[Sulbactam and clavulanic acid: studies of enzyme kinetics and synergism with ampicillin and mezlocillin].

Both inhibitors clavulanic acid and sulbactam exhibit high affinity (Ki-values less than 10(-6) mol/l) to the beta-lactamases of gram-negative bacteria with predominant penicillinase activity and lowered affinity to enzymes with cephalosporinase activity (above all clavulanic acid). As compared to sulbactam there was a stronger synergism of clavulanic acid with ampicillin or mezlocillin in those isolates producing either a plasmid-mediated or a chromosomally-mediated penicillinase. In beta-lactamase-overproducing Klebsiella ssp. variants both inhibitors could protect neither ampicillin nor mezlocillin from breakdown-independent of the synergy observed in the corresponding wild-type strains. Moreover, there was a marked synergism between mezlocillin (but not ampicillin) and sulbactam in beta-lactamase-overproducing Enterobacter cloacae variants. There was no strong relation between the amount of enzyme produced and the resulting synergy between inhibitor and antibiotic among clinical isolates. These observations agree with the results obtained for routinely performed susceptibility testing of amoxycillin/clavulanic acid and ampicillin/sulbactam.