GENERAL CHARACTERISTICS

The organisms discussed in this chapter are environmental inhabitants that are occasionally encountered in human specimens. Most of the organisms originated in the heterogenous group Flavobacterium. However, when subjected to molecular analysis, they did not prove to be closely related and therefore have been reclassified. They are considered together here because they share similar physiologic and morphologic characteristics. Most are yellow-pigmented, oxidase-positive, glucose oxidizers that grow on MacConkey agar. Sphingobacterium spp. have an unusually large amount of sphingophospholipid com pounds in their cell membranes. Sphingobacterium mizutaii, which does not grow on MacConkey agar.

EPIDEMIOLOGY

 As environmental inhabitants, these organisms may be found in various niches (Table 1). Most notable in terms of clinical relevance is their ability to survive in hospital environments, especially in moist areas. Although they are not considered part of normal human flora, these species can colonize a patient’s respiratory tract during hospitalization. This results from exposure to contaminated water or medical devices. Transmission also may occur directly from contaminated pharmaceutical solutions and, in the case of E. meningoseptica, from person to person.

Table1. Epidemiology

Because of their ability to survive well in hospital environments, these organisms have the potential to contaminate laboratory culture media and blood culture systems. Whenever these species are encountered, their clinical significance and the potential for contamination should be seriously considered.

PATHOGENESIS AND SPECTRUM OF DISEASE

As environmental organisms, no specific virulence factors have been identified for these species. However, the ability to survive in chlorinated tap water may give these organisms an edge in their ability to thrive in hospital water systems.

The development of infection basically requires exposure of debilitated patients to a contaminated source, resulting in respiratory colonization (Table 2). Depending on the patient’s health, subsequent infections, such as bacteremia and pneumonia, may develop. These infections are most frequently caused by Elizabethkingia meningoseptica or Myroidesodoratus. Infections of several other body sites, which may or may not be pre ceded by respiratory colonization, have been associated with the other species.

Table2. Pathogenesis and Spectrum of Diseases

Meningitis caused by E. meningoseptica is the most notable infection associated with the organisms listed in Table 2. This life-threatening infection, which may be accompanied by bacteremia, originally gained attention because it occurred in neonates. However, E. meningoseptica meningitis can also occur in compromised adults.

The organism has been implicated in hospital-based out breaks of both meningitis and pneumonia.

LABORATORY DIAGNOSIS

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

SPECIMEN PROCESSING

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

DIRECT DETECTION METHODS

Gram staining is used to detect these organisms in clinical material. The Chryseobacterium spp., E. meningoseptica, and CDC group IIb are medium to long straight rods that often appear as “II-forms” (i.e., cells that appear thin in the center and thicker at the ends). Empedobacter brevis varies in being short to long rods. Sphingobacterium spp. are short straight rods, S. thalpophilum may exhibit II-forms. Rhizobium radiobacter yellow group are slender, medium to long, gram-negative rods. CDC groups EO-3 and EO-4 are coccobacilli, and CDC groups O-1, O-2, and O-3 are short, gram-negative, curved rods.

CULTIVATION

Media of Choice

All genera and CDC groups in this chapter grow well on routine laboratory media such as 5% sheep blood and chocolate agars. They also grow well in the broth of blood culture systems and in common nutrient broths such as thioglycollate and brain-heart infusion.

 Incubation Conditions and Duration

These organisms will produce detectable growth on blood and chocolate agars when incubated at 35°C in either carbon dioxide or ambient air for a minimum of 24 hours. Growth on MacConkey agar is usually detectable within 24 hours of inoculation.

Colonial Appearance

 Table 3 presents descriptions of the colonial appearance and other distinguishing characteristics of each genus on 5% sheep blood and MacConkey agars.

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 uncertain. The key biochemical reactions used to presumptively differentiate among the genera discussed in this chapter are provided in Table 4. However, definitive identification of these organisms often requires a battery of biochemical tests not commonly available in many clinical microbiology laboratories. Therefore, full identification of clinically relevant isolates may require that they be sent to a reference laboratory.

Table4. Key Biochemical and Physiologic Characteristics

Comments Regarding Specific Organisms

The growth of Sphingobacterium spiritivorum and Chryseobacterium spp. is variable on MacConkey agar. Therefore, these organisms often need to be differentiated from the yellow-pigmented, MacConkey-negative, oxidase-positive genera.

Indole and urea hydrolysis are key biochemical tests for distinguishing E. brevis, E. meningoseptica, and Chryseobacterium spp. from Sphingobacterium spp.

SERODIAGNOSIS

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

ANTIMICROBIAL SUSCEPTIBILITY TESTING AND THERAPY

Validated susceptibility testing methods do not exist for these organisms. Although they grow on the media and under the conditions recommended for testing, the ability to grow and the ability to detect important antimicrobial resistances are not the same. Therefore, the lack of validated in vitro susceptibility testing methods does not allow definitive treatment and testing guidelines to be given for any of the organisms listed in Table 5.

Table5. Antimicrobial Therapy and Susceptibility Testing

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 such data. Review Chapter 12 for preferable strategies that can be used to provide susceptibility information and data when validated testing methods do not exist for a clinically important bacterial isolate.

In general, the species considered in this chapter are frequently resistant to β-lactams (including penicillins, cephalosporins, and carbapenems) and aminoglycosides commonly used to treat infections caused by other gram negative bacilli. However, the susceptibility data can vary substantially with the type of testing method used. An unusual feature of many of these species is that they often appear susceptible to, and may be treated with, antimicrobial agents that are usually considered effective against gram-positive bacteria; clindamycin, rifampin, and vancomycin are notable examples.

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 through the use of appropriate sterile technique, infection control, and implementation of effective protocols for sterilization and decontamination of medical supplies.

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مواضيع ذات صلة

Pseudomonas, Burkholderia, and Similar Organisms : Epidemiology

Acinetobacter, Stenotrophomonas, and Similar Organisms : Pathogenesis and Spectrum of Disease

Nocardia, Streptomyces, Rhodococcus, and Similar Organisms: Antimicrobial Susceptibility Testing and Therapy

Erysipelothrix, Lactobacillus, and Similar Organisms : Antimicrobial Susceptibility Testing and Therapy

Erysipelothrix, Lactobacillus, and Similar Organisms : Approach to Identification

Staphylococcus, Micrococcus, and Similar Organisms : Pathogenesis and Spectrum of Disease

Strategies to Control Bacteria at the Environmental Level

Definition and Measurement of Bacterial Death

Growth in Biofilms

Morphologic Changes During Growth

Staining

Endospores