Ochrobactrum anthropi, formerly known as CDC group Vd, is an oxidase-producing, gram-negative, obligately aerobic, non-lactose-fermenting bacillus of low virulence that occasionally causes human infections. It is highly resistant to all -lactams except imipenem. A clinical isolate, SLO74, and six reference strains were tested. MICs of penicillins, aztreonam, and most cephalosporins tested, including cefotaxime and ceftazidime, were >128 g/ml and of cefepime were 64 to >128 g/ml. Clavulanic acid was ineffective and tazobactam had a weak effect in association with piperacillin. Two genes, ampR and ampC, were cloned by inserting restriction fragments of genomic DNA from the clinical strain O. anthropi SLO74 into pBK-CMV to give the recombinant plasmid pBK-OA1. The pattern of resistance to -lactams of this clone was similar to that of the parental strain, except for its resistance to cefepime (MIC, 0.5 g/ml). The deduced amino acid sequence of the AmpC -lactamase (pI, 8.9) was only 41 to 52% identical to the sequence of other chromosomally encoded and plasmid-encoded class C -lactamases. The kinetic properties of this -lactamase were typical for this class of -lactamases. Upstream from the ampC gene, the ampR gene encodes a protein with a sequence that is 46 to 62% identical to those of other AmpR proteins and with an amino-terminal DNA-binding domain typical of transcriptional activators of the Lys-R family. The deduced amino acid sequences of the ampC genes of the six reference strains were 96 to 99% identical to the sequence of the clinical strain. The -lactamase characterized from strain SLO74 was named OCH-1 (gene, bla OCH-I ).The species of the genus Ochrobactrum form two groups: Ochrobactrum anthropi and O. intermedium (25). O. anthropi, formerly classified as CDC group Vd, is a nonfastidious, gramnegative bacillus that is strictly aerobic, oxidase positive, and motile (with peritrichous flagella), does not ferment lactose, and has strong urease activity (8,16). O. anthropi is widespread and is distributed in water and hospital environments. In some cases, it has been isolated from water-based environments in hospitals (antiseptic solutions, dialysis fluids) (12). It has often been found on human clinical material: it often adheres to catheters, but pacemakers, intraocular lenses, and silicon tubing may also become infected (10, 18). Although only weakly virulent, O. anthropi causes hospital-acquired infections, often in immunocompromised hosts (7,11, 14,27). O. anthropi is usually resistant to -lactams, such as broad-spectrum penicillins and oxyimino cephalosporins, except for cefepime in some cases and aztreonam (3). It is generally susceptible to carbapenems and aminoglycosides (19), trimethoprim-sulfamethoxazole (4), ciprofloxacin, and tetracyclines. The most effective antimicrobial agents for treating human infections are imipenem, trimethoprim-sulfamethoxazole, and ciprofloxacin (7), sometimes in conjunction with catheter removal (28). As this bacterium displays extensive resistance to -lactams...