Rhizobium, Ochrobactrum, and Similar Organisms
1. Describe the general characteristics of the organisms discussed in this chapter, including their normal habitat, Gram stain characteristics, and morphology.
2. List the types of diseases associated with each organism.
3. Compare and contrast the Gram stain appearance of the various species.
4. Create an algorithm that outlines the major tests used to differentiate Achromobacter spp., Alcaligenes xylosoxidans, Ochrobactrum anthropi, and Rhizobium radiobacter.
Epidemiology
As environmental organisms, these bacteria are rarely encountered in human specimens or infections. When they are encountered, they are found on contaminated medical devices or are isolated from immunocompromised or debilitated patients. Of the organisms listed in Table 23-1, Rhizobium radiobacter, and O. anthropi are the species most commonly encountered in the clinical setting. Ochrobactrum intermedium is phenotypically indistinguishable from O. anthropi. The other bacteria have rarely been discovered in clinical material, and several have never been established as the cause of human infection.
TABLE 23-1
| Species | Habitat (Reservoir) | Mode of Transmission |
| “Achromobacter” group | Uncertain, probably environmental; may be part of endogenous flora of the ear and gastrointestinal tract | Unknown Nosocomial infections related to contaminated disinfectants, dialysis fluids, saline solution, and water |
| Rhizobium radiobacter | Environmental, soil and plants; not part of human flora | Contaminated medical devices, such as intravenous and peritoneal catheters |
| CDC group EF-4b | Animal oral and respiratory flora; not part of human flora | Animal contact, particularly bites or scratches from dogs and cats |
| Paracoccus yeei | Environmental; not part of human flora | Identified in human peritonitis |
| Psychrobacter immobilis | Environmental, particularly cold climates such as the Antarctic; not part of human flora | Unknown. Rarely found in humans. Has been found in fish, poultry, and meat products |
| CDC group OFBA-1 | Uncertain, probably environmental; not part of human flora | Unknown. Rarely found in humans |
| Ochrobactrum anthropi | Uncertain, probably environmental; found in water and hospital environments; may also be part of human flora | Uncertain. Most likely involves contaminated medical devices, such as catheters or other foreign bodies, or contaminated pharmaceuticals. Also can be acquired in community by puncture wounds |
| Shewanella putrefaciens Shewanella algae |
Environmental and foods; not part of human flora | Unknown, rarely found in humans Isolated from abscesses and wounds |
Pathogenesis and Spectrum of Disease
Because these organisms rarely cause human infections, little is known about what, if any, virulence factors they may produce to facilitate infectivity (Table 23-2). The fact that R. radiobacter and O. anthropi infections frequently involve contaminated medical materials and immunocompromised patients, and rarely, if ever, occur in healthy hosts, suggests that these bacteria have relatively low virulence. One report suggests that R. radiobacter is capable of capsule production. The ability of O. anthropi to adhere to the silicone material of catheters may contribute to this organism’s propensity to cause catheter-related infections. No known virulence factors have been described for CDC group EF-4b. Infection appears to require traumatic introduction by a puncture wound, bite, or scratch, which indicates that the organism itself does not express any invasive properties.
TABLE 23-2
Pathogenesis and Spectrum of Disease
| Species | Virulence Factors | Spectrum of Disease and Infections |
| “Achromobacter” group | Unknown | Rarely isolated from humans. Isolates have been recovered from wounds, blood, respiratory and gastrointestinal tract. |
| Rhizobium radiobacter | Unknown. One blood isolate described as mucoid, suggestive of exopolysaccharide capsule production. | Exposure of immunocompromised or debilitated patient to contaminated medical devices resulting in bacteremia and, less commonly, peritonitis, endocarditis, or urinary tract infection. |
| CDC group EF-4b | Unknown | Infected bite wounds of fingers, hands, or arm leading to cellulitis or abscess formation. Systemic infections are rare. |
| Paracoccus yeei | Unknown | No infections described in humans. Rarely encountered in clinical specimens. |
| Psychrobacter immobilis | Unknown | Rare cause of infection in humans. Has been described in wound and catheter site infections, meningitis, and eye infections. |
| CDC group OFBA-1 | Unknown | Rarely isolated from clinical specimens; found in blood, respiratory, wound, and catheter specimens. |
| Ochrobactrum anthropi | Unknown. Exhibits ability to adhere to silicone catheter material in a manner similar to staphylococci. | Catheter- and foreign body–associated bacteremia. May also cause pyogenic infections, community-acquired wound infections, and meningitis in tissue graft recipients. Patients are usually immunocompromised or otherwise debilitated. |
| Shewanella putrefaciens | Unknown | Clinical significance uncertain; often found in mixed cultures. Has been implicated in cellulites, otitis media, and septicemia; also may be found in respiratory tract, urine, feces and pleural fluid |
For both R. radiobacter and O. anthropi, bacteremia is the most common type of infection (see Table 23-2); peritonitis, endocarditis, meningitis, urinary tract, and pyogenic infections are much less commonly encountered. R. radiobacter is frequently isolated from blood, peritoneal dialysate, urine, and ascitic fluid. Cellulitis and abscess formation typify the infections resulting from the traumatic introduction of CDC group EF-4b into the skin and subcutaneous tissue.
Although other species listed in Table 23-2 may be encountered in clinical specimens, their association with human infection is rare, and their clinical significance in such encounters should be carefully analyzed.
Laboratory Diagnosis
Specimen Collection and Transport
No special considerations are required for specimen collection and transport of the organisms discussed in this chapter. Refer to Table 5-1 for general information on specimen collection and transport.
Direct Detection Methods
Other than Gram staining, no specific procedures are required for direct detection of these organisms in clinical material. Ochrobactrum spp., CDC group OFBA-1, and CDC group Ic are slender, short to long rods, and CDC group O-3 are thin, medium to slightly long, curved rods with tapered ends, resembling a sickle. R. radiobacter is a short, pleomorphic rod. Psychrobacter immobilis, CDC group EF-4b, and Paracoccus yeei are coccobacilli. P. yeei has a characteristic O appearance on Gram staining (Figure 23-1). Shewanella putrefaciens organisms are long, short, or filamentous rods.
