Maternal-infant perinatal transmission of methicillin-resistant and methicillin-sensitive Staphylococcus aureus.

Because of the increasing importance of Staphylococcus aureus (SA), including methicillin-resistant SA (MRSA) in serious neonatal infections, we studied the contribution of perinatal maternal-infant transmission of SA to the colonization and infection of newborn infants. Cultures for SA, including MRSA, were obtained from nares and vagina of women in labor at term. Each mother's infant, if delivered vaginally, was cultured from nares and skin at delivery and again after 48 hours (at discharge). All MRSA and selected SA isolates were studied by pulsed field gel electrophoresis (PFGE). Infants were monitored after discharge for staphylococcal infection for 4 weeks. Of 304 women completing the study, 43 were colonized with SA, and 9/43 had MRSA. Of 252 evaluable infants, 25 were colonized with SA, and 9/25 had MRSA. Six of 252 mother-infant pairs were concordant for SA colonization, and one of these for MRSA. Isolates from five of these six infants were indistinguishable from their mother's isolates by PFGE, including the pair with MRSA. One SA-colonized infant and four noncolonized infants subsequently developed staphylococcal infections during the monitoring period. About 20% of SA isolates in this maternal population were MRSA. Perinatal maternal-infant transmission accounted for 20% of instances of perinatal colonization of infants with SA. Molecular confirmation of perinatal maternal-infant transmission of MRSA was first documented. In this population of term infants, most SA infections in the first 4 weeks of life appeared to result from colonization that occurred after discharge from the nursery.