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In vitro selection of beta-lactam resistance by exposure to second, third, and fourth generation cephalosporins.

The frequency in which in vitro resistance to beta-lactam antibiotics can be selected by prior exposure to a second, third, or fourth generation cephalosporin was examined. A total of 91 isolates of Enterobacteriaceae or Pseudomonas were tested against cefamandole, cefotaxime, moxalactam, cefoperazone, ceftazidime, ceftizoxime, ceftriaxone, or HR 810. Resistance was most commonly observed in members of the genera Enterobacter, Serratia, and Pseudomonas. Resistance developed after exposure to all 8 test antibiotics and stable resistance was maintained in 31% of the originally selected isolates. Cross-resistance was frequently seen, although stable resistance to the fourth generation cephalosporin, HR 810, was not observed in any isolate.