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 (1994) as
described in Yildirim-Aksoy et al. (2007).