90 The Korean Journal of Internal Medicine Vol. 26, No. 1, March 2011 Although the occurrence of ciprofloxacin resistance in E. coli and K. pneumoniae isolates is well-established, lit- tle is known of the epidemiology of P. mirabilis bac- teremia caused by ciprofloxacin-resistant isolates. Thus, in this study, we evaluated the clinical features of ciprofloxacin-resistant P. mirabilis bacteremia to clarify the risk factors for ciprofloxacin resistance in P. mirabilis isolates causing bacteremia. METHODS The database in our clinical microbiology laboratory (Samsung Medical Center, Seoul, Korea; a 1,950-bed, ter- tiary-care university hospital and referral center) was reviewed to identify patients > 16 years of age diagnosed with P. mirabilis bacteremia from October 2000 to July 2009. Only the first episode for each patient was included in the analysis. Antibiotic susceptibility was tested using the VITEK II automated system (bioMérieux, Hazelwood, MO, USA) using the standard modified broth microdilu- tion method. Minimum inhibitory concentration (MIC) breakpoints and quality control protocols were used according to standards established by the Clinical and Laboratory Standards Institute [7]. For the purposes of this study, isolates showing in vitro resistance to cefo- taxime, ceftriaxone, or ceftazidime were classified as ESBL-producing organisms. Strains showing an ‘interme- diate’ result to the tested antibiotics were considered resis- tant. We compared the clinical features of patients with ciprofloxacin-resistant and ciprofloxacin-susceptible P. mirabilis bacteremia. The data collected included age, gender, underlying disease, primary site of infection, acquisition site, and antimicrobial regimen. The presence of the following comorbid conditions was also document- ed: recent operation, corticosteroid use, indwelling uri- nary catheter, L-tube insertion, percutaneous tube use, prior invasive procedure, and prior use of antibiotics. Nosocomial acquisition of infection was defined as an infection that occurred > 48 hours after hospital admis- sion, while infections diagnosed within the first 48 hours of hospitalization were classified as community-onset infection, based on the time the culture samples were obtained. Because many cases of Proteus bacteremia that are present or incubating on admission to the hospital are nonetheless healthcare-associated, we refer to non-noso- comial bacteremia as community-onset rather than com- munity-acquired. Community-onset infection was further classified as community-associated and healthcare-associ- ated, as previously suggested [8]. The site of infection was determined by physicians, based on the isolation of P. mirabilis from the presumed portal of entry and clinical evaluation. Student's t test was used to compare continuous vari- ables, and χ2 or Fisher's exact tests were used to compare categorical variables. A logistic regression analysis was performed to evaluate ESBL production as a factor associ- ated with ciprofloxacin resistance. The odds ratio (OR) and 95% confidential interval (CI) were calculated. All p values were two-tailed, and p < 0.05 was considered to indicate statistical significance. RESULTS Thirty-seven patients with P. mirabilis bacteremia were identified. Their demographic and clinical characteristics are shown in Table 1. The most common underlying dis- eases were neurologic diseases (38%) and solid tumors (30%). The most common site of infection was the urinary tract (35%). Ten (27%) of the 37 patients were infected with ciprofloxacin-resistant isolates and nine (24%) were infected with ESBL-producers. Among the 27 patients Table 1. Clinical characteristics of 37 patients with Proteus mirabilis bacteremia Characteristics No. of patients (%) Age, yr (mean ± SD) 62.4 ± 17.8 Female 22 (59.5) Acquisition site Community-associated 17 (45.9) Healthcare-associated 10 (27.0) Nosocomial infection 10 (27.0) Underlying disease Neurologic disease 14 (37.8) Solid tumor 11 (29.7) Diabetes mellitus 6 (16.2) Cardiovascular disease 3 (8.1) Renal disease 3 (8.1) Primary site of infection Primary bacteremia 12 (32.4) Urinary tract infection 13 (35.1) Intra-abdominal infection 8 (21.6) Pneumonia 4 (10.8) Extended-spectrum β-lactamase production 9 (24.3) Ciprofloxacin resistance 10 (27.0)