[Rapid determination of the synergy of clavulanic acid and beta lactams by measuring the intracellular ATP by bioluminescence].

Twenty clinical isolates of ampicillin- and carbenicillin-resistant or susceptible (two strains) Gram-negative rods, producing at least one beta-lactamase, were examined for susceptibility to a combination of ampicillin or carbenicillin with clavulanic acid (enzymatic inhibitor). Synergy was evaluated by the reduction of the beta-lactam agar dilution MIC and by the measurement of intracellular AYP using firefly bioluminescence. The potentiation effect of clavulanic acid was variable, depending on resistance levels, species and types of beta-lactamase (TEM, SHV-1, CARB, OXA, MAL and Cpase). The synergy was significant, with 10 mg/l of inhibitor for all the strains producing TEM-1, TEM-2 and Carb-2 except for one strain of Serratia marcescens (TEM-2). The synergy was weak for Levinea strains (Citrobacter diversus biotype b), biosynthesizing specific penicillinases as MAL-1. No potentiation effect was observed for strains producing a cephalosporinase, such as Escherichia coli and Pseudomonas aeruginosa. This effect appeared to be variable for strains producing the oxacillin-hydrolysing enzyme (OXA-1), such as E. coli and P. aeruginosa. A positive correlation was clearly demonstrated between the MIC values and the intracellular ATP concentration in bacteria within 2 h. The opportunity of using the firefly assay for the rapid determination of synergy between beta-lactam antibiotics and clavulanic acid is discussed.