Abstract
To determine the susceptibility of group A beta-hemolytic streptococci (GABHS) in the lower St Lawrence region, Quebec to different antibiotics, particularly macrolides, and to compare different antibiogram methods (disk diffusion, E-test and microdilution) and incubation atmospheres (ambient air and 5% carbon dioxide). A total of 384 strains of GABHS isolated from 377 patients (throat 335; other sites 49) from three hospitals in the lower St Lawrence region were analyzed for their susceptibility to erythromycin, clarithromycin, azithromycin, penicillin, clindamycin, cephalothin, rifampin and vancomycin by disk diffusion on Mueller-Hinton (MH) agar supplemented with 5% defibrinated sheep blood (MHB) at 35ºC in 5% carbon dioxide. Strains that were found to be intermediately resistant or resistant to one of the antibiotics by disc diffusion, strains from sites other than throat, and a sample of 97 pharyngeal strains were evaluated by E-test on MHB (35ºC, 5% carbon dioxide) for their susceptibility to the antibiotics erythromycin, clarithromycin, azithromycin, penicillin, clindamycin and ceftriaxone. In addition, minimum inhibitory concentrations (MICs) were determined for erythromycin and azithromycin by broth microdilution using MH broth supplemented with 2.5 % of lysed horse blood (35ºC, ambient air) on strains that were resistant or intermediately resistant to the macrolides (erythromycin, clarithromycin, azithromycin). An evaluation was also carried out on these strains to determine the influence of the incubating atmosphere (ambient air versus 5% carbon dioxide) on susceptibility results obtained by disk diffusion (erythromycin, clarithromycin and azithromycin) and E-test (erythromycin and azithromycin) methods. Nine strains (2%) from nine patients (throat eight, pus one) were resistant to all macrolides as tested by three different techniques (disk diffusion, E-test and microdilution). All strains were susceptible to all the other antibiotics tested. For the strains intermediately resistant or resistant to macrolides, incubation in a 5% carbon dioxide atmosphere was associated with a reduction in the diameter of inhibition determined by disk diffusion (P<0.001) with frequent minor variations in interpretation, and with an increase in the MIC by E-test (P<0.001), which had no impact on interpretation. Resistance of GABHS to macrolides was not common (2%) in the lower St Lawrence Region. GABHS susceptibility to erythromycin seemed to predict the susceptibility to the other macrolides. Significant variation in antibiogram results (disk diffusion and E-test) of GABHS susceptibility to macrolides was related to the incubation atmosphere and may have an impact on the interpretation of disk diffusion results.
Highlights
Antibiotic susceptibility testing – Disk diffusion: All strains were tested by disk diffusion on Mueller-Hinton (MH) agar (Quelab, Montreal, Quebec) supplemented with 5% sheep blood (MHB) in accordance with the National Committee for Clinical Laboratory Standards (NCCLS) (Wayne, Pennsylvania) (5% carbon dioxide, 20 to 24 h) [22]
This study shows that group A beta-hemolytic streptococci (GABHS) resistance to erythromycin, azithromycin or clarithromycin is uncommon in the lower St Lawrence region (LSLR)
It is difficult to extrapolate the prevalence of resistance to other areas of Quebec or Canada, but these results are in agreement with other published North American studies reporting resistance rates of 5% or less
Summary
GABHS strains: GABHS isolates were collected between March 1995 and February 1996 from three hospitals in the LSLR: the Centre Hospitalier Régional de Rimouski, the Centre Hospitalier d’Amqui and the Centre Hospitalier du Grand Portage. Reidentification of all the strains was carried out based on the following criteria: presence of betahemolysis on blood sheep agar, presence of Gram-positive cocci on Gram stain, negative catalase, susceptibility to 0.04 units of bacitracin (DIFCO Laboratories, Detroit, Michigan), resistance to trimethoprim/sulphamethoxazole 1.25 m g/23.75 m g (BBL Sensi-Disk, Becton Dickinson Microbiology Systems, Cockeysville, Maryland), and positive reaction to hydrolysis of L-pyrrolidonyl-beta-naphthylamine (PYR test) (Sigma Chemical Company, St Louis, Missouri) detected by adding N,N-dimethylaminocinnamaldehyde (BioMerieux Vitek Inc, Hazelwood, Missouri). Antibiotic susceptibility testing – Disk diffusion: All strains were tested by disk diffusion on Mueller-Hinton (MH) agar (Quelab, Montreal, Quebec) supplemented with 5% sheep blood (MHB) in accordance with the National Committee for Clinical Laboratory Standards (NCCLS) (Wayne, Pennsylvania) (5% carbon dioxide, 20 to 24 h) [22]. MIC by microdilution: MIC values were determined for erythromycin (Sigma Chemical Company) and azithromycin
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