Antiphagocytic properties of uterine isolates of Streptococcus zooepidemicus and mechanisms of killing in freshly obtained blood of horses.

: A total of 22 clinical streptococcal isolates, predominantly Streptococcus zooepidemicus, associated with endometritis in horses were tested for their ability to withstand the natural bactericidal properties of freshly obtained blood. During a 3-hour incubation in blood from a single horse, 8 of these isolates survived and grew, the remainder were killed. To determine whether this ability to grow extended to blood of other horses, 5 of these growing isolates were tested for their ability to grow in the blood of 5 additional horses. The same 5 horses were used for each isolate. The isolates grew in blood of some of the horses, but were killed in blood of the others. However, the horse's blood that mediated killing was different for each isolate. Killing required leukocytes, but the specificity for killing appeared to reside in plasma, although plasma by itself was not bactericidal. Heat-stable and heat-labile components in plasma, interpreted as antibody and complement, respectively, appeared necessary for killing. Isolates that could grow in fresh blood lost this ability after 10 passages in artificial media. Results of these experiments of phagocytosis in fresh blood may provide helpful insights into the phagocytosis of S zooepidemicus in equine uterine fluid.