Impact of use of multiple antimicrobials on changes in susceptibility of gram-negative aerobes

Evaluation of antimicrobial usage vs, susceptibility relationships typicall y involves single agents. However, susceptibility profiles may be affected by multiple drugs. From 1992 through 1996, we studied relationships between drug usage and the susceptibility (only susceptibility rates of greater th an or equal to 70%) of Acinetobacter anitratus (baumannii), Enterobacter ae rogenes, Escherichia coli, Enterobacter cloacae, Klebsiella pneumoniae, Pse udomonas aeruginosa, Proteus mirabilis, and Serratia marcescens to 22 agent s. Linear regression was used to assess usage of each agent vs. susceptibil ity to it and to all agents. Only relationships with a coefficient of deter mination of greater than or equal to 0.5 and a negative slope were evaluate d and classified as increasing drug use and decreasing susceptibility (up a rrow D, down arrow%S) or decreasing drug use and increasing susceptibility (down arrow D, up arrow%S). The mean numbers (range) of drugs associated wi th a change in susceptibility were 1.7 (0-14) and 0.6 (0-7), respectively, for to, down arrow%S and down arrow D, up arrow%S relationships. Multiple a ntimicrobials are associated with susceptibility to other drugs; thus, surv eillance of these relationships should not be limited to single drugs.