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Eikenella and Similar Organisms

المؤلف:  Patricia M. Tille, PhD, MLS(ASCP)

المصدر:  Bailey & Scotts Diagnostic Microbiology

الجزء والصفحة:  13th Edition , p387-390

2026-05-21

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The organisms listed in Table 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.

Table1. Epidemiology, Pathogenesis, and Spectrum of Disease

This organism also is the “E,” for Eikenella, in the HACEK group of bacteria known to cause subacute bacterial endocarditis. 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 guide lines (Table 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.

Table2. Antimicrobial Therapy and Susceptibility Testing

LABORATORY DIAGNOSIS

SPECIMEN COLLECTION AND TRANSPORT

 No special considerations are required for specimen col lection and transport for the organisms discussed in this chapter

SPECIMEN PROCESSING

 No special considerations are required for processing of the organisms discussed in this chapter.

DIRECT DETECTION METHODS

 Other than Gram stain and microscopic examination, there are no specific procedures for the direct detection of these organisms in clinical material. E. corrodens is a slender, medium-length gram-negative, straight rod with rounded ends. Methylobacterium is a vacuolated, pale staining, short to medium-length gram-negative bacillus that may resist decolorization. Weeksella virosa and Bergeyella zoohelcum are medium to long gram-negative rods with parallel sides and rounded ends that may form “II-forms” (parallel sides) similar to the Sphingobacterium.

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

Table3. Colonial Appearance and Characteristics

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 4 outlines basic criteria useful for differentiating the genera discussed in this chapter.

Table4. Key Biochemical and Physiologic Characteristics

Comments Regarding Specific Organisms

 As previously indicated, Methylobacterium may be differentiated from other pink-pigmented, gram-negative rods by its ability to utilize acetate and its inability to grow at 42° C. Some strains of Methylobacterium weakly oxidize glucose and oxidize xylose.

The most recognizable feature of E. corrodens in culture is the distinctive bleachlike odor. The organism is asaccharolytic (does not utilize glucose or other carbohydrates). The organism is oxidase positive, catalase negative, reduces nitrate to nitrite, and hydrolyzes both ornithine and lysine.

Weeksella and Bergeyella are oxidase and catalase positive. A distinguishing feature of the two bacteria is that they are indole positive, an unusual characteristic for most nonfermentative bacteria. W. virosa is urease negative and B. zoohelcum is urease-positive, pyrrolidonyl aminopeptidase negative, and resistant to colistin. W. virosa will grow on selective media such as modified Thayer martin (MTM) for Neisseria gonorrhoeae but can differentiated from the gonococci using indole and Gram-stain morphology.

SERODIAGNOSIS

Serodiagnostic techniques are not generally used for the laboratory diagnosis of infections caused by the organ isms discussed in this chapter.

PREVENTION

Because these organisms do not generally pose a threat to human health, there are no recommended vaccination or prophylaxis protocols.