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

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 alpha-toxin and culture supernatants from S aureus M60 and an alpha-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 alpha-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 alpha-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 alpha-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 alpha-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.