Ochrobactrum spp. belongs to the family Brucellaceae. The genus Ochrobactrum contains nine species among which only three species, Ochrobactrum anthropi, Ochrobactrum intermedium, and Ochrobactrum pseudintermedium, have been reported to cause human infections. O. anthropi is an aerobic, oxidase-positive, urease-positive, Gram-negative, motile, nonlactose fermenting bacilli which were earlier known as “Achromobacter group Vd.”[1,2] Data related to O. anthropi infections are limited and come mainly from case reports. A review of the literature by Ryan and Pembroke showed only 46 reported cases of bloodstream infections caused by O. anthropi since 1998. Ten cases of death related to O. anthropi have also been described in the literature. Although the organism has relatively low virulence, it can be pathogenic in seriously ill or immunocompromised patients. It is resistant to many antibiotics and difficult to differentiate it from other species of Ochrobactrum genus by conventional methods.
A 6-year-old child with ALL who was admitted to our hospital for chemotherapy developed fever a few days later. Blood from two different sites was sent for culture in paired BACTEC (Becton Dickinson, Sparks, MD, U.S.A.) bottles. Both flagged positive after 2 days. Gram stain showed Gram-negative bacilli. Growth on blood agar showed gray nonhemolytic colonies [Figure 1] and on MacConkey agar showed mucoid nonlactose fermenting colonies [Figure 2]. It was catalase, oxidase, and urease positive [Figure 3]. The organism was identified as O. anthropi by VITEK 2 system (bioMerieux, Marcy l’Etoile, France). Antibiotic susceptibility testing (AST) was done by Kirby–Bauer disk diffusion method and the isolate was susceptible to amikacin, gentamicin, ciprofloxacin, and imipenem but resistant to co-trimoxazole. The child was continued on cefepime and his fever subsided. Although the pathogen is commonly resistant to the antibiotic prescribed, he responded well to the treatment and the antibiotic did not need to be changed probably because of the low virulence of the organism. Repeat blood culture showed no growth and the child was discharged after chemotherapy.
A 34-year-old male with acute promyelocytic leukemia was admitted with high-grade fever with chills. Two sets of blood cultures (BACTEC bottles) were taken at the time of fever and he was started on cefoperazone–sulbactam. Both flagged positive on the same day. Gram stain showed Gram-negative bacilli. Microbiological identification and AST were done like case 1. The isolate was found to be resistant to co-trimoxazole, amikacin, gentamicin, and ciprofloxacin and sensitive to meropenem. Since the patient was afebrile after initiation of antibiotic, the same was given for 3 days and stopped. Repeat blood culture showed no growth and the patient was discharged after chemotherapy.
Unusual organisms that were not thought to have pathogenic potential are now becoming the cause of many serious infections such as endocarditis and septicemia. O. anthropi is such an example. Most human infections were seen in immunocompromised individuals and were associated with the presence of indwelling medical devices such as central venous catheters, drainage tubes, or intraperitoneal catheters. The ability of the organism to adhere to silicone could be the reason for these catheter-associated infections. Patients who were on cytotoxic chemotherapy and had recent transplantation, malignancies are at higher risk of developing infection with such unusual nonfermenting Gram-negative bacilli. The absence of pigment production differentiates Ochrobactrum from Pseudomonas and Flavobacterium spp. Achromobacter, Alcaligenes, and Agrobacterium produces hydrogen sulfide, while Ochrobactrum does not. Therefore, if the isolate is nonfastidious, nonlactose fermenting, oxidase-positive, Gram-negative bacilli which are resistant to beta-lactam antibiotics (except imipenem) it is from the genus Ochrobactrum. Positive urease test, the presence of mucoid colonies, and growth at 45°C on tryptic soy agar along with susceptibility to colistin, tobramycin, and netilmicin help in differentiating O. anthropi from O. intermedium.O. anthropi isolates are susceptible to aminoglycosides, carbapenems, co-trimoxazole, and quinolones but resistant to penicillins, cephalosporins, chloramphenicol, and macrolides.[2,3] However, studies have shown that there is a poor correlation between in vitro susceptibility data with clinical efficacy of the antimicrobial agent. Both of our patients were treated with cephalosporin, to which the organism was supposed to be resistant, but both became afebrile without any complications, probably because of the low virulence of the organism.
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