Rapid method for determining serum bactericidal activity.

To screen large numbers of sera, a method was devised which utilizes the Steers-Foltz replicator which is usually used to determine minimal inhibitory concentration for antibiotics. Each of the wells (9 by 15 mm) of the replicator is filled with 0.06 ml of serum, 0.02 ml of a 10(5) suspension of organisms, and 0.02 ml of diluent (tris(hydroxymethyl)aminomethane-hydrochloride buffer, pH 8.4). The mixtures are incubated for 3 h, and samples are taken at 0, 1, 2, and 3 h by stamping duplicate nutrient agar plates (approximately 0.04 ml from each well). Plates are incubated overnight, and bactericidal activity is estimated by visual inspection of bacterial growth at each site for each sampling time. Results obtained with 28 serum-organism pairs paralleled standard pipetting-pour plate methods. The replicator method for determining bactericidal activity allows for the testing of a large number of samples and requires negligible amounts of serum.