Staphylococcus aureus (S. aureus) is a contagious mastitis pathogen that is extremely difficult to control by treatment alone. The spread of S. aureus can occur through milkers' hands, washcloths, teat cup liners, and flies. Successful control is gained only through prevention of new infections and culling of infected animals.

Corynebacterium bovis is a contagious mastitis-causing pathogen. It is mildly pathogenic, and will usually cause only a mild increase in SCC and a slight reduction in milk production. Efficacious post-milking disinfectants will dramatically reduce the infection rate from C. bovis, and dry cow therapy is very effective in eliminating C. bovis.

Enterobacter spp. are environmental mastitis-causing pathogens. The spread of Enterobacter spp. occur primarily through environmental contact, particularly when teats come in contact with manure or contaminated bedding. The primary way to prevent infection by Enterobacter spp. is by keeping cows clean and dry to decrease exposure, as well as proper teat sanitation prior to milking.

Trueperella pyogenes are environmental pathogens that typically cause an acute, purulent form of mastitis that is sometimes referred to as “summer mastitis.” Trueperella pyogenes are Gram-positive bacteria that can appear like coccobacilli — an intermediate between cocci and bacilli. When plated on blood agar, scant to no growth is visible at 24 hours. However, at 48 hours, tiny, pinpoint, smooth, whitish colonies develop and are surrounded by a narrow zone of hemolysis.