Rhizobium, Ochrobactrum, and Similar Organisms

Objectives

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.

Genera and Species to Be Considered

Current Name	Previous Name
CDC group EF-4b	CDC group EF-4
CDC group Ic
CDC group O-3
CDC group OFBA-1
Ochrobactrum anthropi Ochrobactrum intermedium	CDC group Vd1 -2
Paracoccus yeei	CDC group EO-2
Psychrobacter immobilis (saccharolytic strains)	Part of CDC group EO-2
Rhizobium radiobacter	Agrobacterium radiobacter, CDC group Vd-3
Shewanella putrefaciens Shewanella algae	Alteromonas putrefaciens, Achromobacter putrefaciens, CDC group Ib

*Quotation marks indicate a proposed organism name.

General Characteristics

Most of the organisms discussed in this chapter exist in the environment. CDC group EF-4b inhabits the upper respiratory tract of certain animals. Ochrobactrum anthropi may occasionally inhabit the human gastrointestinal tract. All are nonpigmented, oxidase positive, and oxidize glucose; most grow on MacConkey agar. However, their specific morphologic and physiologic features are somewhat diverse; these are considered later in this chapter in the discussion of laboratory diagnosis.

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

Epidemiology

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

R. radiobacter inhabits the soil, and human infections occur by exposure to contaminated medical devices.

The specific environmental niche of O. anthropi is unknown, but this organism is capable of survival in water, including moist areas in the hospital environment. The organism may also be a transient colonizer of the human gastrointestinal tract. Similar to R. radiobacter, human infections caused by O. anthropi are associated with implantation of intravenous catheters or other foreign bodies in patients with a debilitating illness. Acquisition by contaminated pharmaceuticals and by puncture wounds has also been documented.

The epidemiology of CDC group EF-4b is unlike that of the other bacteria discussed in this chapter. Animals, rather than the environment, are the reservoir, and transmission to humans occurs by dog or cat bites and scratches.

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.