In vitro activity of six beta-lactam antibiotics against non-beta-lactamase producing and producing enterobacteria and antibiotic resistance transfer.

beta-Lactamase production was evaluated by chromogenic cephalosporin 87/312 in 184 enterobacteria isolated from clinical sources. Minimal inhibitory concentrations (MICs) of six beta-lactam antibiotics (Ampicillin, Cephaloridine, Cephalexine, Cefazoline, Cefuroxime, Cefotaxime) were determined on 90 non beta-lactamase producing and 94 beta-lactamase producing strains by a miniaturized dilution method. beta-lactamase production transfer and antibiotic resistance transfer were observed in a receiving E. coli K12 C600 NA- after conjugation with 94 producing strains. Cefotaxime showed high antibacterial activity both against beta-lactamase producing and non producing bacteria. Cefuroxime, Cephalexine and Cefazoline were active only against high percentage of non beta-lactamase producing bacteria. Ampicillin and Cephaloridine showed low antibacterial activity both against producing and non producing bacteria. About 19% (18/94) of beta-lactamase producing bacteria transferred beta-lactamase producing capacity to E. coli K12 by conjugation and a significant increase of MICs of each antibiotic, except that of Cefotaxime, was observed in E. coli K12 that acquired capacity to produce beta-lactamase by conjugation. Resistant strains that produce beta-lactamase transfer antibiotic resistance by conjugation to E. coli in variable percentages to the antibiotics under examination. Antibiotic resistance to Ampicillin, Cephaloridine, Cephalexine and Cefazolin observed in receiving strains after conjugation is not only due to beta-lactamase transfer. All strains acquiring Cefuroxime and Cefotaxime resistance never acquire beta-lactamase producing capacity.