Reduced sensitivity to beta-lactam antibiotics arising during ceftazidime treatment of Pseudomonas aeruginosa infections.

Pseudomonas aeruginosa isolated from two patients with empyema and one with bronchopneumonia became less sensitive after treatment with ceftazidime, while Ps. aeruginosa persisted in a patient with an infected compound fracture of the tibia treated with ceftazidime but did not become less sensitive. The reduction in sensitivity to ceftazidime, which was small, was accompanied by resistance to azlocillin but there are little reduction in sensitivity to carbenicillin. The resistant strains produced increased amounts of the chromosomally-mediated cephalosporinase produced by most isolates of Ps. aeruginosa. Variants with reduced sensitivity to ceftazidime, which resembled those that developed in vivo, were selected in vitro from each of the initial ceftazidime-sensitive isolates.