COMPARATIVE SUSCEPTIBILITY OF CLINICAL GROUP-A, GROUP-B, GROUP-C, GROUP-F, AND GROUP-G BETA-HEMOLYTIC STREPTOCOCCAL ISOLATES TO 24 ANTIMICROBIAL DRUGS

A total of 312 clinical beta-hemolytic streptococcal isolates (Strepto coccus pyogenes, group A = 63; Streptococcus agalactiae, group B = 145 ; group C = 50; group F = 27; group G = 27) were examined for suscepti bility to 23 and 24 antimicrobial drugs with the Bauer-Kirby agar disk diffusion and the agar dilution method, respectively. Sheep blood Mue ller-Hinton agar served as the reference medium. Wilkins-Chalgren agar supported optimal growth of group A and B, but not of all group C, F, and G streptococci. The group A streptococci were susceptible to all beta-lactam antibiotics, clindamycin, chloramphenicol, rifampin, teico planin, and vancomycin, but resistant to cotrimoxazole, fusidic acid, and, except for 2 strains, to fosfomycin. Resistance (R)/intermediate susceptibility (I) rates (R/I%) to ciprofloxacin (0/2%), ofloxacin (1/ 2%), erythromycin (1.6/0%), and clarithromycin (0/1%) were low. Higher resistance rates were noted with tetracyclines (doxycycline 23.8/15.9 %; tetracycline 39.7/3.2%). Among the group B streptococcal isolates, one strain was resistant against oxacillin and of intermediate suscept ibility to penicillin G and cefoxitin. All isolates were susceptible t o teicoplanin and rifampin. Conversely, all group B isolates were resi stant to cotrimoxazole and fusidic acid; 69% and 51% of these isolates were susceptible to fosfomycin and rifampin, respectively. R/I rates of the group B streptococcal isolates were low for ciprofloxacin and o floxacin (0/0.7%), clindamycin (0.7/0%), erythromycin (1.4/3.5%), clar ithromycin (1.4/0%), and chloramphenicol (0.7/0%). Resistance to tetra cyclines was significant (doxycycline: 72.4/2.1%; tetracycline: 74.5/1 .4%). Among the non-A, non-B beta-hemolytic streptococci, 2 group C st rains were resistant to oxacillin and showed intermediate susceptibili ty to penicillin G. All isolates were susceptible to third and fourth- generation cephalosporins, imipenem, chloramphenicol, rifampin, teicop lanin, and vancomycin. R/I rates to the other antimicrobial drugs were : ciprofloxacin (3.9/1.9%), ofloxacin (2.9/1.9%), clindamycin (2.9/1%) , erythromycin (5.8/0%), clarithromycin (3.8/2.9%), and cotrimoxazole (16.4/3.9%). Resistance against tetracyclines was more frequent (doxyc ycline: 18.3/2.9%; tetracycline: 20.2/6.7%). On the basis of various m inor discrepancies between MIC and disk diffusion test results, it is proposed that the current NCCLS inhibition zone (diameter, mm) criteri a indicative of intermediate susceptibility of beta-hemolytic streptoc occi be changed for the following antimicrobial drugs: ampicillin: 22- 27 mm (only for group A and B beta-hemolytic streptococci); ciprofloxa cin: 16-18 mm; clindamycin: 15-18 mm; doxycycline: 17-19 mm; tetracycl ine: 17-19 mm, and erythromycin: 14-19 mm.