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

General Characteristics

The organisms discussed in this chapter are considered together because they are all asaccharolytic, oxidase-positive bacilli that fail to grow on MacConkey agar. Their individual morphologic and physiologic features are presented later in this chapter.

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

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	Not available

^*Validated testing methods include those standard methods recommended by the Clinical and Laboratory Standards Institute (CLSI) and those commercial methods approved by the Food and Drug Administration (FDA).

Laboratory Diagnosis

Cultivation

Media of Choice

Because it is a facultative anaerobe, Eikenella corrodens grows slowly on blood and chocolate agar with small colonies developing within 48 hours. The organism will not grow on MacConkey agar. The organism also displays limited growth in blood culture broth media, thioglycollate broth, and brain-heart infusion broth. The hallmark characteristics for the presence of E. corrodens in culture include the organism’s tendency to pit or corrode the agar, demonstrate a slightly yellow hue after several days, and exude a chlorine bleach odor. Most strains require hemin for growth unless incubated in 5% to 10% CO₂. Detection may be improved using selective media containing clindamycin.

Methylobacterium is also difficult to grow on routine laboratory media producing small colonies in 4 to 5 days on sheep blood agar, modified Thayer-Martin, buffered charcoal-yeast extract, and Middlebrook 7H11 agar. Reports have indicated that improved growth may be achieved using BYCE agar and Sabouraud agar. As previously indicated, the organism is not capable of growth on MacConkey agar. Optimal growth occurs at 15° to 30° C. Methylobacterium produce small, dry, coral pink-pigmented colonies. Pink colonies are also produced by Roseomonas. The two genera can be differentiated by incubation at 42° C. Roseomonas is capable of growth at 42° C, whereas Methylobacterium is temperature sensitive and incapable of growth in increased temperatures. In addition, Methylobacterium can metabolize acetate, and Roseomonas cannot.

Incubation Conditions and Duration

To detect growth on 5% sheep blood and chocolate agars, incubation at 35° to 37° C in carbon dioxide for a minimum of 48 hours is required. In contrast to the other genera, Methylobacterium grows at lower temperatures, as previously indicated.

Colonial Appearance

Table 29-3 describes the colonial appearance and other distinguishing characteristics (e.g., odor and pigment) of each genus on 5% sheep blood agar.

TABLE 29-3

Colonial Appearance and Characteristics

Organism	Medium*	Appearance
Bergeyella zoohelcum	BA	Colonies may be sticky; tan to yellow in color
Eikenella corrodens	BA	Colonies are tiny at 24 hours; mature colonies have moist, clear centers surrounded by flat, spreading growth; colonies may pit or corrode the agar surface; slight yellow pigmentation in older cultures; sharp odor of bleach
Methylobacterium spp.	BA	Pink to coral pigment; does not grow well on blood agar
Weeksella virosa	BA	Small colonies at 24 hours; mature colonies mucoid and adherent with a tan to brown pigment

BA, 5% sheep blood agar.

^*These organisms usually do not grow on MacConkey agar; if breakthrough growth occurs, the organisms appear as non-lactose-fermenters.

Approach to Identification

The ability of most commercial identification systems to accurately identify the organisms discussed in this chapter is limited or, at best, uncertain. Therefore, strategies for identification of these genera are based on the use of conventional biochemical tests. Table 29-4 outlines basic criteria useful for differentiating the genera discussed in this chapter.

TABLE 29-4

Key Biochemical and Physiologic Characteristics

Organism	Catalase	Oxidizes Xylose	Indole	Arginine Dihydrolase
Eikenella corrodens	−	−	−	−
Methylobacterium spp.*	+	+	−	ND
Weeksella virosa	+	−	+	−
Bergeyella zoohelcum	+	−	+	+

ND, No data; +, >90% of strains positive; −, >90% of strains negative.

^*Colonies are pigmented pink and must be differentiated from Roseomonas spp.; Roseomonas spp. usually grow on MacConkey agar and will grow at 42° C.

Data compiled from Weyant RS, Moss CW, Weaver RE, et al, editors: Identification of unusual pathogenic gram-negative aerobic and facultatively anaerobic bacteria, ed 2, Baltimore, 1997, Williams & Wilkins.