Evaluation of different methods for the detection of methicillin resistance in coagulase-negative staphylococci.

The efficacy of 19 agar diffusion methods for the detection of methicillin resistance among coagulase-negative staphylococci (CoNS) within 24 h was evaluated. A total of 359 CoNS isolates were tested, of which 204 were Staphylococcus epidermidis. In 164 isolates, the presence of mecA was investigated; 61 strains were mecA-positive and 103 were mecA-negative by Southern blot analysis. Based on the best agreement shown with the mecA determination (94%) among four agar dilution assays for determining methicillin MIC, an assay with Columbia agar supplemented with NaCl and incubation with a heavy bacterial inoculum of 10(5)-10(6) cfu/spot was used as the reference MIC method. The best agar diffusion results were obtained with a 1 microg oxacillin disc on Columbia agar with 4.5% NaCl supplement. With this method, 99% of S. epidermidis and 94% of non-S. epidermidis were in agreement with the MIC determination. However, Columbia (without NaCl), Mueller-Hinton and Isosensitest agars were almost as useful when a 1 microg oxacillin disc was used. The zone breakpoints for S. epidermidis were, in general, considerably larger than those for other CoNS species and, consequently, differentiation according to species is recommended. Furthermore, resistance to other antibiotics, such as gentamicin and erythromycin, makes methicillin resistance highly likely.

Aminoglycoside resistance among Danish blood culture isolates of coagulase-negative staphylococci.

A sample of 137 coagulase-negative staphylococci isolated from blood cultures in Denmark over a 4-month period during 1992-1993 were tested for aminoglycoside resistance and for the presence of aminoglycoside-modifying enzymes. This was done on the basis of minimum inhibitory concentrations (MICs) measured by agar dilution, inhibition zone diameter by disk diffusion, and DNA dot blot analysis. Using the National Committee for Clinical Laboratory Standards (NCCLS) MIC breakpoints, 5%, 46%, 57%, and 63% of the strains were resistant to netilmicin, amikacin, gentamicin and tobramycin, respectively. The large majority of resistant staphylococci strains produced the bifunctional AAC(6)-III+APH(2") enzyme. The presence of AAC(6)-III+APH(2") explains the high level of resistance to gentamicin, kanamycin and tobramycin. In contrast to our results. Staphylococcus haemolyticus strains are usually reported to be more resistant than Staphylococcus epidermidis strains.

Aminoglycoside resistance mechanisms in Enterobacteriaceae and Pseudomonas spp. from two Danish hospitals: correlation with type of aminoglycoside used.

Sixty-two aminoglycoside-resistant Gram-negative enteric bacteria were isolated over a 3-year period from two hospitals (Bispebjerg and Esbjerg) among a total of almost 270,000 isolates. These hospitals were selected because of their different aminoglycoside policies during the years investigated. At Bispebjerg Hospital the principal aminoglycoside used was tobramycin, while gentamicin was the first choice at Esbjerg Hospital. Escherichia coli was the most frequently found aminoglycoside-resistant species. Among the 61 aminoglycoside-resistant strains studied, resistance was due to aminoglycoside-modifying enzymes in all except two Xanthomonas maltophilia strains. The ANT(2") enzyme occurred significantly more often at Esbjerg Hospital (p = 0.001), while enzymes of the AAC(3) or AAC(6') moieties were more common, but not significantly so, at Bispebjerg Hospital. The phenotypic pattern of aminoglycoside resistance, as determined by disc diffusion, correlated 100% with the ANT(2") and AAC(3)-V (the two most common enzymes among the isolates) genotype of the organisms as established using DNA probes. Median minimum inhibitory concentrations (MICs) (mg/l) for clinically utilized aminoglycosides were: amikacin (1.6), gentamicin (25.0), kanamycin (50.0), netilmicin (1.6-25.0) and tobramycin (12.5-50.0). Isolates from Bispebjerg Hospital revealed significantly higher MICs for netilmicin and tobramycin (p < 0.01) as compared to isolates from Esbjerg Hospital.

Coagulase-negative Staphylococci in Danish blood cultures: species distribution and antibiotic susceptibility.

The distribution and antibiotic susceptibility of coagulase-negative staphylococci (CoNS) isolated from blood cultures was examined in samples from hospitals covering most of Denmark. A total of 499 CoNS isolates were detected in 477 blood cultures from 340 patients and speciated as Staphylococcus epidermidis, 285; Staphylococcus hominis, 61; Staphylococcus haemolyticus, 43; Staphylococcus warneri, 12; Staphylococcus cohnii, 7; Staphylococcus saprophyticus, 4; Staphylococcus capitis, 2 and Staphylococcus lugdunensis, 1. Seventy-eight isolates could not be identified to species level and six were Micrococcus spp. In 108 (22.6%) blood culture sets, more than one CoNS strain were found, as detected by species identification, antibiogram and biotyping. Significantly more blood cultures from patients in university hospitals were drawn from central venous catheters. Comparing university and non-university hospitals, the overall antibiotic susceptibility among CoNS was only slightly different, except for methicillin and amikacin. The prevalence of methicillin-resistant strains was 35.1% in the university hospital strains vs. 25.3% in the non-university hospital strains. The overall prevalence of methicillin resistance was 32%. Great geographic variation in both species distribution and antibiotic resistance was observed. The high prevalence of S. epidermidis makes subtyping of this species important.