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.

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.

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.

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.

Antimicrobial Susceptibility Testing and Therapy

No validated susceptibility testing methods are available for the organisms discussed in this chapter. Although many of these organisms grow on the media and under the conditions recommended for testing of more commonly encountered bacteria (see Chapter 12 for more information on validated testing methods), no standardized reference exists for antimicrobial resistance for these organisms. The lack of validated in vitro susceptibility testing methods does not allow definitive treatment and testing guidelines to be given for the organisms listed in Table 23-5. Although susceptibility data for some of these bacteria can be found in the literature, the lack of understanding of potential underlying resistance mechanisms prohibits the validation of the data. Review Chapter 12 for preferable strategies used to provide susceptibility information and data when validated testing methods do not exist for a clinically relevant bacterial isolate.

Because R. radiobacter and O. anthropi infections are frequently associated with implanted medical devices, therapeutic management of the patient often involves removal of the contaminated material. Although definitive antimicrobial therapies for these infections have not been established, in vitro data suggest that certain agents could be more effective than others (see Table 23-5). Most strains of R. radiobacter are susceptible to cephalosporins, carbapenems, tetracyclines, and gentamicin.

O. anthropi is commonly resistant to all currently available penicillins, cephalosporins, aztreonam, and amoxicillin-clavulanate but usually is susceptible to aminoglycosides, fluoroquinolones, imipenem, tetracycline, and trimethoprim-sulfamethoxazole. O. anthropi (colistin susceptible) may be differentiated from O. intermedium by colistin resistance. This resistance profile is sufficiently consistent with the species, making it potentially useful for confirming the organism’s identification. The organism may also appear susceptible to trimethoprim-sulfamethoxazole and ciprofloxacin, but antimicrobial therapy without removal of the contaminated medical device may not successfully eradicate the organism.

Prevention

Because these organisms are ubiquitous in nature and are not generally a threat to human health, no recommended vaccination or prophylaxis protocols have been established. Hospital-acquired infections are controlled by following appropriate sterile techniques, infection control guidelines and by implementing effective protocols for the sterilization and decontamination of medical supplies.