Latex agglutination-negative methicillin-resistant Staphylococcus aureus recovered from neonates: Epidemiologic features and comparison of typing methods

An unusual strain of methicillin-resistant Staphylococcus aureus (MRSA) was repeatedly isolated from infants in a newborn special care unit (NBSC) and a newborn intensive care unit. Between January 1989 and March 1990, approximately 100 isolates from infected or colonized infants were recovered. Surveillance cultures taken during this time revealed a 20% colonization rate, which was defined as recovery of MRSA from the nares, umbilicus, or groin. Isolates were identified as S. aureus by tube coagulase reactivity and heat-stable nuclease production but were unreactive in a latex agglutination assay. Representative isolates that were collected during the outbreak and that were found to share the latex agglutination assay-negative phenotype were compared by antibiogram (12 isolates), bacteriophage typing (20 isolates), capsular polysaccharide typing (30 isolates), and plasmid as well as chromosomal DNA analyses (20 isolates). All isolates known to be associated with the outbreak had nearly identical antibiograms and were notably susceptible to clindamycin. Staphylococcal bacteriophage typing was not useful in determining the relatedness of the isolates, since the majority were nontypeable. Plasmid pattern analysis revealed one large plasmid (approximately 100 kb) of equivalent size among the isolates. Capsular polysaccharide typing revealed that 14 of 30 isolates tested were type 5. Isolates identified in children at two other hospitals in the city which were also unreactive by the latex agglutination assay and clindamycin susceptible had plasmid and antibiogram patterns identical to those of isolates from the NBSC. Pulsed-field gel electrophoresis of restriction enzyme-digested genomic DNAs from the outbreak isolates demonstrated identical patterns which could be clearly differentiated from those of other unrelated MRSA. The strain from the NBSC is, therefore, unique and underscores the need for caution in interpreting the latex agglutination reactivities of MRSA isolates.