Bacteria belonging to these three genera (together with others such as the genus Helicobacter) are morphologically similar, being Gram-negative curved bacilli. They are enteric pathogens of humans or part of the normal flora. Because of their unusual growth requirements (formate and fumarate needed), they have to be cultured in special media.
Vibrios
The genus Vibrio includes two important human pathogens, but their natural habitat is water. Vibrio cholerae causes cholera, whereas Vibrio parahaemolyticus causes a less severe diarrhoea. The main symptom of cholera is watery diarrhoea that can be fatal as a result of severe dehydration, water and electrolyte loss.
Vibrio cholerae
Habitat and transmission
The habitat is water contaminated with faeces of patients or carriers; there is no animal reservoir. A life-threatening, watery diarrhoea (rice-water stools) is the characteristic disease.
Characteristics
Gram-negative slender bacilli, comma-shaped with pointed ends. Highly motile by means of a single polar flagellum. May be seen directly in stool samples by dark-field microscopy.
Culture and identification
Grows in alkaline conditions (pH 8.5-9.2 approximately): selective media for culture such as thiosulphate citrate-bile salts-sucrose agar (TCBS) medium are based on this property. This, together with biochemical tests and serology, helps identification. Serotyping is based on the somatic O antigens. All diarrhoea-producing strains of V. cholerae are designated as O1 and are subdivided into three major serotypes: the Ogawa, Inaba and El Tor strains.
Pathogenicity
V. cholerae has the ability to colonize the intestinal tract in very high numbers and about 108 cells per millilitre are seen in patients' faeces. The cells attach to but do not invade the intestinal mucosa. Pathogenicity is due to secretion of an enterotoxin, which binds to ganglioside receptors on mucosal cells. After a lag period of 15-45 min, adenylate cyclase is activated and the cyclic adenosine monophosphate concentration inside the intestinal cells increases. This in turn leads to excretion of electrolytes and water and subsequent diarrhoea, leading to severe dehydration.
Treatment and prevention
Intravenous administration of fluids and electrolytes is essential for recovery. Oral administration of a solution containing glucose and electrolytes (oral rehydration therapy) is successful, but the patient must be capable of consuming the liquid by mouth. Severely ill patients are generally too weak to ingest fluids. Antibiotics (usually tetracycline) do not affect the disease outcome once the enterotoxin attaches to the intestinal cells, but they prevent subsequent attacks by reducing the number of toxin-producing V. cholerae cells in the intestine.
Immunization with a whole-cell vaccine is of limited use. New vaccines are under development.
Vibrio parahaemolyticus
This vibrio requires a relatively high salt concentration for growth and is distributed worldwide in marine environments, for example, in South-East Asia. A common agent of acute enteritis associated with the consumption of improperly cooked seafood, it accounts for about half of all cases of food poisoning in Japan.
There is no specific treatment for diarrhoea. The best control measure is the consumption of only thoroughly cooked seafood.
Campylobacters
The genus Campylobacter contains medically important species that are important human pathogens, once classified as vibrios. Campylobacter jejuni is the major human pathogenic species; Campylobacter rectus has been isolated from active periodontal disease sites and has been implicated as a periodontopathogen.
Campylobacter spp.
Habitat and transmission
The natural reservoir is animals. Organisms are acquired from contaminated food and milk.
Characteristics
Curved, seagull-shaped, Gram-negative rods; mobile with a single polar flagellum.
Culture and identification
C. jejuni grows best under microaerophilic (i.e., an environment of 10% oxygen and 10% carbon dioxide) and thermophilic (a temperature of 43°C) conditions in an enriched medium. Further identification is by biochemical tests and antibiotic- susceptibility patterns.
Pathogenicity
Gastroenteritis, especially in children, is the most common human infection caused by Campylobacter species. It resembles dysentery and is usually self-limiting, but may last for several days. The heat-labile enterotoxin of Campylobacter fetus is implicated. Campylobacters may occasionally cause bacteraemia, meningitis, endocarditis, arthritis and urinary tract infection.
C. jejuni has been implicated as the aetiological agent of Guil- lain-Barré syndrome. Some strains of C. rectus isolated from periodontal disease sites produce a cytotoxin similar to that of Aggregatibacter actinomycetemcomitans and stimulate human gingival fibroblasts to produce interleukin-6 and interleukin-8.
Treatment and prevention
No specific therapy is necessary for the mild diarrhoea. Good food and hand hygiene are important.
Helicobacter pylori
This organism (previously classified as a campylobacter) causes a significant proportion of gastritis and duodenal ulcers in
humans; it may play a role in gastric cancer. Antimicrobial therapy eradicates the bacteria from the stomach and resolves many of the ulcers that were formerly thought to be due to gastric acidity. A few studies have demonstrated the presence of this organism, albeit in small numbers, in human supragingival plaque.
Wolinella
Members of the genus Wolinella are curved or helical Gramnegative motile rods. Its darting motility is due to a polar flagellum; they are anaerobes and require formate and fumarate for growth. The main species is Wolinella succinogenes.
Habitat
These organisms are frequently isolated from the oral cavity, especially the gingival sulcus.
Culture and identification
A selective medium is available for culturing the organism from plaque samples. Identification is by colonial characteristics (dry, spreading or corroding colonies), whole-cell protein profiles and serology.
Pathogenicity
Although some studies have shown a high correlation between periodontal disease activity and isolation of Wolinella spp., the pathogenic role is not clear. The organisms can induce alveolar bone loss in gnotobiotic rats. A possible periodontal pathogen.
Key facts
• Vibrios are small, comma-shaped, Gram-negative, oxidase-positive bacteria that prefer an alkaline growth environment.
• Vibrio cholerae is the major pathogen in the genus and is responsible for cholera epidemics, especially in the developing world.
• Campylobacter jejuni is a thermophilic, microaerophilic vibrio that causes human diarrhoeal illness.
• Helicobacter pylori causes a significant proportion of gastritis and duodenal ulcers in humans, and may play a role in gastric cancer.
Review questions (answers on p. 365)
Please indicate which answers are true, and which are false.
16.1 Vibrio cholerae:
A. are Gram-negative, highly motile slightly curved rods
B. grow well in alkaline media
C. pathogenicity is by means of invasion of the intestinal mucosa
D. cause dysentery
E. whole-cell vaccine is effective in preventing the disease
16.2 Wolinella spp.:
A. are often isolated from plaque biofilms
B. are major periodontal pathogens
C. are implicated in gastritis
D. require folate and fumarate for growth
E. form dry spreading colonies
16.3 Campylobacter spp.:
A. are implicated in food poisoning
B. are isolated from active sites of periodontal infection
C. grow best under strict anaerobic conditions
D. are thermophilic
E. are Gram-negative curved bacilli
Further reading
Brooks, J. F., Carroll, K. C., Butel, J. S., et al. (Eds.), (2013). Enteric Gram negative rods (Vibrios, Campylobacters, Helicobacter and associated bacteria) (Chapter 17). In Jawetz, Melnick & Adelberg's medical microbiology (26th ed., pp. 255-263). New York: McGraw Hill. [e-Book].
Macuch, P. J., & Tanner, A. C. (2000). Campylobacter species in health, gingivitis, and periodontitis. Journal of Dental Research, 79, 785-792.