Bovine-associated CNS species resist phagocytosis differently

Abstract
Background: Coagulase-negative staphylococci (CNS) cause usually subclinical or mild clinical bovine mastitis,
which often remains persistent. Symptoms are usually mild, mostly only comprising slight changes in the
appearance of milk and possibly slight swelling. However, clinical mastitis with severe signs has also been reported.
The reasons for the differences in clinical expression are largely unknown. Macrophages play an important role in
the innate immunity of the udder. This study examined phagocytosis and killing by mouse macrophage cells of
three CNS species: Staphylococcus chromogenes (15 isolates), Staphylococcus agnetis (6 isolates) and Staphylococcus
simulans (15 isolates). Staphylococcus aureus (7 isolates) was also included as a control.
Results: All the studied CNS species were phagocytosed by macrophages, but S. simulans resisted phagocytosis
more effectively than the other CNS species. Only S. chromogenes was substantially killed by macrophages.
Significant variations between isolates were seen in both phagocytosis and killing by macrophages and were more
common in the killing assays. Significant differences between single CNS species and S. aureus were observed in
both assays.
Conclusion: This study demonstrated that differences in the phagocytosis and killing of mastitis-causing staphylococci
by macrophages exist at both the species and isolate level.