Antibacterial activity in vitro and regression studies for ceftazidime and ceftriaxone

The antibacterial activity in vitro of ceftazidime and ceftriaxone was investigated against 575 recent clinical isolates. Both cephalosporins displayed excellent activity against most of the pathogens tested, in particular Gram-negative bacteria, including Pseudomonas aeruginosa. Apart from the Pseudomonas group, Acinetobacter calcoaceticus and Campylobacter jejuni ceftriaxone was slightly to moderately more active than ceftazidime overall. Ceftriaxone was moderately active against Streptococcus faecalis. Regression lines for the two antibiotics were almost identical. Corresponding to differences in susceptibility, the zone sizes differed for the two drugs with respect to certain bacterial groups, e.g. Pseudomonas sp. and enterococci. Therefore, the two cephalosporins cannot substitute for each other in disc susceptibility testing. Breakpoints for disc tests around 8-16 micrograms/ml, as suggested in the literature, appear too high considering the beneficial pharmacokinetic properties of the two drugs.