Infant S. aureus Colonization Rises After Birth in Cases of Maternal Nasal Carriage


WASHINGTON — Staphylococcus aureus colonization increases significantly in the first 2 months of life and appears to be positively associated with maternal carriage, based on a study of 200 mother-neonate pairs.

At delivery, S. aureus detection among 165 infants was 8%; at discharge, detection among 190 infants was 7%. However, at age 2 months, 33% of infants were colonized with S. aureus—17% with methicillin-resistant S. aureus (MRSA), Dr. Clarence B. (Buddy) Creech II said at the jointly held annual Interscience Conference on Antimicrobial Agents and Chemotherapy and the annual meeting of the Infectious Diseases Society of America.

“In those mothers who had no evidence of nasal colonization at 2 months, the vast majority of their infants also remained uncolonized. However, among those mothers who had methicillin-susceptible S. aureus (MSSA) nasal colonization [at 2 months], 51% of their infants were nasally colonized with MSSA.” Of mothers with MRSA nasal colonization, 45% of their infants had nasal MRSA colonization at 2 months.

“We were able to detect MRSA carriage in a significant number of mothers and infants; however, we have not observed many infections in this cohort,” Dr. Creech said in an interview. The findings demonstrate that staphylococci are commensal organisms and part of normal flora. “The question that remains is whether certain strains of staphylococci, in certain hosts, are more likely to cause disease than others.”

Pregnant women were recruited at the time of group B streptococcus (GBS) screening between gestational weeks 35 and 37. At that time, nasal swabs for S. aureus and rectovaginal swabs for GBS and S. aureus were taken. Nasal swabs were repeated in the mother on the day of delivery and at 2 months post partum. Neonatal nasal and umbilical swabs were obtained within 2 hours of birth and nasal swabs were repeated on the day of discharge and at 2 months.

At the time of the presentation, 431 pregnant women had been enrolled. Dr. Creech, of the pediatric infectious diseases department at Vanderbilt University, Nashville, Tenn., presented data on the first 200 mother-neonate pairs with data out to the 2-month period. Mothers were primarily recruited from academic private practice and an obstetric resident clinic serving an inner-city Medicare population in Nashville.

Nasal S. aureus colonization of mothers in pregnancy was 23.5%—12% for MSSA and 11.5% for MRSA. Rectovaginal S. aureus colonization of mothers during pregnancy was 17%—13% for MSSA and 4% for MRSA. It was not always possible to obtain neonatal swabs within 2 hours of delivery. In all, 165 infants were evaluable at delivery. Neonatal MSSA detection at delivery (not necessarily colonization) was 5% and MRSA detection was 3%; 190 infants were evaluable at discharge. Neonatal MSSA detection at discharge was 2%; MRSA detection was 5%.

Of the 136 mothers who did not have S. aureus colonization during enrollment, 96% of their infants did not have S. aureus detected at delivery, 3% of infants had MSSA, and 1% had MRSA. Of the 21 mothers who had MSSA colonization at enrollment, 67% of their infants did not have S. aureus detected at delivery, 19% of infants had MSSA, and 14% had MRSA. Of the eight women who had MRSA colonization during enrollment, 62% of their infants did not have S. aureus detected at delivery, 13% of infants had MSSA, and 25% had MRSA.

Two babies developed disease during the first 5–7 weeks. One infant had an abscess that required drainage and intravenous antibiotics. The other infant developed purulent conjunctivitis. The isolates from these infections matched very closely the USA300 strain with staphylococcal cassette chromosome mecIV and were Panton-Valentine leukocidin toxin positive.

With regard to women colonized with S. aureus during pregnancy, “there [are] no data to suggest that prophylactic antibiotics that cover S. aureus would be of any benefit. Many infants in whom MRSA was detected within 2 hours of birth had no evidence of S. aureus at discharge or at 2 months follow-up,” he said.

Dr. Creech disclosed financial relationships with several pharmaceutical companies.

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