Mastitis neonatorum of the left breast, positive for methicillin-resistant Staphylococcus aureus (MRSA): On the basis of the patient's history, physical findings, and laboratory data, the healthcare team diagnosed mastitis neonatorum (acute mastitis) with an underlying abscess. The abscess was incised and drained, and the patient was treated with antibiotics. On further questioning, the mother and father both reported that they had recent wounds, which were drained. Their cultures were positive for MRSA. The patient's wound culture grew MRSA that was sensitive to clindamycin but resistant to erythromycin. A D-test demonstrated no inducible clindamycin resistance; the patient was discharged home with a 14-day regimen of clindamycin.

Mastitis neonatorum is an uncommon neonatal skin or soft tissue infection of the breast. The condition is most common in female babies, it occurs during the first 4 weeks of life, and it is typically unilateral. Causes of infection include contact with an infected mother or other family members, infected healthcare workers, or contaminated breast milk. The most likely mechanism of entry is direct inoculation through the skin, and the most common organism is S aureus. Although S aureus is colonized in approximately 40-90% of neonates by the age of 5 days, skin and soft tissue infections are relatively infrequent. However, the increased prevalence of community-acquired and nosocomial-acquired MRSA appears to be raising the number of clinical infections.

Abscesses should be drained, and appropriate antibiotics may be needed. Evidence suggests that isolated abscesses do not require antibiotic treatment, though an antibiotic regimen is typically recommended if surrounding cellulitis is present. Cephalexin and erythromycin, which were previously effective treatments, are becoming increasingly unsuccessful in eradicating cellulitis. When cultures are positive for MRSA infection, when antibiotics fail in the setting of identified MRSA resistance, or in areas where the prevalence of MRSA is high, treatment should include a regimen known to be successful against MRSA in that geographic location. Commonly effective antibiotics include clindamycin, trimethoprim-sulfamethoxazole, and rifampin.

Although mastitis neonatorum rarely results in serious bacterial infections, possible complications include bacteremia, sepsis, and meningitis. A case in the literature reports an infant with acute mastitis and a subsequent brain abscess. Given the increasing antibiotic-resistance patterns and the relatively immature immune system of a neonate, patients should be closely monitored for clinical improvement.