Which organism is a common cause of mastitis, skin wounds, and abscesses and is also found in the environment?

• A. Proteus vulgaris
• B. Staphylococcus aureus
• C. Escherichia coli
• D. Streptococcus spp.

Answer: (B)

Staphylococcus aureus is the organism that best fits this combination because it is a leading cause of mastitis in animals, and it also commonly causes skin wounds and abscesses due to its pyogenic (pus-forming) infections. It repeatedly shows up in environments associated with animals and human handling—on skin, in bedding, milking equipment, and other surfaces—so it can be readily transmitted to udders and wounds. In the lab, S. aureus is a Gram-positive coccus that grows in clusters and is coagulase-positive, which helps distinguish it from other Staphylococcus species and from many environmental bacteria.

Proteus species, while capable of wound infections in some contexts, are more often linked to urinary tract infections and contaminated surfaces rather than a classic trio of mastitis, skin wounds, and abscesses. Escherichia coli is a common mastitis pathogen and can cause abscesses, but it’s not as characteristically tied to skin wounds and environmental persistence in the way Staphylococcus aureus is. Streptococcus species can cause skin infections and mastitis as well, but the combination of frequent environmental presence with the specific association to mastitis and abscesses points most strongly to Staphylococcus aureus.