An investigation of phagocytosis and intracellular killing of Staphylococcus intermedius by canine neutrophils in vitro

Bacterial infection (pyoderma) of the canine skin is largely caused by Staphylococcus intermedius and may be a primary, idiopathic disease or secondary to a range of other dermatological disorders. The immune response to this organism involves phagocytosis and killing by neutrophils. In this study, the effect of opsonisation of S. intermedius by clinical sera (n = 60) and sera from normal dogs (n = 20) was investigated by in vitro phagocytosis and killing assays. All sera from dogs with pyoderma were significantly more efficient at opsonisation for phagocytosis than normal dog sera (p < 0.001). Further, fresh serum was more effective at opsonisation of S. intermedius than heat-inactivated serum in this assay (p < 0.001). The results indicate that opsonisation is enhanced by serum from dogs with pyoderma; therefore the humoral response in clinical cases of canine pyoderma is, in terms of phagocytosis, protective. The small but significant reduction in phagocytosis of S. intermedius opsonised by heat-inactivated serum compared to fresh frozen serum suggests that complement plays a role, although minor, in opsonisation. An in vitro killing assay showed that normal canine neutrophils were able to effectively kill phagocytosed S. intermedius over a 90-min time course, but there was no significant enhancement of killing by prior incubation of the bacteria with any clinical serum as opposed to normal sera. This suggests that the humoral response to S. intermedius does not affect intracellular killing per se.