EVALUATION OF COAGULASE-NEGATIVE STAPHYLOCOCCAL ISOLATES FROM SERIAL NASOPHARYNGEAL CULTURES OF PREMATURE-INFANTS

The present study was undertaken to determine whether very low birth weight infants in a neonatal intensive care unit became colonized with virulent strains of coagulase-negative staphylococci (C-S) over time (i.e., those characterized as Staphylococcus epidermidis, slime positive, and/or multiply antibiotic resistant), and if so, whether the initial colonizing strains developed these characteristics or whether the strains themselves changed. Nasopharyngeal (NP) cultures were obtained weekly on 28 very low birth weight (<1750 g) infants hospitalized for a mean of 8 wk (range 4-15 wk). There were 105 isolates of C-S recovered from 96 cultures that were characterized by species, biotype, antibiotic susceptibility pattern, and slime production (screening parameters). Isolates from the same infant with highly similar screening parameters then underwent phage typing and plasmid analysis to increase the likelihood of establishing strain identity. C-S colonization rose from 12% on admission to 75% by wk 2, then gradually declined to 30% by wk 6 and remained stable through wk 10. There were no significant differences among C-S isolates from wk 1 compared with wk 10 of hospital stay with respect to distribution of species, slime positive, or multiply antibiotic resistant strains. One biotype of S. epidermidis was recovered from 46% of study infants, but only one infant was colonized with a predominant biotype of S. epidermidis throughout hospitalization. Thirteen pairs of isolates recovered from 12 of the infants on two or more wk were found to be identical by phage typing and plasmid analysis. Only seven of these 13 pairs of isolates had concordant results for all the screening parameters. There were six instances of minor changes in the biotype of strains, two instances of strains becoming resistant to an additional antibiotic, and no instances of identical strains changing slime production from 1 wk to another. In summary, although one biotype of S. epidermidis was recovered from 46% of study infants, there was marked variability among C-S isolates from individual infants from wk to wk. There was no trend towards individual strains acquiring virulence characteristics nor did these characteristics seem to convey a selective advantage to C-S bacteria colonizing infants in this study. Rather, many different strains of multiply antibiotic resistant C-S were prevalent in the neonatal intensive care unit (NICU) with approximately 50% producing slime throughout the study period. © 1990.