Effects of antibiotics on phagocyte recruitment, function, and morphology in the bovine mammary gland during the early nonlactating period

The effects of 2 antibiotic preparations administered intramammarily on phagocyte recruitment, function, and morphology were evaluated at the beginning of the nonlactating period. Twelve cows with no clinical or microbiologic evidence of mastitis were assigned to 1 of 2 treatment groups. At the end of lactation, 1 of the antibiotic preparations was infused in a fore- and hind quarter of each cow; the remaining quarters were untreated controls. One group was given benzathine cephapirin; the second group was given sodium novobiocin. Secretion samples were collected from 1 treated and 1 control quarter at 16 hours, and from the remaining 2 quarters at 64 hours after treatment. Total and differential somatic cell counts were determined, and morphology of mammary polymorphonuclear neutrophils (PMN) and macrophages was observed by transmission electron microscopy. In vitro ingestion and killing of Staphylococcus aureus by mammary PMN and macrophages were assessed by fluorescent microscopy, using acridine orange stain. Cells resident in a fixed volume of secretion were incubated with a known concentration of S aureus. Total cell and PMN concentrations were higher in treated than in control quarters. Neutrophils were the predominant cell type in both treated and control quarters over the sampling period. As measured in this study, in vitro ingestion and killing of S aureus by individual PMN from treated quarters was reduced. Antibiotic treatment also increased the proportion of morphologically abnormal phagocytes. There were significant correlations among PMN ingestion, killing, and morphology. However, increased PMN concentrations tended to compensate for the reduced phagocytic function of individual cells. Therefore, efficacy of antibiotic treatment of nonlactating cows may depend, at least in part, on increased PMN concentration, which may tend to compensate for reduced phagocytic function. Compared with PMN, macrophages appeared to have only a minor role in phagocytosis of bacteria.