RESEARCH Multidrug-resistant Acinetobacter baumannii Aharon Abbo,* Shiri Navon-Venezia,* Orly Hammer-Muntz,* Tami Krichali,* Yardena Siegman-Igra,* and Yehuda Carmeli* To understand the epidemiology of multidrug-resistant antimicrobial agents, contributes to the organism’s fitness (MDR) Acinetobacter baumannii and define individual risk and enables it to spread in the hospital setting. factors for multidrug resistance, we used epidemiologic The nosocomial epidemiology of this organism is com- methods, performed organism typing by pulsed-field gel plex. Villegas and Hartstein reviewed Acinetobacter out- electrophoresis (PFGE), and conducted a matched case- breaks occurring from 1977 to 2000 and hypothesized that control retrospective study. We investigated 118 patients, on 27 wards in Israel, in whom MDR A. baumannii was iso- endemicity, increasing rate, and increasing or new resist- lated from clinical cultures. Each case-patient had a control ance to antimicrobial drugs in a collection of isolates sug- without MDR A. baumannii and was matched for hospital gest transmission. These authors suggested that length of stay, ward, and calendar time. The epidemiologic transmission should be confirmed by using a discriminato- investigation found small clusters of up to 6 patients each ry genotyping test (15). The importance of genotyping with no common identified source. Ten different PFGE tests is illustrated by outbreaks that were shown by classic clones were found, of which 2 dominated. The PFGE pat- epidemiologic methods and were thought to be caused by tern differed within temporospatial clusters, and antimicro- a single isolate transmitted between patients; however, bial drug susceptibility patterns varied within and between when molecular typing of the organisms was performed, a clones. Multivariate analysis identified the following signifi- cant risk factors: male sex, cardiovascular disease, having more complex situation of multiple unrelated strains caus- undergone mechanical ventilation, and having been treated ing the increasing rates of infections by A. baumannii was with antimicrobial drugs (particularly metronidazole). discovered (16–18). Penicillins were protective. The complex epidemiology may Almost 25 years ago, researchers observed acquired explain why the emergence of MDR A. baumannii is difficult resistance of A. baumannii to antimicrobial drugs com- to control. monly used at that time, among them aminopenicillins, ureidopenicillins, first and second-generation cephalo- cinetobacter baumannii has emerged as an important sporins, cephamycins, most aminoglycosides, chloram- Anosocomial pathogen (1–5). Hospital outbreaks have phenicol, and tetracyclines (19). Since then, strains of A. been described from various geographic areas (6–9), and baumannii have also gained resistance to newly developed this organism has become endemic in some of them. The antimicrobial drugs. Although multidrug-resistant (MDR) role of the environmental contamination in the transmis- A. baumannii is rarely found in community isolates, it sion of nosocomial infections in general and in A. bau- became prevalent in many hospitals (20). MDR A. bau- mannii infections in particular is well recognized (10,11). mannii has recently been established as a leading nosoco- A. baumannii does not have fastidious growth require- mial pathogen in several Israeli hospitals, including our ments and is able to grow at various temperatures and pH institution (21,22). Several locally contained small out- conditions (12). The versatile organism exploits a variety breaks of MDR A. baumannii occurred in our institution of both carbon and energy sources. These properties during the late 1990s. In 1999, however, the incidence of explain the ability of Acinetobacter species to persist in MDR A. baumannii isolation had doubled compared to the either moist or dry conditions in the hospital environment, previous 2 years, and the organism became endemic in thereby contributing to transmission (13,14). This hardi- many wards (unpub. data). ness, combined with its intrinsic resistance to many The likelihood of isolation of A. baumannii from a hos- pitalized patient is related to temporospatial (extrinsic, *Tel Aviv Sourasky Medical Center, Tel Aviv, Israel ecologic characteristics) factors such as colonization 22 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 11, No. 1, January 2005 Multidrug-resistant Acinetobacter baumannii pressure (23), nurse-to-patient ratio, and other ward char- variables occurring before inclusion in the study (culture acteristics and to individual patient risk factors (character- day for case-patients and match day for controls) were ana- istics). The current study was designed to examine the lyzed as possible risk factors. A. baumannii was isolated occurrence and spread of A. baumannii within our institu- from clinical specimens submitted to the microbiology tion, as well as to define individual risk factors for isola- laboratory and identified by using the Gram-Negative tion of this organism. Identification Panel (Microscan, Dade Behring Inc., Sacramento, CA, USA). This system may not distinguish Methods between closely related genotypic strains of Acinetobacter, and thus, some of these organisms may belong to these Hospital Setting, Data Collection, closely related strains. Susceptibilities were determined by and Microbiologic Testing automated microdilution broth testing (Neg/Urine Combo This study was performed at the Tel-Aviv Sourasky panel, Dade Behring Inc.). Resistance to imipenem and Medical Center, Israel, a 1,200–bed tertiary care teaching meropenem was confirmed by using Kirby-Bauer disk dif- hospital with 70,000 admissions annually. Approximately fusion, according to the National Council for Clinical 82,500 clinical microbiologic cultures are processed annu- Laboratory Standards (NCCLS) guidelines. A. baumannii ally. We designed this as a matched case-control study to isolates were collected prospectively and stored at –70°C identify the individual risk factors for having MDR A. bau- for further work-up. mannii. We also performed epidemiologic investigations and genetic typing of the organisms to clarify the spread of Analysis of Chromosomal DNA this nosocomial pathogen. by Pulsed-Field Gel Electrophoresis (PFGE) Case-patients were defined as patients from whom Isolates of our patients, when available, were kept MDR A. baumannii was isolated from any clinical culture frozen at –70ºC and genetically characterized with PFGE. (not surveillance cultures) during a 6-month period, from DNA preparation and cleavage with 20 U of ApaI endonu- January 1, 2001, to June 30, 2001. A control patient was clease (New England Biolabs, Beverly, MA, USA) were matched to each study patient on temporospatial factors as preformed as previously described (26). Electrophoresis previously described (24). Briefly, controls were randomly was performed in a 1% agarose gel (BMA products) pre- chosen from the list of patients who stayed on the same pared and run in 0.5 x Tris-borate-EDTA buffer on a ward in the same calendar month as the matched case- CHEF-DR III apparatus (Bio-Rad Laboratories, Hercules, patient and who were hospitalized for at least the same CA, USA). The initial switch time was 5 s, the final switch number of days by the day the culture yielded MDR A. time was 35 s, and the run time was 23 h at 6 V/cm. Gels baumannii in the study patient. Controls were not MDR A. were stained in ethidium bromide, destained in distilled baumannii positive (i.e., the patient’s samples were cul- water, and photographed by using a Bio-Rad GelDoc 2000 tured, and either non-MDR A. baumannii or no A. bau- camera. DNA patterns were analyzed visually and by using mannii was isolated, or the patient’s samples were never Diversity software (Bio-Rad). PFGE DNA patterns were cultured). Random control selection was performed by cre- compared and interpreted according to the criteria of ating a list of all possible controls, assigning each candi- Tenover et al. (27). The obtained PFGE DNA patterns date a random number, and choosing the highest random were used to cluster the clones of the A. baumannii clini- number (without replacement). cal isolates that were included in the study. Case-patients and control patients were included only once in the study. Data were collected from the patients’ Definitions records and from hospital computerized databases into a We defined A. baumannii as MDR when the organism pre-prepared electronic questionnaire (Microsoft Access, was resistant to all studied agents (including Microsoft Corp., Redmond, WA, USA). The parameters piperacillin/tazobactam, cefepime, ceftazidime, aztreon- registered for each patient (case-patients and controls) am, ciprofloxacin, gentamicin, tobramycin), but we were age, sex, habits of smoking and alcohol consumption allowed susceptibility to amikacin, ampicillin-sulbactam, before hospitalization, cause and ward of hospitalization, imipenem, meropenem, and minocycline. Infection was transfer from another institution or ward within our insti- defined according to the Centers for Disease Control and tution, intensive care unit (ICU) stay, underlying disorders, Prevention guidelines and modified to include community- immunosuppressive therapy, severity of illness as defined acquired infections and to exclude asymptomatic bacteri- by the McCabe score (25), functional capacity and neuro- uria (28). logic condition at time of isolation of A. baumannii, Foley Standard criteria were used to define underlying