Problems with the laboratory identification and evaluation of coagulase-negative staphylococci

During a 105 day period in mid 1984, 796 isolates of coagulase-negative staphylococci were recovered from routine specimens handled in the Microbiology Laboratory, Dunedin Hospital. Of these isolates 504 (63.3%) were from wounds, 170 (21.4%) from urines, 58 (7.3%) from intravascular catheter tips, 44 (5.5%) from blood cultures and 20 (2.5%) from sputa. Presumptive identification of 315 consecutive isolates revealed 175 (55.6% of total) strains of Staphylococcus epidermidis, 44 (14.0%) S. capitis, 36 (11.4%) S. haemolyticus/hominis, 29 (9.2%) S. warneri, 19 (6.0%) S. simulans and 12 (3.8%) members of the S. saprophyticus group. Using laboratory generated criteria, 44.8%, 25.9% and 4.5% of coagulase-negative isolates from catheter tips, urines and blood cultures respectively, were deemed pathologically potentially significant. Although more common, S. epidermidis did not appear to be significantly more virulent than other members of the epidermidis species group or S. simulans; 67% of the S. saprophyticus group isolates from urine were considered pathologically significant. Antibiograms on the 796 coagulase-negative isolates revealed 63.2% resistant to penicillin, 22.6% to methicillin, 34.8% to cephradine, 27.5% to gentamicin, and 14.4% to erythromycin; multiple resistance was common. Methicillin resistance was a feature of S. saprophyticus group strains. With isolates from catheter tips and blood cultures, a significantly higher percentage of those regarded as significant were gentamicin resistant. Apart from penicillin, antibiotic resistance was not so marked in strains of coagulase-positive staphylococci recovered over the same period.