This study was performed at the Philadelphia Veteran’s Administration Medical Center (PVAMC) long term care facility (LTCF). Approval was obtained from the Institutional Review Boards of the University of Pennsylvania and the PVAMC. This study is based on data originally collected as part of a study investigating the epidemiology of GI tract colonization with fluoroquinolone-resistant and fluoroquinolone-susceptible E. coli
in LTCF residents [8
]. As described previously, LTCF residents were recruited for this study between March and July 2002. All residents were considered eligible for inclusion and were enrolled if informed consent could be obtained. For enrolled subjects, one rectal swab was obtained at enrollment.
Swabs were inoculated onto MacConkey agar without antimicrobial additives (nonselective medium). Up to 25 lactose-positive colonies (as available) were arbitrarily selected and inoculated onto nonselective media and MacConkey agar containing 8 μg/ml of ofloxacin. If fewer than 25 colonies were present, all were sampled. If greater than 25 colonies were present, 25 colonies were selected at random. Antimicrobial susceptibilities and species identification were confirmed by automated testing (Vitek, BioMerieux, Hazelwood, MO) [8
Chromosomal DNA was digested with Xba
I and resolved by pulsed-field gel electrophoresis (PFGE) using the CHEF DR II System (Bio-Rad, Hercules, CA) to determine genetic relatedness [8
]. Strain identity was interpreted according to established criteria [9
To determine risk factors for colonization with multiple distinct strains of E. coli
, we conducted a cross sectional study. Cases were defined as those subjects for whom two or more distinct strains of E. coli
were identified while controls were defined as subjects harboring only one strain of E. coli
. The computerized VA medical record was reviewed for potential risk factors. Data obtained included age, gender, race, the number of days from LTCF admission to study enrollment, and prior hospitalizations. Presence of a decubitus ulcer, surgical wound and indwelling devices (i.e., tracheostomy, urinary catheter, and feeding tube) were ascertained. Data on co-morbid conditions included renal insufficiency, malignancy, diabetes, congestive heart failure, coronary artery disease, dementia, and depression were also assessed. Finally, pharmacy records were reviewed for all antibiotic prescriptions for one year prior to study entry. Overall antibiotic exposure as well as exposure to specific agents and classes was assessed [10
Bivariable analyses were performed to determine the association between each variable and the outcome of interest (i.e., colonization with multiple E. coli strains). Categorical variables were compared using Fishers exact test while continuous variables were analyzed by the Wilcoxon rank sum test. Odds ratios (OR) and 95% confidence intervals (CI) were calculated to determine both the strength of any association and the precision of the estimate of the effect. Multivariable analyses were considered to further evaluate those variables found to be significantly associated with the outcome on unadjusted analyses. All statistical calculations were performed using STATA version 10.0 (Stata Corp, College Station TX).