In vitro activity of ceftazidime in combination with other antibiotics

189 bacterial strains were investigated for their in vitro sensitivity against ceftazidime (alone and in combination with another antibiotic). Moreover, the possibility to prevent development of secondary bacterial resistance as observed in subcultures at subinhibitory antibiotic concentrations, was studied using specific antibiotic combinations. Of 115 staphylococcal strains (91 strains of Staphylococcus aureus, 24 strains of Staphylococcus epidermidis), 2% were sensitive, 82% were moderately sensitive and 16% were resistant to ceftazidime. On combining ceftazidime with vancomycin, synergism was found in 61% of the strains, and secondary resistance to ceftazidime could be prevented with this combination. The combination of ceftazidime and clindamycin showed synergism in 26% and an additive effect in 48% of the strains. Secondary resistance to ceftazidime did not develop with this combination in subcultures at subinhibitory concentrations in which loss of activity was only minimal with clindamycin alone. Rifampicin and fusidic acid were highly active against staphylococci. In combination with ceftazidime, only weak synergism or additive effects were seen in most strains; no antagonism could be observed. In subcultures at subinhibitory concentrations, secondary resistance to rifampicin and fusidic acid developed rapidly and could be partially prevented by adding ceftazidime. Of 60 Pseudomonas aeruginosa strains, 84% were sensitive, 13% were moderately sensitive and 2% were resistant to ceftazidime. Synergism was most frequently observed when ceftazidime was combined with tobramycin. Using this combination, secondary resistance of Pseudomonas strains to ceftazidime did not develop. When ceftazidime was combined with piperacillin, synergism was observed in most strains, but the development of secondary resistance in vitro was not prevented.(ABSTRACT TRUNCATED AT 250 WORDS)