[A new method for evaluation of beta-lactamase production of resistant enterobacteriaceae strains (author's transl)].

Resistant strains of Enterobacteriaceae were investigated with a chemical and microbiological test on beta-lactamase activity. The chemical method was needed as screening-test basing on the enzymatic hydrolysis of the chromogenic cephalosporin compound 87/312. The microbiological test is a modified cup plate method using Staph. aureus SG 511 as sensitive test strain for penicillins and cephalosporins. In one of the two cups of the blood agar plate 20 microliter of the beta-lactam antibiotic, in the other cup 20 microliter of the supernatant of the 24 h broth of the bacterial strain was pipettet. If the lactamase in the supernatant neutralises the antibiotic, a halfemoonlike blank is forming on the left site of the inhibition zone. An unchangeable inhibition zone demonstrates stability of the antibiotic to the enzyme. Each of 10 ampicillin- and/or cephalothin-resistant strains of E. coli, Klebsiella, Enterobacter and Serratia were investigated against 6 penicillins (penicillin G, ampicillin, ticarcillin, mezlocillin, azlocillin, bay k 4999) and 6 cephalosporins (cefoxitin, cefuroxime, cefamandol, cephalothin, cefazolin, cefradine). The microbiological method allows the statement, against which beta-lactam antibiotic the enzyme is effective. A correlation between the beta-lactamase activity and the MIC values in Enterobacteriaceae was not found.