IN-VITRO SELECTION OF RESISTANCE TO 4 BETA-LACTAMS AND AZITHROMYCIN IN STREPTOCOCCUS-PNEUMONIAE

Selection of resistance to amoxicillin (with or without clavulanate), cefaclor, cefuroxime, and azithromycin among six penicillin G- and azi thromycin-susceptible pneumococcal strains and among four strains with intermediate penicillin sensitivities (azithromycin MICs, 0.125 to 4 mu g/ml) aas studied by performing 50 sequential subcultures in medium with sub-MICs of these antimicrobial agents. For only one of the six penicillin-susceptible strains did subculturing in medium with amoxici llin (with or without clavulanate) lead to an increased MIG, with the MIC rising from 0.008 to 0.125 mu g/ml. Five of the six penicillin-sus ceptible strains showed increased azithromycin MICs (0.5 to >256.0 mu g/ml) after 17 to 45 subcultures. Subculturing in medium with cefaclor did not affect the cefaclor MCs of three strains but and led to incre ased cefaclor MICs (from 0.5 to 2.0 to 4.0 mu g/ml) for three of the s ix strains,,vith MICs of other beta-lactams rising 1 to 3 twofold dilu tions. Subculturing in cefuroxime led to increased cefuroxime MICs (fr om 0.03 to 0.06 mu g/ml to 0.125 to 0.5 mu g/ml) for all six strains w ithout significantly altering the MICs of other beta-lactams, except f or one strain, which developed an increased cefaclor MIC. Subculturing in azithromycin did not affect beta-lactam MICs, Subculturing of the four strains with decreased penicillin susceptibility in amoxicillin ( with or without clavulanate) or cefuroxime did not select for p-lactam resistance. Subculturing of one strain in cefaclor led to an increase in MIC from 0.5 to 2.0 mu g/ml after 19 passages. In contrast to stra ins that were initially azithromycin susceptible, which required >10 s ubcultures for resistance selection, three of four strains with azithr omycin MICs of 0.125 to 4.0 mu g/ml showed increased MICs after 7 to 1 3 passages, with the MICs increasing to 16 to 32 mu g/ml. All azithrom ycin-resistant strains were clarithromycin resistant. With the excepti on of strains that contained mefE at the onset, no strains that develo ped resistance to azithromycin contained ermB or mefE, genes that have been found in macrolide-resistant pneumococci obtained from clinic pa tients.