2.7. Bacterial challenge and antibody titer
A frozen stock-culture of S. iniae (ARS 98-60) from an outbreak of streptococcal disease in Nile tilapia was grown in tryptic soy broth (TSB) at 25 °C with shaking at 100 rpm for 24 h.
The concentration of the culture was adjusted to an optical density of 1.0 measured on a Shimadzu UV-1601 spectrophotometer (Shimadzu Scientific Instruments, Inc., Norcross, Georgia) at 540 nm to give a S. iniae concentration of 1×109 colony forming units (CFU)/mL.
At the end of the 12-week feeding period, twenty remaining fish per aquarium were randomly selected and intra-peritoneally (IP) injected with 0.1 mL of 1×106 cfu/mL of S. iniae (1×105 cfu/fish) using a tuberculin syringe.
They continued to receive their respective diets.
Fish were monitored and mortality was recorded twice daily for 15 days following injection and dead fish were removed.
At the end of the S. iniae challenge trial, blood samples were collected from four randomly chosen surviving fish and serum was collected following centrifugation.
Agglutinating antibody titers against S. iniae in pre- and post-challenge serum samples were determined by modifying the method of Chen and Light as described in Yildirim-Aksoy et al.