respective isoelectric points of 4.1, 6; 3.5-4, 8; and 3.5, 8.
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