Bovine mammary epithelial cell invasion by Streptococcus uberis.

Streptococcus uberis commonly causes bovine mastitis; however, the pathogenesis of this infection is poorly understood. In this study, the ability of S. uberis to invade mammary epithelial cells in culture was investigated. Two strains of S. uberis isolated from bovine mammary secretions were capable of invading bovine mammary epithelial cells in vitro at different levels, suggesting strain differences in invasiveness. Invasion required microfilaments but not microtubular cytoskeletal elements. No morphological changes in epithelial cells were observed for up to 24 h postinfection, suggesting no cellular injury. Strains of S. uberis evaluated were genetically distinct and differed phenotypically in expression of potential virulence factors. Whether a single factor or combination of factors was responsible for differences in invasiveness was not addressed in this study. These data provide a foundation for a better understanding of the processes used by S. uberis to invade epithelial cells. Epithelial cell invasion may be a potentially important mechanism in the pathogenesis of S. uberis mastitis.