Multiply Antibiotic Resistant Gram-Negative Bacilli in a Long Term Care Facility A Case Control Study of Patient rick Factors and Prior Antibiotic Use

OBJECTIVE To determine the relation between prior exposure to specific antimicrobials and acquisition of gram-negative bacilli resistant to multiple beta-lactam and aminoglycoside antibiotics among long-term-care patients. DESIGN Case-control study. Cases were patients from whom multiply resistant Enterobacteriaceae or Pseudomonas aeruginosa were isolated; controls were patients from whom nonresistant bacteria of the same species were isolated. Prospectively defined risk factors included underlying illness, activity level, presence of decubitus ulcers, presence of indwelling devices, and prior exposure to specific antimicrobial agents. Resistant and control isolates of P aeruginosa were compared using pulsed-field gel electrophoresis (PFGE) of genomic DNA after digestion with XbaI. SETTING 390-bed long-term Veterans' Affairs facility. RESULTS We identified 35 patients with multiply resistant Enterobacteriaceae and 24 patients with multiply resistant P aeruginosa. Of the resistant Enterobacteriaceae, 87% of isolates were resistant to piperacillin, 55% to ceftazidime, and 90% to gentamicin. Acquisition of multiply resistant Enterobacteriaceae was associated with presence of decubitus ulcers (odds ratio [OR], 12.2; 95% confidence interval [CI95], 3.3-44.2; P = .0002) and prior receipt of ampicillin (OR, 13.7; CI95, 2.2-84; P = .005). Of resistant isolates of P aeruginosa, 88% were resistant to piperacillin, 25% to ceftazidime, 42% to imipenem, and 67% to ciprofloxacin. Isolation of a multiply resistant P aeruginosa was associated with total days of antimicrobial exposure (OR, 1.07; CI95, 1.01-1.12; P = .011) and not with prior receipt of any individual agent. Eleven multiply resistant isolates shared a common PFGE pattern. CONCLUSIONS In our long-term-care facility, acquisition of multiply resistant Enterobacteriaceae was associated with the presence of decubitus ulcers and prior exposure to ampicillin. Acquisition of resistant P aeruginosa was associated with total antibiotic exposure. Molecular typing of P aeruginosa isolates implicated patient-to-patient transmission of a limited number of resistant strains.