2.4 Serum bactericidal activity (SBA)
against different strains was measured as follows. The blood samples collected from clinically healthy horses for routine veterinary control purposes were used. The samples were collected into a yellow-cap vacuum tube with clot activator (Elamed, Moscow, Russia). The serum was separated by centrifugation at 1600 × g for 10 min. The serum was stored at +4 oC and used for the tests within 24 h. For the SBA assessment, the wells of 96-well plate containing 175 μl of LB medium and 25 μl of the serum were inoculated with 5 μl of the corresponding strain mid-log phase culture (OD600 = 0.6) and the plates were incubated at 37oC in an automated plate reader with agitation. The OD600 was recorded every 30 min. In the control experiment the same volume of physiological saline replaced the serum. The whole experiment was triplicated.