Eikenella and Similar Organisms

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Eikenella and Similar Organisms

Genera and Species to Be Considered

Current Name Previous Name
Eikenella corrodens  
Methylobacterium spp. Pseudomonas mesophilica, Pseudomonas extorquens, Vibrio extorquens
Weeksella virosa CDC group IIf
Bergeyella zoohelcum Weeksella zoohelcum, CDC group IIj

Epidemiology, Spectrum of Disease, and Antimicrobial Therapy

The organisms listed in Table 29-1 are not commonly associated with human infections, but they are occasionally encountered in clinical specimens. Eikenella corrodens is normal flora of the human oral cavity. The organism is a facultative anaerobe, nonmotile, gram-negative rod. Among the organisms considered in this chapter, it is the organism most frequently isolated and is usually found in mixed infections resulting from human bites or clenched-fist wounds. The organism can be isolated from dental plaque and has been implicated in periodontitis, osteomyelitis, bite wound infections, bacteremia, and endocarditis. It is an opportunistic pathogen predominantly in immunocompromised patients, causing abscesses and infections, and may lead to death. Patients with diabetes are often at risk for Eikenella infections as a result of the daily microtrauma to their skin via glucose monitoring, insulin injections, and the potential for introduction of the organism from oral secretions by licking or biting their skin. The organism is often the cause of soft tissue infections in intravenous drug abusers who lick the injection site.

TABLE 29-1

Epidemiology, Pathogenesis, and Spectrum of Disease

Organism Habitat (Reservoir) Mode of Transmission Virulence Factors Spectrum of Disease and Infections
Eikenella corrodens Normal human flora of mouth and gastrointestinal tract Person to person involving trauma associated with human teeth incurred during bites or clenched-fist wounds incurred as a result of facial punches; infection may be a result of the patient’s endogenous strains (e.g., endocarditis) Unknown; opportunistic organism usually requires trauma for introduction into normally sterile sites; also may enter bloodstream to cause transient bacteremia or be introduced by intravenous drug abuse Human bite wound infections, head and neck infections, and aspiration pneumonias as part of mixed infection; can also cause endocarditis that is slow to develop and indolent (i.e., sub acute); less commonly associated with brain and intra-abdominal abscesses
Methylobacterium spp. Found on vegetation and occasionally in the hospital environment; not considered normal human flora Uncertain; probably involves contaminated medical devices such as catheters Unknown; an opportunistic organism probably of low virulence
Uncommon cause of infection
Bacteremia and peritonitis in patients undergoing chronic ambulatory peritoneal dialysis (CAPD)
Weeksella virosa Uncertain; probably environmental; not considered normal human flora Uncertain; rarely found in clinical material Unknown; role in human disease is uncertain Asymptomatic bacteruria; also isolated from female genital tract
Bergeyella zoohelcum Normal oral flora of dogs and other animals; not considered normal human flora Bite or scratch of dog or cat Unknown; an opportunistic organism that requires traumatic introduction to normally sterile site Dog and cat bite wound infections


This organism also is the “E,” for Eikenella, in the HACEK group of bacteria known to cause subacute bacterial endocarditis (see Chapter 68 for more information regarding endocarditis and bloodstream infections). HACEK is an acronym used to represent the slow-growing gram-negative bacilli associated with endocarditis. The additional members of the HACEK group of bacteria include Aggregatibacter aphrophilus, Actinobacillus actinomycetemcometans, Cardiobacterium hominis, and Kingella kingae.

Methylobacterium sp. bacteria are gram-negative bacilli predominantly found in water and soil. There are currently 20 recognized species. They can be opportunistic pathogens but are considered to be of low virulence as most human infections are associated with immunocompromised patients. M. mesophilicum and M. zatmanii are the two species most commonly isolated from clinical samples. Methylobacterium spp. are chlorine resistant and have been isolated from water-distribution systems.

The rarity with which these organisms are encountered in the clinical laboratory and the lack of validated in vitro susceptibility testing methods do not provide enough data to recommend definitive treatment guidelines (Table 29-2). Although ß-lactamase production has been described in E. corrodens, this species is usually susceptible to penicillin and other ß-lactam antimicrobials. Penicillin-resistant strains of E. corrodens have been identified.

TABLE 29-2

Antimicrobial Therapy and Susceptibility Testing

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Organism Potential Resistance to Therapeutic Options Therapeutic Options Validated Testing Methods*
Eikenella corrodens Often susceptible to penicillins, quinolones, cephalosporins, and trimethoprim-sulfamethoxazole May produce beta-lactamases; usually resistant to clindamycin, metronidazole, and aminoglycosides See CLSI document M45, section on “HACEK” organisms
Methylobacterium spp. No guidelines Unknown Not available
Weeksella virosa and Bergeyella zoohelcum No guidelines; potentially active agents include beta-lactams and quinolones Susceptibility to tetracycline, aminoglycosides, and trimethoprim-sulfamethoxazole