Superimposed Intramammary Infection with Staphylococcus hominis Under Different Pre-Existing Infection Conditions in Bovine Udder: Bacterial Dynamics, Inflammatory Response and Oxidative Stress

Microbial interactions control mammary gland inflammation and disease severity. This study examined bacterial dynamics, inflammatory responses, and oxidative stress following Staphylococcus hominis (SH) superinfection in cows with chronic subclinical mastitis. Mammary quarters were assigned to varying SH doses: COW1 received 106 and 105 CFU/mL (two quarters each); COW2 received 106 CFU/mL (two quarters) and PBS control (one quarter); COW3 received 104 and 105 CFU/mL (two quarters each); and COW4 received 104 CFU/mL (two quarters). On Day 0, quarters received 5 mL of their assigned preparations. Milk and clinical parameters were evaluated during a pre-challenge baseline and monitored for 8 days post-inoculation. Results showed post-infection mastitis severity depended heavily on prior infections. SH alone functioned as a mild pathogen, inducing short-term inflammation with elevated somatic cell counts and IL-6 levels primarily on Days 1 to 3, but did not independently cause severe tissue damage. The intense immune response, elevated malondialdehyde (MDA) levels, and severe tissue damage were primarily driven by pre-existing major mastitis pathogens. Although S. hominis is inherently a mild pathogen, high bacterial loads synergistically exacerbate oxidative stress and clinical mastitis symptoms in co-infected quarters.