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Latex agglutination-negative methicillin-resistant Staphylococcus aureus recovered from neonates: epidemiologic
A R Wanger1, S L Morris, C Ericsson
1Department of Pathology and Laboratory Medicine, University of Texas Medical School, Houston 77025.
Abstract:
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.
Insights
An unusual methicillin-resistant Staphylococcus aureus (MRSA) strain caused an outbreak in newborn units, showing a 20% colonization rate. This unique MRSA strain requires careful interpretation of latex agglutination test results.
Area of Science:
- Microbiology
- Infectious Diseases
- Neonatal Care
Background:
- An unusual strain of methicillin-resistant Staphylococcus aureus (MRSA) was identified in infants within newborn special care and intensive care units.
- Surveillance revealed a significant 20% colonization rate among infants, with MRSA isolated from nares, umbilicus, or groin.
Purpose of the Study:
- To characterize an unusual MRSA strain causing an outbreak in neonatal units.
- To investigate the genetic relatedness and unique phenotypic properties of the outbreak MRSA isolates.
Main Methods:
- Isolates were identified using standard microbiological tests (tube coagulase, heat-stable nuclease) and a latex agglutination assay.
- Molecular typing methods included antibiogram, bacteriophage typing, capsular polysaccharide typing, plasmid analysis, and pulsed-field gel electrophoresis (PFGE).
Main Results:
- The MRSA isolates were unreactive in the latex agglutination assay but susceptible to clindamycin.
- Molecular analyses, including PFGE, demonstrated that outbreak isolates were genetically identical and distinct from other MRSA strains.
- Identical plasmid and antibiogram patterns were observed in MRSA isolates from other hospitals in the city.
Conclusions:
- The MRSA strain identified in the newborn units is unique and warrants caution when interpreting latex agglutination test results.
- The findings highlight the importance of comprehensive molecular characterization for identifying and tracking unique microbial strains in healthcare settings.