The changing ecology of hospital bacteria and the selective role of cephalosporins.

More than 12,800 clinical isolates from 115,373 in-patient specimens obtained at the Sir Charles Gairdner Hospital, Perth, Western Australia, were identified and analysed statistically for relationships with usage of three generations of cephalosporins over the 5-year period from July 1984 to June 1989. A positive relationship between cephalosporin usage and significantly increasing isolation rates for those species capable of producing chromosomal beta-lactamases was observed. Simultaneously, a small increase in the isolation frequency of non-chromosomal beta-lactamase-producing strains was noted and no correlation with cephalosporin usage was demonstrated. The trend toward predomination in the hospital environment of strains possessing substantial cephalosporin resistance has implications for future antimicrobial policy, choice of empiric therapy and the predictive value of standard antimicrobial susceptibility tests.