ANTIBIOTIC SUSCEPTIBILITY OF ALPHAHEMOLYTIC AND NONHEMOLYTIC STREPTOCOCCI FROM PATIENTS AND HEALTHY-ADULTS TO 24 ANTIMICROBIAL DRUGS

A total of 278 alpha- and nonhemolytic streptococcal isolates (patient s, n = 116; healthy adults, n = 162) were examined for susceptibility to 23 and 24 antimicrobial drugs with the Bauer-Kirby agar disk diffus ion and the agar dilution method, respectively. Wilkins-Chalgren mediu m compared favorably with sheep blood Mueller-Hinton agar, the referen ce medium, for 58 representative streptococcal isolates. In terms of m inimal inhibitory concentrations (MICs), all 278 isolates were suscept ible to teicoplanin and vancomycin. None of the isolates revealed high -level gentamicin resistance. All isolates were resistant to fusidic a cid. Twelve Streptococcus mitis isolates, all from patients, were resi stant to penicillin G and variably to other antibiotics. Oxacillin dis ks failed to predict penicillin G susceptibility. In general, patient isolates were more frequently resistant to beta-lactam antibiotics and fluoroquinolones; conversely, isolates from healthy carriers were sli ghtly more resistant to macrolide antibiotics. Specifically, susceptib ility rates were: penicillin G 79.1%, oxacillin 87.8%, ampicillin 66.9 %, piperacillin 98.2%, cefoxitin 76.6%, cefuroxime 96.8%, cefotaxime 9 8.6%, ceftriaxone 98.6%, cefepime 98.6%, imipenem 98.2%, ciprofloxacin 59.7%, ofloxacin 89.2%, doxycycline 65.8%, tetracycline 56.8%, clinda mycin 87.8%, erythromycin 59%, clarithromycin 74.9%, chloramphenicol 9 8.9%, cotrimoxazole 97.9%, rifampin 97.5%, and fosfomycin 2.2%. On the basis of numerous minor discrepancies between disk diffusion and agar dilution test results for certain antibiotics, it is proposed that th e current NCCLS inhibition zone (diameter, mm) criteria indicative of intermediate susceptibility of alpha- and nonhemolytic streptococci be changed for the following antimicrobial drugs: ampicillin = I = 22-27 mm; ciprofloxacin = I = 16-18 mm; clindamycin = I = 15-18 mm; doxycyc line = I = 17-19 mm; tetracycline = I = 17-19 mm; erythromycin = I = 1 4-19 mm, and cotrimoxazole = I = 11-13 mm. It is recommended to exclud e both cefoxitin and doxycycline (substitute = tetracycline) disks fro m test batteries for non-group A, B beta-hemolytic and alpha-/nonhemol ytic streptococcal isolates.