Vibrio, Aeromonas, Chromobacterium, and Related Organisms

Objectives

1. Describe the general characteristics of the organisms discussed in this chapter, including natural habitat, route of transmission, Gram stain reactions, and cellular morphology.

2. Describe the media used to isolate Vibrio spp. and the organisms’ colonial appearance.

3. Explain the physiologic activity of the cholera toxin and its relationship to the pathogenesis of the organism.

4. Describe the clinical significance of Aeromonas spp., Chromobacterium sp., and Vibrio spp. other than Vibrio cholerae.

5. Correlate the patient’s signs and symptoms and laboratory data to identify an infectious agent.

Genera and Species to Be Considered

Current Name	Previous Name
Aeromonas caviae complex A caviae A media
Aeromonas hydrophila complex A hydrophila subsp. hydrophila A hydrophila subsp. dhakensis A bestiarum A salmonicida
Aeromonas veronii complex A veronii biovar sobria A veronii biovar veronii A jandaei A schubertii
Chromobacterium violaceum
Photobacterium damselae	Vibrio damsela
Grimontia hollisae	CDC group EF-13; Vibrio hollisae
Vibrio alginolyticus	Vibrio parahaemolyticus biotype 2
Vibrio cholerae
Vibrio cincinnatiensis
Vibrio fluvialis	CDC group EF-6
Vibrio furnissii
Vibrio harveyi	Vibrio carchariae
Vibrio metschnikovii	CDC enteric group 16
Vibrio mimicus	Vibrio cholerae (sucrose negative)
Vibrio parahaemolyticus	Pasteurella parahaemolyticus
Vibrio vulnificus	CDC group EF-3

General Characteristics

The organisms discussed in this chapter are considered together because they are all oxidase-positive, glucose-fermenting, gram-negative bacilli capable of growth on MacConkey agar. Their individual morphologic and physiologic features are presented later in this chapter in the discussion of laboratory diagnosis. Other halophilic organisms, such as Halomonas venusta and Shewanella algae, require salt but do not ferment glucose, as do the halophilic Vibrio spp.

Aeromonas spp. are gram-negative straight rods with rounded ends or coccobacillary facultative anaerobes that occur singly, in pairs, or in short chains. They are typically oxidase and catalase positive and produce acid from oxidative and fermentative metabolism. Chromobacterium violaceum is a facultative anaerobic, motile, gram-negative rod or cocci.

The family Vibrionaceae includes six genera, three of which are discussed in this chapter. The Photobacterium and Grimontia each include a single species. The genus Vibrio consists of 10 species of gram-negative, facultative anaerobic, curved or comma-shaped rods. Most species are motile and are catalase and oxidase positive except Vibrio metschnikovii. All Vibrio spp. require sodium for growth and ferment glucose.

Many aspects of the epidemiology of Vibrio spp., Aeromonas spp., and C. violaceum are similar (Table 26-1). The primary habitat for most of these organisms is water; generally, brackish or marine water for Vibrio spp., freshwater for Aeromonas spp., and soil or water for C. violaceum. Aeromonas spp. may also be found in brackish water or marine water with a low salt content. None of these organisms are considered part of the normal human flora. Transmission to humans is by ingestion of contaminated water, fresh produce, meat, dairy products, or seafood or by exposure of disrupted skin and mucosal surfaces to contaminated water.

TABLE 26-1

Epidemiology

Species	Habitat (Reservoir)	Mode of Transmission
Vibrio cholerae	Niche outside of human gastrointestinal tract between occurrence of epidemics and pandemics is uncertain; may survive in a dormant state in brackish or saltwater; human carriers also are known but are uncommon	Fecal-oral route, by ingestion of contaminated washing, swimming, cooking, or drinking water; also by ingestion of contaminated shellfish or other seafood
V. alginolyticus	Brackish or saltwater	Exposure to contaminated water
V. cincinnatiensis	Unknown	Unknown
Photobacterium damsela	Brackish or saltwater	Exposure of wound to contaminated water
V. fluvialis	Brackish or saltwater	Ingestion of contaminated water or seafood
V. furnissii	Brackish or saltwater	Ingestion of contaminated water or seafood
Grimontia hollisae	Brackish or saltwater	Ingestion of contaminated water or seafood
V. metschnikovii	Brackish, salt and freshwater	Unknown
V. mimicus	Brackish or saltwater	Ingestion of contaminated water or seafood
V. parahaemolyticus	Brackish or saltwater	Ingestion of contaminated water or seafood
V. vulnificus	Brackish or saltwater	Ingestion of contaminated water or seafood
Aeromonas spp.	Aquatic environments around the world, including freshwater, polluted or chlorinated water, brackish water and, occasionally, marine water; may transiently colonize gastrointestinal tract; often infect various warm- and cold-blooded animal species	Ingestion of contaminated food (e.g., dairy, meat, produce) or, water; exposure of disrupted skin or mucosal surfaces to contaminated water or soil; traumatic inoculation of fish fins and or fishing hooks
Chromobacterium violaceum	Environmental, soil and water of tropical and subtropical regions. Not part of human flora	Exposure of disrupted skin to contaminated soil or water

The epidemiology of the most notable human pathogen in this chapter, Vibrio cholerae, is far from being fully understood. This organism causes epidemics and pandemics (i.e., worldwide epidemics) of the diarrheal disease cholera. Since 1817 the world has witnessed seven cholera pandemics. During these outbreaks the organism is spread among people by the fecal-oral route, usually in environments with poor sanitation.

The niche that V. cholerae inhabits between epidemics is uncertain. The form of the organism shed from infected humans is somewhat fragile and cannot survive long in the environment. However, evidence suggests that the bacillus has survival, or dormant, stages that allow its long-term survival in brackish water or saltwater environments during interepidemic periods. These dormant stages are considered viable but nonculturable. Asymptomatic carriers of V. cholerae have been documented, but they are not thought to be a significant reservoir for maintaining the organism between outbreaks.

Pathogenesis and Spectrum of Disease

As a notorious pathogen, V. cholerae elaborates several toxins and factors that play important roles in the organism’s virulence. Cholera toxin (CT) is primarily responsible for the key features of cholera (Table 26-2). Release of this toxin causes mucosal cells to hypersecrete water and electrolytes into the lumen of the gastrointestinal tract. The result is profuse, watery diarrhea, leading to dramatic fluid loss. The fluid loss results in severe dehydration and hypotension that, without medical intervention, frequently lead to death. This toxin-mediated disease does not require the organism to penetrate the mucosal barrier. Therefore, blood and the inflammatory cells typical of dysenteric stools are notably absent in cholera. Instead, “rice water stools,” composed of fluids and mucous flecks, are the hallmark of cholera toxin activity.

TABLE 26-2

Pathogenesis and Spectrum of Diseases

Species	Virulence Factors	Spectrum of Disease and Infections
Vibrio cholerae	Cholera toxin; zonula occludens (Zot) toxin (enterotoxin); accessory cholera enterotoxin (Ace) toxin; O1 and O139 somatic antigens, hemolysin/cytotoxins, motility, chemotaxis, mucinase, and toxin coregulated pili (TCP) pili.	Cholera: profuse, watery diarrhea leading to dehydration, hypotension, and often death; occurs in epidemics and pandemics that span the globe. May also cause nonepidemic diarrhea and, occasionally, extra intestinal infections of wounds, respiratory tract, urinary tract, and central nervous system
V. alginolyticus	Specific virulence factors for the non–V. cholerae species are uncertain.	Ear infections, wound infections; rare cause of septicemia; involvement in gastroenteritis is uncertain
V. cincinnatiensis		Rare cause of septicemia
P. damsela		Wound infections and rare cause of septicemia
V. fluvialis		Gastroenteritis
V. furnissii		Rarely associated with human infections
Grimontia hollisae		Gastroenteritis; rare cause of septicemia
V. metschnikovii		Rare cause of septicemia; involvement in gastroenteritis is uncertain
V. mimicus		Gastroenteritis; rare cause of ear infection
V. vulnificus		Wound infections and septicemia; involvement in gastroenteritis is uncertain
Aeromonas spp.	Aeromonas spp. produce various toxins and factors, but their specific role in virulence is uncertain.	Gastroenteritis, wound infections, bacteremia, and miscellaneous other infections, including endocarditis, meningitis, pneumonia, conjunctivitis, and osteomyelitis
Chromobacterium violaceum	Endotoxin, adhesins, invasins and cytolytic proteins have been described.	Rare but dangerous infection. Begins with cellulitis or lymphadenitis and can rapidly progress to systemic infection with abscess formation in various organs and septic shock

V. cholerae is divided into three major subgroups; V. cholerae O1, V. cholerae O129, and V. cholerae non-O1. The somatic antigens O1 and O139 associated with the V. cholerae cell envelope are positive markers for strains capable of epidemic and pandemic spread of the disease. Strains carrying these markers almost always produce cholera toxin, whereas non-O1/non-O139 strains do not produce the toxin and hence do not produce cholera. Therefore, although these somatic antigens are not virulence factors per se, they are important virulence and epidemiologic markers that provide important information about V. cholerae isolates. The non-O1/non-O139 strains are associated with nonepidemic diarrhea and extraintestinal infections.

V. cholerae produces several other toxins and factors, but the exact role of these in disease is still uncertain (see Table 26-2). To effectively release toxin, the organism first must infiltrate and distribute itself along the cells lining the mucosal surface of the gastrointestinal tract. Motility and chemotaxis mediate the distribution of organisms, and mucinase production allows penetration of the mucous layer. Toxin coregulated pili (TCP) provide the means by which bacilli attach to mucosal cells for release of cholera toxin.

Depending on the species, other vibrios are variably involved in three types of infection: gastroenteritis, wound infections, and bacteremia. Although some of these organisms have not been definitively associated with human infections, others, such as Vibrio vulnificus, are known to cause fatal septicemia, especially in patients suffering from an underlying liver disease.

Aeromonas spp. are similar to Vibrio spp. in terms of the types of infections they cause. Although these organisms can cause gastroenteritis, most frequently in children, their role in intestinal infections is not always clear. Therefore, the significance of their isolation in stool specimens should be interpreted with caution. Severe watery diarrhea has been associated with Aeromonas strains that produce a heat-labile enterotoxin and a heat-stable enterotoxin. In addition to diarrhea, complications of infection with Aeromonas spp. include hemolytic-uremic syndrome and kidney disease.

C. violaceum is not associated with gastrointestinal infections, but acquisition of this organism by contamination of wounds can lead to fulminant, life-threatening systemic infections.

Laboratory Diagnosis

Specimen Collection and Transport

Because no special considerations are required for isolation of these genera from extraintestinal sources, the general specimen collection and transport information provided in Table 5-1 is applicable. However, stool specimens suspected of containing Vibrio spp. should be collected and transported only in Cary-Blair medium. Buffered glycerol saline is not acceptable, because glycerol is toxic for vibrios. Feces is preferable, but rectal swabs are acceptable during the acute phase of diarrheal illness.

Specimen Processing

No special considerations are required for processing of the organisms discussed in this chapter. Refer to Table 5-1 for general information on specimen processing.

Direct Detection Methods

V. cholerae toxin can be detected in stool using an enzyme-linked immunosorbent assay (ELISA) or a commercially available latex agglutination test (Oxoid, Inc., Odgensburg, New York), but these tests are not widely used in the United States.

Microscopically, vibrios are gram-negative, straight or slightly curved rods (Figure 26-1). When stool specimens from patients with cholera are examined using dark-field microscopy, the bacilli exhibit characteristic rapid darting or shooting-star motility. However, direct microscopic examination of stools by any method is not commonly used for laboratory diagnosis of enteric bacterial infections.