[The antibiotic agent azthreonam: studies on plasmid-dependent resistance formation]

Two mechanisms can be assumed that are responsible for the stability of azthreonam against the attack of beta-lactamases: first, azthreonam lacks binding to the enzyme protein and second, rate of hydrolysis is extremely low, resulting even in inhibition of enzyme activity. Enzyme kinetics were studied from highly purified enzymes: azthreonam did not bind to the TEM-1 enzyme, whereas the compound revealed a time-dependent inhibition of the chromosomally mediated Enterobacter cloacae enzyme. With various cephalosporins as substrates marked discrepancies in the mode of inhibition were observed: cefacedone revealed merely noncompetitive inhibition, cefazolin and cefoperazone demonstrated mixed-inhibition kinetics, and with cephalothin as the substrate enzyme kinetics strongly suggested binding of a second azthreonam molecule. With cefacedone as the substrate KI was ascertained to be 0.8 X 10(-7) mol/l. Conjugative transfer of the TEM-mediating plasmids R6K and RP1 resulted in 20 to 50-fold increase of the MIC's for the Enterobacter cloacae and Citrobacter freundii recipients, in spite of the fact that azthreonam does not serve as a substrate for the TEM-1 enzyme. These findings have likely to be attributed to an impaired penetration of the compound through the "outer membrane" of the recipient strains.