Effect of multiplicity of infection on the evasion of neutrophil killing by Streptococcus agalactiae isolated from clinical mastitis bovine.

Streptococcus agalactiae (S. agalactiae) causes intramammary infection in dairy cows. Increased neutrophils and a high bacterial load are important characteristics of bovine bacterial mastitis. We hypothesized that the multiplicity of infection (MOI) of S. agalactiae in bovine mastitis plays an important role in bacterial pathogenicity by modulating the neutrophil response to promote bacterial survival. Neutrophils from BALB/c mice were infected with the bovine mastitis isolate of S. agalactiae SAG-FX17 at various MOIs, and neutrophil responses were investigated. Infecting neutrophils with SAG-FX17 at an MOI of 1 induced reactive oxygen species (ROS) and neutrophil extracellular traps (NETs) formation. Bacteria at an MOI of 10 suppressed neutrophil responses, including ROS bursts, NET formation, and cell necrosis, which are conducive to bacterial multiplication within 30 min postinfection. In addition, neutrophils are destroyed by SAG-FX17 at an MOI of 100 or greater. This study identified the MOIs related to the ROS and NET suppression caused by SAG-FX17, and the findings suggested that interventions to decrease bacterial loads before the MOI of 10 could be necessary and effective to harness the power of innate immune response to eliminate pathogens.