Staphylococcus aureus is highly resistant and virulent in milk

Staphylococcus aureus is frequently found in human and veterinary infections. It is named for the golden-yellow pigment it produces, which contributes to its virulence and protects it against oxidative stress. Its ability to withstand antibiotic treatment makes it a major clinical challenge. To better understand the factors underlying this resistance, the scientists investigated the growth of the bacterium in two environments relevant to infection: milk and serum¹. Produced by the mammary glands, milk provides a stable, nutrient-rich environment in which S. aureus can thrive and cause mastitis (infections of the mammary glands). Serum is the environment in which S. aureus develops during infection.

Milk: an ideal environment for Staphylococcus aureus

Staphylococcus aureus has very different properties depending on whether it grows in milk or serum. The scientists found, in particular, that the lipid composition of its membrane changes. When grown in milk, S. aureus incorporates milk lipids into its membrane instead of producing its own. As a result, it develops an envelope that is about three times thicker than that of bacteria grown in serum or laboratory culture media. Milk lipids also stimulate pigment production, making S. aureus more virulent and more resistant to oxidative stress. The scientists also tested the various types of antibiotics used to treat mastitis. When grown in milk, S. aureus becomes much more robust and resistant to the different antibiotics tested. This can be explained by the fact that the antibiotics target the enveloppe²of S. aureus, which is thicker when the bacterium has grown in milk, or cause oxidative stress, against which it is better protected thanks to its higher pigment production.

These findings suggest a unique relationship between milk lipids and the severity of Staphylococcus aureus infections. The bacterium's remarkable robustness in milk may indicate a need to adapt antibiotic treatment protocols for the treatment of mastitis when milk is abundant, particularly in humans and domestic animals. Research is underway to develop combination therapies that combine an antibiotic with a molecule that prevents the bacterium from incorporating lipids from its environment.

1. Serum is the liquid component of blood after the cells and clotting proteins have been removed.
2. The envelope comprises the bacterial membrane and cell wall.