Antibacterial activity of cefotaxime, desacetylcefotaxime, and the combination of the two.

Cefotaxime (CTX) is metabolized to desacetylcefotaxime (dCTX), which is antibacterially active. The combination of CTX and dCTX is synergistic in a 1:1 ratio, but in this study, a combination of CTX:dCTX in a fixed ratio of 5:1 was not found synergistic when tested on 120 recently isolated clinical strains. Zone diameters of disks containing pure CTX or dCTX 30 micrograms or the combination of CTX/dCTX in micrograms: 25/5, 20/10, 15/15, and 10/20, were compared. The correlation between the zone size of the pure CTX disk and the 25/5, 20/10, and 15/15 disks were high with correlation coefficients (r) 0.98 to 0.96. Regardless of the metabolism and the potential synergy, no differences were found in the zone sizes comparing the pure CTX disk with the combination with a ratio from 5:1 to 1:1. The clinical microbiological laboratory can go on using the pure 30 micrograms CTX disk for routine susceptibility testing.