Loss of antimicrobial susceptibility in aerobic gram-negative bacilli repeatedly isolated from patients in intensive-care units.

Abstract

To study the loss of antimicrobial susceptibility in repeat (same patient, same bacterial species, and same site) aerobic gram-negative bacilli (AGNB) isolated from individual patients during their stay in the intensive-care unit (ICU). A 792-bed, tertiary-care community hospital with a total of 107 adult, pediatric, and neonatal ICU beds. An observational prospective study performed November 1992 through mid-July 1993. Of 594 consecutive AGNB from 287 ICU patients, 117 isolates (20%) from 55 patients (19%) were repeat isolates, with the majority obtained from respiratory secretions (83%). Pseudomonas aeruginosa and Enterobacter species accounted for 61% of the isolates. Forty-two (36%) of the repeat isolates from 24 patients (44%) had > or = 4-fold increase in minimum inhibitory concentration to at least one antibiotic and no longer were considered fully susceptible based on National Committee on Clinical Laboratory Standards criteria. Loss of antimicrobial susceptibility often developed within several (median 8) days and was associated only infrequently with simultaneous transition from colonization to infection in the individual patient. Use of certain beta-lactam antibiotics was associated with increasing resistance to several other antibiotics in the same class. Concurrent use of beta-lactams and aminoglycosides did not prevent loss of antimicrobial susceptibility to the former in repeat isolates. We conclude that loss of antimicrobial susceptibility in repeat AGNB isolated from ICU patients is common, usually is not associated with transition from colonization to infection, and often is associated with prior use of antibiotics. Minimizing antibiotic use in ICU patients should help reduce the risk of antimicrobial resistance in repeat AGNB isolates.