To understand the epidemiology of multidrug-resistant (MDR) Acinetobacter baumannii and define individual risk factors for multidrug resistance, we used epidemiologic methods, performed organism typing by pulsed-field gel electrophoresis (PFGE), and conducted a matched case- control retrospective study. We investigated 118 patients, on 27 wards in Israel, in whom MDR A. baumannii was iso- lated from clinical cultures. Each case-patient had a control without MDR A. baumannii and was matched for hospital length of stay, ward, and calendar time. The epidemiologic investigation found small clusters of up to 6 patients each with no common identified source. Ten different PFGE clones were found, of which 2 dominated. The PFGE pat- tern differed within temporospatial clusters, and antimicro- bial drug susceptibility patterns varied within and between clones. Multivariate analysis identified the following signifi- cant risk factors: male sex, cardiovascular disease, having undergone mechanical ventilation, and having been treated with antimicrobial drugs (particularly metronidazole). Penicillins were protective. The complex epidemiology may explain why the emergence of MDR A. baumannii is difficult to control. A cinetobacter baumannii has emerged as an important nosocomial pathogen (1–5). Hospital outbreaks have been described from various geographic areas (6–9), and this organism has become endemic in some of them. The role of the environmental contamination in the transmis- sion of nosocomial infections in general and in A. bau- mannii infections in particular is well recognized (10,11). A. baumannii does not have fastidious growth require- ments and is able to grow at various temperatures and pH conditions (12). The versatile organism exploits a variety of both carbon and energy sources. These properties explain the ability of Acinetobacter species to persist in either moist or dry conditions in the hospital environment, thereby contributing to transmission (13,14). This hardi- ness, combined with its intrinsic resistance to many antimicrobial agents, contributes to the organism’s fitness and enables it to spread in the hospital setting. The nosocomial epidemiology of this organism is com- plex. Villegas and Hartstein reviewed Acinetobacter out- breaks occurring from 1977 to 2000 and hypothesized that endemicity, increasing rate, and increasing or new resist- ance to antimicrobial drugs in a collection of isolates sug- gest transmission. These authors suggested that transmission should be confirmed by using a discriminato- ry genotyping test (15). The importance of genotyping tests is illustrated by outbreaks that were shown by classic epidemiologic methods and were thought to be caused by a single isolate transmitted between patients; however, when molecular typing of the organisms was performed, a more complex situation of multiple unrelated strains caus- ing the increasing rates of infections by A. baumannii was discovered (16–18). Almost 25 years ago, researchers observed acquired resistance of A. baumannii to antimicrobial drugs com- monly used at that time, among them aminopenicillins, ureidopenicillins, first and second-generation cephalo- sporins, cephamycins, most aminoglycosides, chloram- phenicol, and tetracyclines (19). Since then, strains of A. baumannii have also gained resistance to newly developed antimicrobial drugs. Although multidrug-resistant (MDR) A. baumannii is rarely found in community isolates, it became prevalent in many hospitals (20). MDR A. bau- mannii has recently been established as a leading nosoco- mial pathogen in several Israeli hospitals, including our institution (21,22). Several locally contained small out- breaks of MDR A. baumannii occurred in our institution during the late 1990s. In 1999, however, the incidence of MDR A. baumannii isolation had doubled compared to the previous 2 years, and the organism became endemic in many wards (unpub. data). The likelihood of isolation of A. baumannii from a hos- pitalized patient is related to temporospatial (extrinsic, ecologic characteristics) factors such as colonization Multidrug-resistant Acinetobacter baumannii Aharon Abbo,* Shiri Navon-Venezia,* Orly Hammer-Muntz,* Tami Krichali,* Yardena Siegman-Igra,* and Yehuda Carmeli* RESEARCH 22 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 11, No. 1, January 2005 *Tel Aviv Sourasky Medical Center, Tel Aviv, Israel