Role of milk fractions, serum, and divalent cations in protection of mammary epithelial cells of cows against damage by Staphylococcus aureus toxins

Abstract

Objective - To determine the effect of milk and blood serum constituents on cytotoxicity of Staphylococcus aureus on mammary epithelial cells. Design - In vitro incubation of cells with cytotoxic agents and milk and serum constituents. Sample Population - Mammary cells, milk, and blood obtained from 3 cows. Procedure - Staphylococcal α-toxin and culture supernatants from S aureus M60 and an α-toxin-negative mutant of M60 were incubated with bovine mammary epithelial cells in the presence of milk fractions, serum, and divalent cations. Propidium iodide fluorescence was used as a measure of cell damage. Results - Skim milk and milk whey inhibited S aureus cytotoxic agents Skim milk protected against α-toxin damage to a greater extent than milk whey. Serum from an adult animal was more protective than was fetal serum. Milk fat and serum albumin had no protective effect. Divalent calcium and Mg2+ were more effective inhibitors of mammary epithelial cell damage caused by α-toxin than of damage attributable to M60 culture supernatant. Divalent calcium and Mg2+ at concentrations similar to those of free Ca2+ and Mg2+ in normal bovine milk decreased cytotoxic damage attributable to α-toxin. However, concentrations similar to those of total Ca2+ and Mg2+ in normal milk were required to decrease cell damage caused by M60 culture supernatant. The α-toxin-negative mutant was less cytotoxic than the M60 parent strain. Conclusions - Casein, as well as Ca2+ and Mg2+ in bovine milk, inhibit the cytotoxic effect of S aureus on mammary epithelial cells.