FINAL DIAGNOSIS: AGROBACTERIUM RADIOBACTER
The genus Agrobacterium are aerobic, gram negative, peritrichous bacilli which are mainly plant pathogens found in soil all over the world. Agrobacterium tumefaciens and Agrobacterium radiobacter are two species that have been isolated from human specimens although only A. radiobacter is associated with clinical symptoms (1).
Despite A. radiobacter's association with significant clinical symptoms, it is considered to be of low virulence and there have been no reports of mortality from the organism alone. The only difference between the two species is the presence of a tumor-producing plasmid, the Ti plasmid, in A. tumefaciens which results in a tumorous growth in plants (2).
Agrobacterium radiobacter is infrequently recognized in clinical specimens however it has been associated with immunocompromised patients that have implanted medical devices or transcutaneous catheters (1,3,4). It was first isolated in 1967 but was not considered pathogenic (5). Approximately 22 cases of clinically significant A. radiobacter infections have been reported since then, resulting in 2 cases of urinary tract infections (UTI), 5 cases of peritonitis, 14 cases of bacteremia, and one case of endocarditis (1). Both cases of UTIs were associated with nephrostomy tubes, all 5 cases of peritonitis were associated with intraperitoneal catheters, 10 of 14 cases of bacteremia were associated with central venous catheters and the only case of endocarditis was associated with a prosthetic heart valve (1).
In addition to the reported cases above, we have isolated the organism from other patients as part of a mixed flora with unknown pathogenicity.
Biochemically, Agrobacterium is characterized by the production of catalase, oxidase, urease and gas from several carbohydrates including glucose, fructose, arabinose, xylose, lactose, maltose, and sucrose among others (6,7).
The antimicrobial susceptibility pattern is variable as the organism has been shown to form several antibiotic-inactivating enzymes (8). Consequently, antimicrobial therapy should be guided by minimum inhibitory concentration (MIC) testing as the disk diffusion breakpoints have not been determined. In addition to antimicrobial therapy, removal of any foreign body or device should be considered.
MICs on limited isolates suggest that ticarcillin, cefoxitin, ceftriaxone, cefotaxime, gentamicin, and ciprofloxacin have the greatest efficacy on A. radiobacter. These limited studies also showed significant variation in susceptibility within classes of antibiotics (1).
Contributed by David Sholevar, MD., Carla Baxter and A. William Pasculle, Sc.D.