Vibrio cholerae

Ms. Neha Sharma

Introduction 

  • Vibrio cholerae is a Gram-negative, curved (comma-shaped), motile bacterium.

  • It is the causative agent of cholera, an acute water-borne diarrheal disease.

  • The disease is characterized by profuse watery diarrhea, vomiting, and rapid dehydration.

  • Transmission occurs mainly through contaminated water and food.

  • Cholera is common in areas with poor sanitation and unsafe drinking water.

  • Only specific serogroups, O1 and O139, are responsible for epidemic and pandemic cholera.

  • The organism colonizes the small intestine and produces cholera toxin.

  • Cholera toxin causes electrolyte imbalance and massive fluid loss from intestinal cells.

  • If untreated, cholera can lead to hypovolemic shock and death within hours.

  • Vibrio cholerae remains a major public health problem in many developing countries.

 

General Character

  • Genus: Vibrio
  • Key Species:
    • Vibrio cholerae (causes cholera)
    • Vibrio parahaemolyticus (causes gastroenteritis)
    • Vibrio vulnificus (associated with wound infections and gastroenteritis)
  • Family: Vibrionaceae
  • Gram Staining: Vibrio species are Gram-negative bacteria, appearing pink due to their thin peptidoglycan layer and outer membrane.
  • Shape and Arrangement:
    • Shape: Comma-shaped (curved rods).
    • Arrangement: Typically found as single cells.
  • Oxygen Requirements: Vibrio species are facultative anaerobes.

 

Morphology

  • Vibrio cholerae is a Gram-negative bacillus.

  • The organism is curved or comma-shaped, sometimes appearing as S-shaped forms.

  • Size is approximately 1.5–3 µm in length and 0.5 µm in width.

  • Cells are usually arranged singly or in pairs.

  • It is actively motile due to the presence of a single polar flagellum.

  • Shows characteristic darting (shooting-star) motility in hanging drop preparation.

  • On Gram staining, it appears as pink, curved rods.

  • The organism is non-spore forming.

  • It is non-capsulated (classical strains).

  • No branching or filament formation is seen.

 

Cultural Characteristics

  • Enrichment Medium

    • Alkaline peptone water is used as an enrichment medium.

    • The organism multiplies rapidly within 6–8 hours.

    • Growth appears as a thin surface pellicle due to its aerobic nature.

    Selective and Differential Medium

    • TCBS agar (Thiosulfate–Citrate–Bile salts–Sucrose agar) is the medium of choice.

    • Produces large, smooth, yellow colonies due to sucrose fermentation.

    • Bile salts suppress the growth of normal intestinal flora.

    Simple Media

    • On nutrient agar, colonies are smooth, moist, translucent, and circular.

    • On blood agar, V. cholerae is usually non-hemolytic.

  • Growth Media:
    • Thiosulfate-Citrate-Bile Salts-Sucrose (TCBS) Agar: Selective for Vibrio; V. cholerae produces yellow colonies due to sucrose fermentation, while V. parahaemolyticus produces green colonies.
    • MacConkey Agar: Vibrio may appear as colorless colonies due to non-lactose fermentation.
    • Alkaline Peptone Water: Enrichment broth used for isolating Vibrio species from fecal samples.
  • Colony Appearance:
    • On TCBS agar, V. cholerae forms yellow colonies, while V. parahaemolyticus appears green.
  • Temperature and pH Range:
    • The optimal growth temperature is around 37°C, and Vibrio species can tolerate alkaline pH (up to pH 9).

Biochemical Reactions

  • Catalase Test: Vibrio species are catalase-positive.
  • Oxidase Test: Vibrio is oxidase-positive, which helps differentiate it from Enterobacteriaceae.
  • Lactose Fermentation: V. cholerae does not ferment lactose.
  • Indole Production: V. cholerae is indole-positive, while V. parahaemolyticus is typically indole-negative.
  • Methyl Red Test: V. cholerae is methyl red-positive.
  • Voges-Proskauer Test: Negative for V. cholerae and V. parahaemolyticus.

 

Pathogenicity

  • Virulence Factors:
    • Cholera Toxin (CT): Produced by V. cholerae, it disrupts electrolyte and water balance in the intestines, leading to severe diarrhoea.
    • Type IV Pili: Enhance adherence to intestinal epithelial cells.
    • Capsule: Protects against phagocytosis and aids in biofilm formation.
  • Clinical Infections:
    • Vibrio cholerae: Causes cholera, characterized by profuse watery diarrhoea (“rice-water” stools), dehydration, and electrolyte imbalances. Rapid rehydration is crucial for treatment.
    • Vibrio parahaemolyticus: Associated with gastroenteritis, often from consumption of undercooked seafood, leading to diarrhoea, abdominal cramps, and nausea.
    • Vibrio vulnificus: Causes wound infections and gastroenteritis, particularly in individuals with liver disease or immunocompromised states. It can lead to severe, rapidly progressing infections.

 

Laboratory Diagnosis

1. Specimen Collection

  • Fresh stool sample (preferred)

  • Rectal swab (if stool not available)

  • Specimen should be collected before antibiotic therapy

  • If delay is expected, transport in Cary–Blair transport medium

2. Direct Microscopy

  • Hanging drop preparation:

    • Shows characteristic darting / shooting-star motility

  • Gram staining:

    • Curved, comma-shaped Gram-negative bacilli

  • Microscopy is rapid but not confirmatory

3. Enrichment Culture

  • Alkaline peptone water (pH 8.5–9.5):

    • Enhances growth of V. cholerae

    • Incubated for 6–8 hours

    • Growth appears as a surface pellicle

4. Culture on Selective Media

  • Subculture from enrichment medium onto:

    • TCBS agar

  • Colony characteristics on TCBS:

    • Large, smooth yellow colonies

    • Due to sucrose fermentation

  • Helps differentiate V. cholerae from other enteric bacteria

5. Biochemical Identification

Key tests include:

  • Oxidase test – Positive

  • Indole test – Positive

  • String test – Positive

  • Sugar fermentation tests:

    • Glucose: Acid only

    • Sucrose: Positive

    • Lactose: Negative

  • Urease – Negative

  • Nitrate reduction – Positive

6. Serological Identification

  • Slide agglutination test using:

    • V. cholerae O1 antisera

    • V. cholerae O139 antisera

  • Confirms epidemic-causing strains

  • Important during surveillance and outbreaks

7. Molecular Methods

  • PCR for detection of:

    • Cholera toxin genes (ctxA, ctxB)

  • Used in reference and research laboratories

  • Highly sensitive and specific

8. Rapid Diagnostic Tests (RDTs)

  • Immunochromatographic tests available

  • Useful in field settings

  • Results within minutes

  • Must be confirmed by culture

 

Antibiotic Resistance

1. Purpose of Antibiotic Therapy

  • Antibiotics do not replace rehydration therapy

  • They help to:

    • Reduce duration of diarrhea

    • Decrease volume of stool

    • Shorten bacterial shedding

    • Limit transmission during outbreaks

2. Commonly Used Antibiotics

Traditionally effective drugs include:

  • Tetracycline

  • Doxycycline

  • Azithromycin

  • Ciprofloxacin

  • Erythromycin (especially in children and pregnant women)

3. Emerging Resistance Pattern

  • Increasing resistance has been reported against:

    • Tetracycline

    • Ampicillin

    • Cotrimoxazole

    • Nalidixic acid

    • Fluoroquinolones (reduced susceptibility)

  • Multidrug-resistant (MDR) strains are increasingly isolated

4. Mechanisms of Antibiotic Resistance

  • Plasmid-mediated resistance

  • Presence of integrons and transposons

  • Efflux pumps reducing intracellular drug concentration

  • Target site mutations (e.g., DNA gyrase mutations for fluoroquinolones)

  • Horizontal gene transfer from other enteric bacteria

5. Laboratory Detection of Resistance

  • Antibiotic susceptibility testing by:

    • Kirby–Bauer disk diffusion method

  • Interpretation as per CLSI guidelines

  • Surveillance important during outbreaks

6. Clinical and Public Health Significance

  • Resistance leads to:

    • Prolonged illness

    • Increased transmission

    • Limited treatment options

  • Empirical therapy should be guided by local resistance patterns

Prevention

1. Safe Water Supply

  • Use safe and treated drinking water

  • Boiling or chlorination of water before use

  • Protection of water sources from fecal contamination

  • Regular monitoring of water quality

2. Sanitation and Sewage Disposal

  • Proper disposal of human excreta

  • Functional sewage and drainage systems

  • Prevention of open defecation

  • Safe disposal of waste during outbreaks

3. Personal Hygiene

  • Frequent hand washing with soap and clean water

    • Before eating

    • After using the toilet

  • Maintaining cleanliness of utensils and food preparation areas

4. Food Hygiene

  • Consumption of freshly cooked food

  • Avoid raw or undercooked seafood

  • Washing fruits and vegetables with safe water

  • Proper storage of food to prevent contamination

5. Health Education

  • Community awareness about:

    • Modes of transmission

    • Early symptoms of cholera

    • Importance of early treatment

  • Promotion of ORS use at household level

6. Vaccination

  • Oral cholera vaccines (OCV) available:

    • Killed whole-cell vaccines

  • Used in:

    • Endemic areas

    • Outbreak settings

    • High-risk populations

  • Vaccination complements but does not replace sanitation

7. Surveillance and Early Detection

  • Prompt reporting of cases

  • Laboratory confirmation and serotyping

  • Monitoring antimicrobial resistance

  • Rapid response teams during outbreaks

8. Case Management and Isolation

  • Early diagnosis and immediate rehydration therapy

  • Isolation of infected individuals when possible

  • Disinfection of patient excreta and contaminated materials

9. Public Health Measures During Outbreaks

  • Mass chlorination of water sources

  • Distribution of ORS and zinc supplements

  • Temporary treatment centers

  • Community-level vaccination campaigns

 

MCQs

1. Vibrio cholerae is classified as which type of bacterium?

A. Gram-positive cocci
B. Gram-negative curved bacilli
C. Gram-positive rods
D. Acid-fast bacilli

2. The disease caused by Vibrio cholerae is:

A. Typhoid
B. Dysentery
C. Cholera
D. Gastroenteritis

3. The natural habitat of Vibrio cholerae is mainly:

A. Soil
B. Air
C. Water bodies
D. Skin

4. The characteristic shape of Vibrio cholerae is:

A. Straight rod
B. Spiral
C. Comma-shaped
D. Cocci in chains

5. Motility in Vibrio cholerae is due to:

A. Peritrichate flagella
B. Lophotrichate flagella
C. Single polar flagellum
D. Axial filament

6. The best method to demonstrate motility of Vibrio cholerae is:

A. Gram staining
B. Ziehl–Neelsen staining
C. Hanging drop preparation
D. Capsule staining

7. Vibrio cholerae grows best at which pH?

A. Acidic (pH 5)
B. Neutral (pH 7)
C. Alkaline (pH 8.5–9.5)
D. Highly acidic (pH 3)

8. Enrichment medium commonly used for Vibrio cholerae is:

A. Selenite F broth
B. Alkaline peptone water
C. MacConkey broth
D. Robertson cooked meat medium

9. Selective medium of choice for isolating Vibrio cholerae is:

A. Blood agar
B. MacConkey agar
C. TCBS agar
D. Chocolate agar

10. Colonies of Vibrio cholerae on TCBS agar appear:

A. Green
B. Colorless
C. Yellow
D. Black

11. Yellow colonies on TCBS agar indicate fermentation of:

A. Lactose
B. Glucose
C. Sucrose
D. Mannitol

12. Vibrio cholerae is oxidase:

A. Negative
B. Weakly positive
C. Positive
D. Variable

13. Which sugar is NOT fermented by Vibrio cholerae?

A. Glucose
B. Sucrose
C. Mannitol
D. Lactose

14. String test in Vibrio cholerae is:

A. Negative
B. Weakly positive
C. Positive
D. Variable

15. Epidemic cholera is caused mainly by serogroups:

A. O2 and O5
B. O1 and O139
C. O3 and O4
D. O8 and O10

16. The major virulence factor of Vibrio cholerae is:

A. Endotoxin
B. Cholera toxin
C. Hemolysin
D. Capsule

17. Cholera toxin acts by increasing intracellular:

A. cGMP
B. Calcium
C. cAMP
D. Sodium

18. Target receptor for cholera toxin on intestinal cells is:

A. CD4 receptor
B. GM1 ganglioside
C. Toll-like receptor
D. Integrin

19. The characteristic stool in cholera is described as:

A. Bloody stool
B. Mucoid stool
C. Rice-water stool
D. Green stool

20. The main site of action of Vibrio cholerae is:

A. Stomach
B. Colon
C. Small intestine
D. Liver

21. Most important life-saving treatment in cholera is:

A. Antibiotics
B. Antidiarrheal drugs
C. Rehydration therapy
D. Antiemetics

22. Specimen of choice for laboratory diagnosis is:

A. Blood
B. Urine
C. Stool
D. CSF

23. Transport medium commonly used for stool samples is:

A. Stuart medium
B. Cary–Blair medium
C. Amies medium
D. Venkatraman medium

24. Which test confirms epidemic strains of V. cholerae?

A. Oxidase test
B. String test
C. Slide agglutination test
D. Indole test

25. PCR detects which gene of Vibrio cholerae?

A. toxA
B. ctx gene
C. spa gene
D. mecA

26. Which antibiotic resistance pattern is increasingly seen in V. cholerae?

A. Penicillin only
B. Multidrug resistance
C. No resistance
D. Only macrolide resistance

27. Antibiotics in cholera are mainly used to:

A. Replace ORS
B. Prevent dehydration
C. Shorten duration and shedding
D. Cure asymptomatic carriers only

28. Most effective preventive measure against cholera is:

A. Antibiotic prophylaxis
B. Safe drinking water
C. Bed nets
D. Isolation only

29. Oral cholera vaccines are:

A. Live attenuated only
B. Toxoid vaccines
C. Killed whole-cell vaccines
D. DNA vaccines

30. Vibrio cholerae is a:

A. Spore-forming organism
B. Acid-fast organism
C. Non-spore forming organism
D. Encapsulated organism

31. Vibrio cholerae is normally:

A. Capsule positive
B. Capsule negative
C. Acid fast
D. Branching

32. Blood agar culture of V. cholerae usually shows:

A. Beta hemolysis
B. Alpha hemolysis
C. No hemolysis
D. Double zone hemolysis

33. Which condition favors outbreaks of cholera?

A. Cold climate
B. Good sanitation
C. Floods and overcrowding
D. High altitude

34. Vibrio cholerae belongs to which family?

A. Enterobacteriaceae
B. Vibrionaceae
C. Pseudomonadaceae
D. Neisseriaceae

35. Cholera is mainly transmitted through:

A. Air droplets
B. Direct skin contact
C. Contaminated food and water
D. Insect bite

Answer Key

  1. B

  2. C

  3. C

  4. C

  5. C

  6. C

  7. C

  8. B

  9. C

  10. C

  11. C

  12. C

  13. D

  14. C

  15. B

  16. B

  17. C

  18. B

  19. C

  20. C

  21. C

  22. C

  23. B

  24. C

  25. B

  26. B

  27. C

  28. B

  29. C

  30. C

  31. B

  32. C

  33. C

  34. B

  35. C