LOSS OF ANTIMICROBIAL SUSCEPTIBILITY IN AEROBIC GRAM-NEGATIVE BACILLIREPEATEDLY ISOLATED FROM PATIENTS IN INTENSIVE-CARE UNITS

OBJECTIVE: To study the loss of antimicrobial susceptibility in repeat (same patient, same bacterial species, and same site) aerobic gram-ne gative bacilli (AGNB) isolated from individual patients during their s tay in the intensive-care unit (ICU). SETTING: A 792-bed, tertiary-car e community hospital with a total of 107 adult, pediatric, and neonata l ICU beds. METHOD: An observational prospective study performed Novem ber 1992 through mid-July 1993. RESULTS: Of 594 consecutive AGNB from 287 ICU patients, 117 isolates (20%) from 55 patients (19%) were repea t 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 patien ts (44%) had greater than or equal to 4-fold increase in minimum inhib itory concentration to at least one antibiotic and no longer were cons idered fully susceptible based on National Committee on Clinical Labor atory Standards criteria. Loss of antimicrobial susceptibility often d eveloped within several (median 8) days and was associated only infreq uently with simultaneous transition from colonization to infection in the individual patient. Use of certain beta-lactam antibiotics was ass ociated with increasing resistance to several other antibiotics in the same class. Concurrent use of beta-lactams and aminoglycosides did no t prevent loss of antimicrobial susceptibility to the former in repeat isolates. CONCLUSION: We conclude that loss of antimicrobial suscepti bility in repeat AGNB isolated from ICU patients is common, usually is not associated with transition from colonization to infection, and of ten is associated with prior use of antibiotics. Minimizing antibiotic use in ICU patients should help reduce the risk of antimicrobial resi stance in repeat AGNB isolates.