Mechanism of synergistic action of a combination of ampicillin and dicloxacillin against a beta-lactamase-producing strain of Citrobacter freundii

Antimicrobial combinations in the therapy of infections due to gram-negative bacilli

Antimicrobial combinations are used most frequently to provide broad-spectrum coverage; however, they are also frequently employed to enhance antimicrobial activity (synergism). Although there is extensive in vitro documentation of synergism for many antibiotic combinations, a clear advantage for these combinations has been difficult to demonstrate in clinical studies. Several types of combinations have been useful in clinical medicine and frequently result in synergism. These include combinations of a cell wall-active agent with an aminoglycosidic aminocyclitol, combinations of a beta-lactamase inhibitor with a beta-lactam, and combinations of agents that inhibit sequential steps in a metabolic pathway. Given its spectrum of activity, aztreonam will often be used with clindamycin or a beta-lactam antibiotic. Combinations of beta-lactams may be synergistic via several mechanisms. However, these combinations also exhibit significant potential for antagonism when used against gram-negative bacilli and, therefore, require careful evaluation prior to clinical use.

Development of resistance to cephalosporins in clinical strains of Citrobacter spp

The predominant beta-lactam antibiogram of Citrobacter freundii resembles that of Enterobacter cloacae in demonstrating resistance to cephalothin and cefoxitin with susceptibility to the newer cephalosporins. Four representative strains of C. freundii were reversibly induced to high-level beta-lactamase production by cefoxitin, and mutants with stable, high-level production were selected with cefamandole. The mutants were resistant to several second- and third-generation cephalosporins. Comparisons of isoelectric points and substrate profiles of beta-lactamases from wild-type, induced wild-type, and mutant organisms suggested a close relationship to those from E. cloacae and indicated that C. freundii mutants, like those of E. cloacae, were derepressed for production of beta-lactamase. One primary isolate of C. freundii resembled the mutants in all characteristics. In contrast, most strains of Citrobacter diversus were susceptible to all cephalosporins, and two representative strains showed neither inducible nor mutational resistance. Cefoxitin induction to enhanced beta-lactamase production was demonstrated in a cephalothin-resistant isolate, and a derepressed mutant was selected with cefotaxime. The beta-lactamase from this C. diversus strain differed substantially in substrate profile from that of E. cloacae and C. freundii.

beta-Lactam resistance in Serratia marcescens: comparison of action of benzylpenicillin, Apalcillin, Cefazolin, and ceftizoxime

The intrinsic mechanisms of resistance to beta-lactam antibiotics in Serratia marcescens IFO 12648 were investigated, comparing the action of benzylpenicillin, apalcillin, cefazolin, and ceftizoxime. The minimal inhibitory concentrations for this strain were 1,600, 3.13, 6,400, and 0.05 microgram/ml, respectively. The addition of ethylenediaminetetraacetic acid markedly reduced the minimal inhibitory concentrations of benzylpenicillin and cefazolin, whereas those of apalcillin and ceftizoxime were not influenced. S. marcescens IFO 12648 produced only a low level of beta-lactamase activity constitutively, and the production was considerably increased by the addition of benzylpenicillin. Cefazolin was hydrolyzed rapidly by beta-lactamase activity, whereas benzylpenicillin, apalcillin, and ceftizoxime were poorly hydrolyzed. Peptidoglycan synthesis in ether-treated strain IFO 12646 cells was inhibited by a concentration of ceftizoxime markedly lower than that of cefazolin and by a concentration of apalcillin moderately lower than that of benzylpenicillin.