Combination of aztreonam and cefotaxime against CTX-M-15 type β-lactamases: A mechanism based effective therapeutic approach.

CTX-M-15 type β-lactamases are a class of enzymes which hydrolyzes cefotaxime and aztreonam (a monobactam) antibiotics. The emergence of CTX-M-15 producing Enterobacteriaceae member is a major threat to public health. The objective of the study was to check the potency of aztreonam and cefotaxime in combination against β-lactamase producing strains and to monitor the mechanism behind their interaction. FICI results showed the synergistic effect of aztreonam-cefotaxime against CTX-M-15 producing strain. The expressed and purified CTX-M-15 protein was used as the source of enzyme. Kinetic studies confirmed that the catalytic efficiency of the CTX-M-15 enzyme was decreased to about 78% when it was treated with aztreonam then with cefotaxime in 5 and 10 times molar ratio, respectively, in comparison to the studies where efficiency was enhanced by 33% when cefotaxime was taken alone. Fluorescence study showed that aztreonam binding with CTX-M-15 was an endothermic and spontaneous process with Ka of the order of 104 M-1. CD spectroscopic study showed conformational changes upon aztreonam/aztreonam-cefotaxime binding with CTX-M-15. The study concludes that aztreonam in combination with cefotaxime synergistically inhibits CTX-M-15 efficiency significantly. Hence the combination of a monobactam and cephalosporin can be used as the potential therapeutic candidates against β-lactamase producing CTX-M-15 strains.