Susceptibility of group A beta-hemolytic streptococci in the lower St Lawrence region, Quebec

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.