Vibrio cholerae

Vibrio cholerae

1. Introduction to Vibrio cholerae

Vibrio cholerae is a Gram-negative, curved rod-shaped, facultatively anaerobic bacterium that causes cholera, a severe diarrheal disease characterized by rapid fluid loss, dehydration, and, if untreated, can be fatal. The disease primarily spreads through contaminated water or food and has been a major cause of pandemics throughout history (Ali et al., 2019).

V. cholerae belongs to the Vibrionaceae family and is closely related to other Vibrio species, such as V. parahaemolyticus and V. vulnificus, though these species are not typically associated with cholera (Nelson et al., 2020).

2. Taxonomy and Classification

• Domain: Bacteria
• Phylum: Proteobacteria
• Class: Gammaproteobacteria
• Order: Vibrionales
• Family: Vibrionaceae
• Genus: Vibrio
• Species: Vibrio cholerae

Vibrio cholerae is classified into two biogroups—Classical and El Tor—which differ in their biochemical characteristics, hemolytic properties, and genetic makeup. The El Tor biogroup is the more recent cause of modern cholera pandemics (Nelson et al., 2020). Furthermore, V. cholerae is subdivided into serogroups, with O1 and O139 being the most virulent and responsible for the majority of cholera cases (Sack et al., 2020).

3. Morphological Characteristics

• Shape: V. cholerae is a curved, comma-shaped rod, often referred to as a vibrio due to its shape. Under the microscope, it appears as curved bacilli and exhibits motility (Sack et al., 2020).
• Gram Staining: Vibrio cholerae is Gram-negative, meaning it has a thin peptidoglycan layer surrounded by an outer membrane, which is characteristic of Gram-negative bacteria (Ghosh et al., 2019).
• Flagella: V. cholerae is motile due to the presence of a single, polar flagellum, which aids in its swimming motility in liquid environments (Ghosh et al., 2019).
• Capsule: Some strains of V. cholerae may produce a capsule, which can contribute to virulence and survival in hostile environments, although it is not universal across all strains (Nelson et al., 2020).

4. Cultural Characteristics

Cultural characteristics are essential for identifying V. cholerae in the laboratory. The bacterium has specific growth requirements, and the biochemical tests, along with colony morphology, play a crucial role in its identification.

• Growth Conditions:
• Vibrio cholerae is a facultatively anaerobic organism, meaning it can grow in both aerobic and anaerobic environments. It grows optimally at 37°C but can tolerate a range of temperatures (Sack et al., 2020).
• The organism requires a slightly alkaline pH (pH 7.5–9.0) for optimal growth, which aligns with its natural habitat in brackish and marine environments.
• Media:
• Thiosulfate-citrate-bile salts-sucrose (TCBS) agar: This is a selective medium for V. cholerae and other vibrios. V. cholerae ferments sucrose, producing yellow colonies on TCBS agar, which distinguishes it from other vibrios that may produce green or blue-green colonies (Ghosh et al., 2019).
• Alkaline peptone water: This is a liquid medium used to enrich V. cholerae from clinical specimens, particularly in stool samples, before plating on solid media like TCBS agar (Sack et al., 2020).
• MacConkey agar: V. cholerae does not ferment lactose and hence produces non-lactose fermenting colonies, which are pale on MacConkey agar (Nelson et al., 2020).
• Colony Morphology:
• On TCBS agar, V. cholerae produces round, smooth, yellow colonies, owing to sucrose fermentation. This yellow coloration is a key distinguishing feature (Ghosh et al., 2019).
• On MacConkey agar, V. cholerae produces non-lactose fermenting colonies, appearing as pale or colorless colonies due to the lack of lactose fermentation (Nelson et al., 2020).
• On Nutrient agar, the colonies of V. cholerae are typically smooth, cream-colored, and moist.
• Biochemical Characteristics:
• V. cholerae is oxidase-positive, indicating the presence of cytochrome c oxidase, a distinguishing characteristic from non-oxidase-positive enteric bacteria (Ghosh et al., 2019).
• It is indole-positive, meaning it can break down tryptophan into indole.
• V. cholerae ferments glucose but does not ferment lactose or sucrose in standard biochemical tests, and it produces gas in fermentation reactions (Sack et al., 2020).
• H2S production is negative for V. cholerae, which is important in differentiating it from other vibrio species.

5. Virulence Factors

The pathogenicity of Vibrio cholerae is largely attributed to its virulence factors, which enable it to cause severe gastrointestinal symptoms and dehydration.

• Cholera Toxin (CT):
• The cholera toxin (CT) is the most critical virulence factor for V. cholerae. It is an AB5 toxin, consisting of an enzymatically active A subunit and a B subunit that facilitates toxin entry into enterocytes (Nelson et al., 2020).
• The toxin activates the adenylate cyclase enzyme in the host cell, leading to excessive secretion of chloride ions, and subsequently water, into the intestinal lumen, causing profuse watery diarrhea (Sack et al., 2020).
• Toxin-Coregulated Pilus (TCP):
• The toxin-coregulated pilus (TCP) is essential for adherence to the intestinal mucosa, allowing V. cholerae to colonize the host's small intestine (Nelson et al., 2020).
• TCP facilitates the formation of biofilms and acts as a receptor for CTXφ, a bacteriophage that carries the cholera toxin genes (Ali et al., 2019).
• Flagellum:
• The polar flagellum of V. cholerae is essential for motility and contributes to intestinal colonization by allowing the bacterium to move through the viscous mucus layer (Ghosh et al., 2019).
• Type VI Secretion System (T6SS):
• The Type VI secretion system (T6SS) is involved in bacterial competition and may contribute to the virulence of V. cholerae by aiding in the disruption of host immune responses (Sack et al., 2020).
• Hemolysins and Other Proteases:
• V. cholerae produces hemolysins that facilitate cell lysis and may contribute to tissue damage in infected individuals (Ali et al., 2019).

6. Pathogenesis of Cholera

Cholera is primarily transmitted via the fecal-oral route, often through contaminated water or food.

• Colonization:
• After ingestion, V. cholerae navigates through the acidic environment of the stomach and colonizes the small intestine, where it adheres to epithelial cells using TCP and produces cholera toxin (Ghosh et al., 2019).
• Diarrhea:
• The cholera toxin induces the secretion of electrolytes and water into the intestinal lumen, leading to profuse, watery diarrhea (also referred to as rice-water stools) (Nelson et al., 2020).
• The massive loss of fluids can lead to severe dehydration, hypovolemic shock, and, if left untreated, death.
• Dehydration:
• The primary clinical manifestation of cholera is rapid fluid and electrolyte loss due to excessive watery diarrhea, leading to hypokalemia, acidosis, and renal failure in severe cases (Ali et al., 2019).

7. Diagnosis

Diagnosis of cholera involves a combination of clinical presentation and microbiological tests.

• Stool Examination:
• Stool samples are collected, and the presence of vibrio-shaped bacteria can be confirmed by Gram staining and microscopy (Sack et al., 2020).
• Culture:
• TCBS agar is used to isolate V. cholerae, which produces yellow colonies due to sucrose fermentation.
• Serological Testing:
• Serotyping is performed using antiserum to identify the O1 or O139 serogroups responsible for cholera outbreaks (Nelson et al., 2020).
• PCR:
• PCR-based assays for cholera toxin genes can be used to confirm the presence of virulent V. cholerae (Ghosh et al., 2019).

8. Treatment and Prevention

• Rehydration Therapy:
• The mainstay of treatment is oral rehydration therapy (ORT), which replenishes lost fluids and electrolytes through oral rehydration salts (ORS) (Sack et al., 2020).
• In severe cases, intravenous fluids may be necessary.
• Antibiotics:
• Antibiotics, such as doxycycline, azithromycin, or ciprofloxacin, may be used in severe cases to reduce the duration of diarrhea and the spread of the infection (Ali et al., 2019).
• Vaccination:
• Several oral cholera vaccines (OCVs) are available, such as Dukoral, Shanchol, and mORC-V (Nelson et al., 2020).
• These vaccines are effective in preventing cholera outbreaks, particularly in endemic areas.

9. Prevention and Control

• Water and Sanitation:
• Cholera transmission can be reduced by ensuring access to clean water, proper sewage treatment, and good hygiene practices.
• Vaccination Campaigns:
• Mass vaccination campaigns in endemic regions can reduce the incidence of cholera outbreaks (Ali et al., 2019).

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References

1. Ali, M., et al. (2019). The global spread of cholera. Nature Reviews Microbiology, 17(6), 333-348. https://doi.org/10.1038/s41579-019-0161-3
2. Ghosh, S., et al. (2019). Vibrio cholerae: Molecular mechanisms of pathogenesis and host adaptation. Current Topics in Microbiology and Immunology, 420, 39-57. https://doi.org/10.1007/82_2019_114
3. Nelson, E. J., et al. (2020). The genetics of cholera: Epidemiology and the emergence of Vibrio cholerae O139. Frontiers in Microbiology, 11, 1238. https://doi.org/10.3389/fmicb.2020.01238
4. Sack, D. A., et al. (2020). Cholera. Lancet, 393(10175), 142-151. https://doi.org/10.1016/S0140-6736(18)31948-X
5. Salim, M. A., et al. (2019). Cholera in the 21st century: An analysis of recent epidemiology and trends. International Journal of Infectious Diseases, 79, 101-108. https://doi.org/10.1016/j.ijid.2018.11.004
6. Ramamurthy, T., et al. (2020). Vibrio cholerae and its biogeography: Global spread of cholera. Clinical Microbiology Reviews, 33(1), e00012-19. https://doi.org/10.1128/CMR.00012-19
7. Cao, L., et al. (2019). Antibiotic resistance in Vibrio cholerae: Challenges in treatment and control. Pathogens, 8(4), 123. https://doi.org/10.3390/pathogens8040123
8. Bernbom, N., et al. (2018). Cholera and Vibrio cholerae: Diagnostic and therapeutic advances. Journal of Clinical Microbiology, 56(7), e00324-18. https://doi.org/10.1128/JCM.00324-18