Vibrio cholerae

Vibrio cholerae
TEM image
Scientific classification
Kingdom: Bacteria Phylum: Proteobacteria Class: Gamma Proteobacteria Order: Vibrionales Family: Vibrionaceae Genus: Vibrio Species: V. cholerae
Binomial name
Vibrio cholerae Pacini 1854

Vibrio cholerae is a gram negative bacterium with a curved-rod shape that causes cholera in humans. V. cholerae and other species of the genus Vibrio belong to the gamma subdivision of the Proteobacteria.  There are two major strains of V. cholerae, classic and El Tor, and numerous other serogroups.

V. cholerae was first isolated as the cause of cholera by Italian anatomist Filippo Pacini in 1854, but his discovery was not widely known until Robert Koch, working independently thirty years later, publicized the knowledge and the means of fighting the disease.

Habitat

V. cholerae occurs naturally in the plankton of fresh, brackish, and salt water, attached primarily to copepods in the zooplankton. Coastal cholera outbreaks typically follow zooplankton blooms. This makes cholera a typical zoonosis.

Pathogenesis

V. cholerae colonizes the gastrointestinal tract, where it adheres to villous absorptive cells via pili, and secretes a binary toxin, called cholera toxin (CT). The two CT subunits are named A and B, and are synthesised in a 1:5 ratio. B subunits bind and internalize A subunits, which are processed to A1. The A1 form catalyses ADP ribosylation from NAD to the regulatory component of adenylate cyclase, thereby activating it. Increased adenylate cyclase activity increases cyclic AMP (cAMP) synthesis causing massive fluid and electrolyte efflux, resulting in diarrhea.

CT is encoded by the ctxAB genes on a specific filamentous bacteriophage. Transduction of this phage is dependent upon bacterial expression of the Toxin Coregulated Pilus (TCP), which is encoded by the V. cholerae pathogenicity island (VPI). VPI is generally only present in virulent strains and is laterally transferred. VPI was originally thought to encode a filamentous phage responsible for transfer. This theory was discredited by a study of 46 diverse V. cholerae isolates which found no evidence of VPI phage production. The generalized transduction phage CP-T1 has been shown to transduce the entire VPI which is then integrated at the same chromosomal location. Also, VPI has been shown to excise and circularize to produce pVPI via a specialised mechanism involving VPI-encoded recombinases. It is not known whether pVPI is involved in CP-T1 transduction or if it is perhaps a component of an alternative VPI mobilization mechanism.

Additionally, it produces two different proteases called chitinase and mucinase. Chitinase is responsible for the ability of Vibrio cholerae to enter copapods. Mucinase is a non-specific protease that assists entry into the human gastro-intestinal tract.

Finally, Vibrio cholerae produces what is called a ZOT toxin, termed as "Zona Occludans Toxin". This toxin specifically attacks the zona occludans or "tight" junctions joining epithelial cells.

Treatment

Water and electrolyte replacement are necessary in treating cholera. In some cases, tetracycline may be used. A vaccine is available outside the US, but it is short-lived and not currently recommended by the CDC. Fluoroquinolones such as norfloxacin are used, but resistance has been reported..

Treatment

Antimicrobial regimen

• 1. WHO

• Note: Antibiotic treatment for cholera patients with severe dehydration only
• Adults

• Preferred regimen: Doxycycline 300 mg po single dose
• Alternative regimen: Tetracycline 12.5 mg/kg PO qid for 3 days

• Pediatric

• Under 12 years old

• Preferred regimen: Erythromycin 12.5 mg/kg PO qid for 3 days

• Over 12 years old

• Preferred regimen: Doxycycline 300 mg po single dose
• Alternative regimen: Tetracycline 12.5 mg/kg PO qid for 3 days

• 2. Pan American Health Organization

• Note: Antibiotic treatment for cholera patients with moderate or severe dehydration
• 2.1 Adult

• Preferred regimen: Doxycycline 300 mg po single dose
• Alternative regimen (1): Ciprofloxacin 1 g PO single dose
• Alternative regimen (2): Azithromycin 1 g PO single dose

• 2.2 Pediatric

• 2.2.1 Children over 3 year, who can swallow tablets

• Preferred regimen (1): Erythromycin 12.5 mg/kg/ PO qid for 3 days
• Preferred regimen (2): Azithromycin 20 mg/kg PO in a single dose
• Alternative regimen (1): Ciprofloxacin suspension or tablets 20 mg/kg PO single dose
• Alternative regimen (2): Doxycycline suspension or tablets 2-4 mg/kg PO single dose
• Note: Although doxycycline has been associated with a low risk of yellowing of the teeth in children, its benefits outweigh its risks

• 2.2.2 Children under 3 year, or infants who cannot swallow tablets

• Preferred regimen (1): Erythromycin suspension 12.5 mg/kg/ PO qid for 3 days
• Preferred regimen (2): Azithromycin suspension 20 mg/kg PO single dose
• Alternative regimen (1): Ciprofloxacin suspension 20 mg/kg PO single dose
• Alternative regimen (2): Doxycycline syrup 2-4 mg/kg PO single dose

• 2.3 Pregnancy

• Preferred regimen (!): Erythromycin 500 mg/ PO qid for 3 days
• Preferred regimen (2): Azithromycin 1 g PO single dose

External links

• Copepods and cholera in untreated water

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