Acquisition of Multidrug-Resistant Gram-Negative Bacteria: Incidence and Risk Factors within a Long-Term Care Population

Background and Objective.An improved understanding of the transmission dynamics of multidrug-resistant (MDR) gram-negative bacteria and the mechanism of acquisition in long-term care facilities (LTCFs) could aid in the development of prevention strategies specific to LTCFs. We thus investigated the incidence of acquisition of these pathogens among an LTCF population.Design.Prospective cohort study.Setting.Three separate wards at a 600-bed LTCF in metropolitan Boston, Massachusetts, during the period October 31, 2006, through October 22, 2007.Participants.One hundred seventy-two LTCF residents.Methods.A series of rectal samples were cultured to determine acquisition of MDR gram-negative bacteria, defined as absence of MDR gram-negative bacterial colonization at baseline and de novo recovery of MDR gram-negative bacteria from a follow-up culture. Molecular typing was performed to identify genetically linked strains. A nested matched case-control study was performed to identify risk factors associated with acquisition.Results.Among 135 residents for whom at least 1 follow-up culture was performed, 52 (39%) acquired at least 1 MDR gram-negative organism during the study period. Thirty-two residents (62%) had not been colonized at baseline and had acquired at least 1 MDR gram-negative species at follow-up culture, and 20 residents (38%) were colonized at baseline and had acquired at least I MDR gram-negative species at follow-up culture. The most common coresistance pattern was resistance to extended-spectrum penicillins, ciprofloxacin, and gentamicin (57 isolates [42.5%]). Genetically related strains of MDR gram-negative bacteria were identified among multiple residents and between roommates. On conditional logistic regression analysis, antibiotic exposure during the study period was significantly associated with acquisition of MDR gram-negative bacteria (odds ratio, 5.6 [95% confidence interval, 1.1-28.7];P= .04).Conclusions.Acquisition of MDR gram-negative bacteria occurred frequently through resident-to-resident transmission. Existing infection control interventions need to be reevaluated.