A 24-year-old woman is breastfeeding 3 weeks after giving birth to a normal term infant. She notices fissures in the skin around her left nipple. Over the next 3 days, a 5-cm region near the nipple becomes erythematous and tender. Purulent exudate from a small abscess drains through a fissure. Which of the following organisms is most likely to be cultured from the exudate?

Staphylococcal acute mastitis typically produces localized abscesses, whereas streptococcal infections tend to spread throughout the breast because streptococci often produce streptolysins. Acute mastitis can be associated with the first few months of breastfeeding. Candida may cause some local skin irritation, but is likely to become invasive only in immunosuppressed patients. Lactobacillus acidophilus is the organism used to produce fermented nonhuman milk. Listeriosis can be spread by contaminated food, including milk products, not by human milk.