Enhanced Detection Sensitivity and Dual Colonization of Staphylococcus aureus in a Swedish Neonatal Intensive Care Unit

Background: Local resistance patterns guide empirical antibiotic therapy, but the sensitivity of Staphylococcus aureus screening and multiclonal colonization remain insufficiently explored. This study investigates the prevalence and dual colonization of S. aureus during an outbreak of tobramycin-resistant S. aureus (TRSA) of spa type t084 in a neonatal intensive care unit.Methods: From May 11th, 2012 through 2015, samples from anterior nares, umbilicus and perineum were obtained from infants (n=870) and antibiotic susceptibility testing was done. All TRSA isolates were spa typed, and all S. aureus isolates, regardless of resistance pattern, were spa typed during 2014. Medical records from neonates with TRSA growth in clinical cultures were reviewed. To investigate clinical TRSA infections, the observation period was extended through 2019.Results: Overall S. aureus colonization was 49 %. Screening sensitivity was 81 % (nares), 65 % (umbilicus) and 71 % (perineum); nares plus umbilicus yielded 96 % sensitivity. Dual colonization with both TRSA and tobramycin-susceptible S. aureus occurred in 7.8 % of infants and with fusidic acid-resistant plus tobramycin-susceptible strains in 1.9 %. In 2014, when all S. aureus isolates were spa typed, dual colonization was detected in 13 %. Among the 27 infants with S. aureus infections, all had nasal colonization, 74 % and 90 % had umbilical and perineal colonization, respectively.Conclusions: The rate of dual colonization was high, suggesting that screening for S. aureus colonization in infants should include samples from the nares as well as either the umbilicus or perineum to ensure accurate detection and resistance determination.