IN-VITRO SELECTION OF ONE-STEP MUTANTS OF STREPTOCOCCUS-PNEUMONIAE RESISTANT TO DIFFERENT ORAL BETA-LACTAM ANTIBIOTICS IS ASSOCIATED WITH ALTERATIONS OF PBP2X

Many oral penicillins and cephalosporins are used to treat clinical in fections caused by Streptococcus pneumoniae, Therefore, using differen t beta-lactams as selectors, we estimated the frequencies of one-step mutations leading to resistance, Resistant mutants were obtained from penicillin-susceptible, intermediately resistant, and penicillin resis tant strains, For cefixime, cefuroxime, cefpodoxime, cefotaxime, and c eftriaxone, the frequencies of mutation ranged from 10(-6) to 10(-8) w hen resistant mutants were selected at 2- to 8-fold the MIG, and the M ICs increased 2- to 16-fold. For ampicillin, ampicillin-sulbactam, amo xicillin, amoxicillin-clavulanic acid, cefaclor, and loracarbef, the f requencies of mutation were about 10(-7) to 10(-8), and the MICs incre ased twofold at most, One to three resistance profiles of the resultin g mutants were selected for each of the selecting antibiotics. Among t hose, some showed resistance to the cephalosporins associated with a 2 - to 32-fold increase in susceptibility to the penicillins. Competitio n experiments showed a decreased affinity of PBP2x for cefpodoxime in all mutants, Ln some mutants that were more susceptible to amoxicillin , a decreased affinity of PBP2x for cefpodoxime was associated with an increased affinity for amoxicillin and a particular substitution of a lanine for threonine at position 550 just after the KSG triad, From th ese results we infer (i) that among the beta-lactams tested the penici llins, cefaclor, and loracarbef selected one-step resistant mutants le ss frequently and that they achieved a lower level of resistance, and (ii) that mutants with different profiles may have acquired different point mutations in PBP2x.