Agar dilution method for detection of inducible clindamycin resistance in Staphylococcus spp.

We describe the development and validation of an agar dilution method for the detection of inducible clindamycin resistance by using 227 previously characterized erythromycin-resistant, clindamycin-susceptible Staphylococcus sp. isolates. Mueller-Hinton agar with defibrinated horse blood containing a range of erythromycin concentrations (1 to 8 mg/liter) combined with clindamycin at 0.5 mg/liter was used to determine the optimal concentration that produced growth of inducible isolates while inhibiting that of isolates without the inducible phenotype. A concentration of clindamycin of 0.5 mg/liter with erythromycin at 1 mg/liter was the optimal combination for detection of inducible resistance and resulted in a sensitivity of 100% (95% confidence interval [CI], 97.9 to 100) and a specificity of 100% (95% CI, 93.0 to 100). Attention must be paid to ensuring that a sufficient inoculum has been used, since an inoculum below the standard 10(7) bacteria/ml may result in false-negative results. This method has been incorporated into routine use in our laboratory.

In vitro antibacterial activity of beta-lactams and non-beta-lactams against Streptococcus pneumoniae isolates from Sydney, Australia.

AIMS: This study was undertaken to determine the antimicrobial resistance patterns of strains of Streptococcus pneumoniae from Sydney, Australia, comparing penicillin-susceptible, -intermediate and -resistant isolates. METHODS: Non-duplicate cultures of S. pneumoniae were collected from 1 January to 31 December 2002 in the three penicillin-susceptibility categories. Minimum inhibitory concentrations (MICs) of 19 antibacterial agents were determined by agar dilution based on the National Committee for Clinical Laboratory Standards (NCCLS) methodology. Overall for 2002, 687 non-duplicate isolates were obtained, of which 190 (28%) were intermediate or resistant to penicillin. From this set, 183 isolates were selected for study: 88 (48%) in the penicillin-susceptible group (MIC or= 2.0 mg/L). RESULTS: Resistance to non-beta-lactams was more common in penicillin-intermediate or -resistant strains. Multidrug resistance (resistance to >or= 2 non-beta-lactams) was found in 3% of penicillin-susceptible, 52% of penicillin-intermediate and 87% of penicillin-resistant isolates. Erythromycin resistance was seen in 22% of the penicillin-susceptible strains but increased significantly to 60% and 89% in the penicillin-intermediate and resistant strains, respectively. Clindamycin, tetracycline and trimethoprim/sulfamethoxazole showed similar diminished activity in penicillin-intermediate and -resistant strains; 64, 84 and 91% of the penicillin-resistant isolates were resistant to clindamycin, tetracycline and to trimethoprim/sulfamethoxazole, respectively. Chloramphenicol resistance was comparatively low level except 19% of the penicillin-resistant strains were resistant. Ciprofloxacin MICs for 14 strains were raised (MICs 4-16 mg/L); three of these were penicillin-susceptible, one penicillin-intermediate and 10 penicillin-resistant. Only one isolate was resistant to moxifloxacin and to gatifloxacin. Resistance to rifampicin, vancomycin, oritavancin, or linezolid was not detected. Twenty-three isolates were intermediate and one resistant to quinupristin/dalfopristin - 22 of these were penicillin resistant. CONCLUSIONS: Streptococcus pneumoniae isolates from Sydney are commonly resistant to beta-lactams and available non-beta-lactam agents, especially if they are penicillin non-susceptible. Resistance to moxifloxacin and gatifloxacin is still rare, but some isolates were non-susceptible to quinupristin/dalfopristin. It is important to continue to survey resistance patterns to recognise emerging resistances which affect the selection of empirical antimicrobials to treat infections with S. pneumoniae.

Cefoxitin resistance as a surrogate marker for the detection of methicillin-resistant Staphylococcus aureus.

OBJECTIVES: To evaluate the usefulness of cefoxitin when used as a surrogate marker for the detection of methicillin resistance. PATIENTS AND METHODS: Eight hundred and seventy-one strains of Staphylococcus aureus, collected from eight tertiary referral centres serving diverse socio-economic populations, were included in the study using NCCLS disc diffusion and the agar dilution methods. RESULTS: Using cefoxitin and NCCLS criteria for disc diffusion, the sensitivity and specificity for recognizing methicillin resistance were both 100%. Similar results were obtained when the strains were tested by the agar dilution method. The cefoxitin MICs for methicillin-susceptible strains were < or = 4 mg/L. CONCLUSIONS: Testing with cefoxitin as a surrogate marker for the detection of methicillin resistance was very accurate with both disc diffusion and agar dilution methods. Such testing clearly distinguished methicillin-resistant strains of S. aureus from methicillin-susceptible strains.