Yersinia

Dr. Neha Sharma

Introduction 

  • Yersinia is a genus of Gram-negative bacteria belonging to the family Enterobacteriaceae.

  • Members of this genus are short, rod-shaped (coccobacillary) organisms.

  • Yersinia species are facultative anaerobes.

  • The genus includes important human pathogens responsible for enteric and systemic infections.

  • Yersinia pestis is the causative agent of plague, a historically important and potentially fatal disease.

  • Yersinia enterocolitica causes yersiniosis, primarily a gastrointestinal infection.

  • Yersinia species are zoonotic pathogens, with animals acting as natural reservoirs.

  • They can survive and multiply at low temperatures, especially Y. enterocolitica.

  • Pathogenicity is mediated by virulence factors such as capsules, endotoxins, and Yersinia outer proteins (Yops).

  • Yersinia infections remain of significant medical and public health importance worldwide.

General Character

  • Genus: Yersinia
  • Key Species:

    • Yersinia pestis – causes plague

    • Yersinia enterocolitica – causes yersiniosis

  • Family:
    Enterobacteriaceae
  • Gram Staining:
    Yersinia species are Gram-negative bacteria, appearing pink on Gram staining due to the presence of a thin peptidoglycan layer and an outer membrane.
  • Shape and Arrangement:

  • Shape: Rod-shaped (bacilli)

  • Arrangement: Typically found as single cells or in short chains

  • Oxygen Requirements:
    Yersinia species are facultative anaerobes, capable of growing in both the presence and absence of oxygen.

 

Morphology

  • Size:
    Small to medium-sized bacteria, approximately 0.5–0.8 µm × 1–3 µm.

  • Shape:
    Short, plump rod-shaped (coccobacillary) bacilli.

  • Gram Reaction:
    Gram-negative, staining pink due to thin peptidoglycan layer and outer membrane.

  • Arrangement:
    Occur singly, in pairs, or occasionally in short chains.

  • Bipolar Staining:
    Show characteristic bipolar (safety-pin) appearance, especially in clinical specimens and when stained with Wayson, Giemsa, or Leishman stains.

  • Motility:

    • Yersinia pestis: Non-motile

    • Yersinia enterocolitica: Motile at 22–25°C, non-motile at 37°C

  • Spores:
    Non-spore forming.

  • Capsule:

    • Yersinia pestis produces a protein capsule (F1 antigen) at 37°C, contributing to virulence.

    • Other species are generally non-capsulated.

  • Flagella:
    Present in motile species (Y. enterocolitica) at lower temperatures; absent in Y. pestis.

 

Cultural Characteristics

  • Oxygen Requirement:
    Yersinia species are facultative anaerobes and can grow in the presence or absence of oxygen.

  • Temperature Requirement:
    Grow over a wide temperature range (4°C–40°C).
    Optimal growth occurs at 25–28°C.
    Yersinia enterocolitica can multiply at refrigerator temperature (cold enrichment).

  • Growth on Ordinary Media:
    Grow well on simple media such as nutrient agar and peptone water.

  • Blood Agar:

    • Colonies are small, smooth, grayish-white.

    • Yersinia pestis is non-hemolytic.

  • MacConkey Agar:

    • Non-lactose fermenting colonies (pale colonies).

    • Y. enterocolitica may show delayed lactose fermentation in some strains.

  • Selective Media:

    • CIN agar (Cefsulodin–Irgasan–Novobiocin agar):
      Produces characteristic “bull’s-eye” colonies (deep red center with transparent border), especially by Y. enterocolitica.

  • Broth Culture:

    • In broth, Y. pestis shows turbidity with flocculent deposits and may form “stalactite growth” when grown undisturbed.

  • Special Features:

    • Yersinia pestis grows slowly at 37°C compared to other Enterobacteriaceae.

    • Colony morphology varies with temperature and species.

 

Biochemical Reactions

  • Carbohydrate Fermentation:

    • Glucose: Fermented with acid production only (no gas).

    • Lactose: Not fermented (non-lactose fermenter); some strains of Y. enterocolitica may show delayed fermentation.

    • Sucrose: Usually not fermented (species dependent).

    • Mannitol: Fermented by Y. enterocolitica.

  • Oxidase Test: Oxidase negative.

  • Catalase Test: Catalase positive.

  • Indole Test:

    • Yersinia pestis: Indole negative

    • Yersinia enterocolitica: Indole positive (most strains)

  • Urease Test:

    • Y. pestis: Urease negative

    • Y. enterocolitica: Urease positive (rapid urease producer)

  • Methyl Red (MR) Test: Positive.

  • Voges–Proskauer (VP) Test: Negative.

  • Citrate Utilization: Negative.

  • Hydrogen Sulfide (H₂S) Production: Negative.

  • Nitrate Reduction:
    Positive (reduces nitrate to nitrite).

  • Motility Test:

    • Y. pestis: Non-motile

    • Y. enterocolitica: Motile at 22–25°C, non-motile at 37°C

  • Phenylalanine Deaminase Test: Negative.

 

Pathogenicity

1. Yersinia pestis (Plague)

  • Disease Caused:
    Plague – occurs in three clinical forms:

    • Bubonic plague (most common)

    • Septicemic plague

    • Pneumonic plague (most severe and highly fatal)

  • Source & Transmission:

    • Natural reservoir: Rodents

    • Vector: Rat flea (Xenopsylla cheopis)

    • Pneumonic plague spreads by respiratory droplets.

  • Major Virulence Factors:

    • F1 capsular antigen: Anti-phagocytic protein capsule.

    • V and W antigens: Help survival inside macrophages.

    • Endotoxin (LPS): Causes fever, shock, and DIC.

    • Plasminogen activator: Facilitates spread in tissues.

    • Type III secretion system: Injects Yop proteins that inhibit phagocytosis.

  • Pathogenesis:
    After entry, organisms multiply in lymph nodes causing painful swollen lymph nodes (buboes) → septicemia → dissemination to lungs and other organs.

2. Yersinia enterocolitica (Yersiniosis)

  • Disease Caused:
    Yersiniosis, mainly affecting the gastrointestinal tract.

  • Mode of Transmission:

    • Ingestion of contaminated food (pork, milk) or water.

    • Can grow at refrigerator temperature.

  • Clinical Manifestations:

    • Acute gastroenteritis (diarrhea, fever, abdominal pain)

    • Mesenteric lymphadenitis (mimics acute appendicitis)

    • Terminal ileitis

    • Post-infectious complications:

      • Reactive arthritis

      • Erythema nodosum

  • Virulence Factors:

    • Invasin protein: Facilitates entry into intestinal epithelial cells.

    • Yersinia outer proteins (Yops): Inhibit phagocytosis.

    • Heat-stable enterotoxin (Yst): Causes diarrhea.

    • Lipopolysaccharide (LPS): Endotoxic effects.

3. Yersinia pseudotuberculosis

Causes mesenteric lymphadenitis and ileitis, similar to Y. enterocolitica. Can produce scarlatiniform rash and systemic illness.

Laboratory Diagnosis

1. Specimen Collection

Yersinia pestis (Plague)

  • Bubonic plague:
    Aspirate from bubo (lymph node)

  • Septicemic plague:
    Blood

  • Pneumonic plague:
    Sputum / throat swab

Yersinia enterocolitica

  • Stool sample (most common)

  • Blood (in septicemia)

  • Mesenteric lymph node biopsy (rare)

2. Microscopic Examination

  • Gram Staining:
    Shows Gram-negative, short plump bacilli.

  • Special Staining:

    • Wayson / Giemsa / Leishman stain

    • Demonstrates bipolar (“safety-pin”) appearance, especially in Y. pestis.

3. Culture

Media Used

  • Blood agar:
    Small, grayish, non-hemolytic colonies.

  • MacConkey agar:
    Non-lactose fermenting pale colonies.

  • CIN agar (selective medium):
    Especially for Y. enterocolitica – produces bull’s-eye colonies.

  • Broth culture:
    Y. pestis may show flocculent or stalactite growth.

Temperature

  • Optimal growth at 25–28°C.

  • Y. enterocolitica can be isolated by cold enrichment at 4°C.

4. Biochemical Identification

  • Oxidase: Negative

  • Catalase: Positive

  • Glucose: Fermented (acid only)

  • Urease:

    • Y. pestis – Negative

    • Y. enterocolitica – Positive

  • Indole:

    • Y. pestis – Negative

    • Y. enterocolitica – Positive

  • Motility:

    • Y. pestis – Non-motile

    • Y. enterocolitica – Motile at 22–25°C

5. Serological Tests

  • Antigen Detection:
    Detection of F1 antigen of Y. pestis by:

    • ELISA

    • Immunochromatographic tests

  • Antibody Detection:
    Rising antibody titers in paired sera.

6. Molecular Methods

  • PCR:
    Detection of specific genes for rapid and sensitive diagnosis, especially in plague outbreaks.

7. Animal Inoculation

  • Inoculation in guinea pig or mouse; animal dies rapidly in Y. pestis infection.

8. Safety Considerations

  • Y. pestis is a highly infectious organism.

  • Laboratory work must be done in biosafety level-3 (BSL-3) laboratories.

 

Antibiotic Resistance

1. General Features

  • Yersinia species show intrinsic resistance to certain antibiotics.

  • Resistance may be chromosomal or plasmid-mediated.

  • Antimicrobial susceptibility testing (AST) is recommended for all clinical isolates.

2. Intrinsic (Natural) Resistance

  • Penicillin G

  • First-generation cephalosporins

  • Ampicillin (common resistance due to β-lactamase production)

3. Acquired Resistance

  • Resistance genes may be carried on plasmids or transposons.

  • Increasing resistance reported to:

    • Tetracyclines

    • Chloramphenicol

    • Trimethoprim-sulfamethoxazole (in some strains)

  • Multidrug-resistant strains have been reported, especially in enteric Yersinia.

4. Species-Specific Resistance Patterns

Yersinia pestis

  • Generally susceptible to:

    • Streptomycin

    • Gentamicin

    • Doxycycline

    • Ciprofloxacin

  • Rare strains show resistance due to plasmid-mediated genes, especially in endemic regions.

Yersinia enterocolitica

  • Frequently resistant to:

    • Ampicillin

    • Amoxicillin

    • First-generation cephalosporins

  • Usually sensitive to:

    • Fluoroquinolones

    • Aminoglycosides

    • Third-generation cephalosporins

5. Mechanisms of Resistance

  • β-lactamase production → resistance to penicillins and early cephalosporins.

  • Efflux pumps → reduced intracellular antibiotic concentration.

  • Plasmid-encoded resistance genes → multidrug resistance.

  • Altered target sites → reduced drug binding.

6. Clinical & Public Health Importance

  • Empirical therapy should be based on local susceptibility patterns.

  • Emergence of resistant Y. pestis strains is a serious public health concern due to its potential for outbreaks.

  • Rational antibiotic use and surveillance are essential.

 

Prevention

1. Prevention of Plague (Yersinia pestis)

A. Reservoir and Vector Control

  • Rodent control in endemic areas.

  • Flea control using insecticides to prevent transmission.

  • Avoid handling dead rodents without protective measures.

B. Personal Protective Measures

  • Use of protective clothing and gloves when handling animals.

  • Respiratory protection (masks) for healthcare workers and close contacts in pneumonic plague.

C. Isolation and Surveillance

  • Isolation of suspected cases, especially pneumonic plague.

  • Active surveillance and early case detection in endemic regions.

  • Contact tracing and chemoprophylaxis for close contacts.

D. Chemoprophylaxis

  • Doxycycline or ciprofloxacin for close contacts of confirmed cases.

E. Vaccination

  • No widely available effective vaccine for general use.

  • Older killed vaccines are no longer recommended.

2. Prevention of Yersiniosis (Yersinia enterocolitica)

A. Food Safety Measures

  • Proper cooking of pork (common source).

  • Avoid consumption of unpasteurized milk and dairy products.

  • Wash fruits and vegetables thoroughly.

B. Water and Sanitation

  • Use safe drinking water.

  • Maintain proper hand hygiene, especially after handling raw meat.

C. Cold Storage Awareness

  • Y. enterocolitica can grow at refrigerator temperature → avoid prolonged storage of contaminated food.

D. Hospital Infection Control

  • Standard infection control precautions in healthcare settings.

  • Proper disinfection of contaminated surfaces and equipment.

3. General Preventive Measures

  • Health education in endemic and high-risk areas.

  • Early diagnosis and prompt treatment to prevent spread.

  • Reporting of cases to public health authorities.

 

MCQs

1. Yersinia belongs to which family?

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

2. Yersinia species are:

A. Gram-positive cocci
B. Gram-negative bacilli
C. Acid-fast bacilli
D. Spirochetes
Answer: B

3. The causative agent of plague is:

A. Yersinia enterocolitica
B. Yersinia pseudotuberculosis
C. Yersinia pestis
D. Yersinia ruckeri
Answer: C

4. The typical staining appearance of Yersinia pestis is:

A. Chain formation
B. Acid-fast rods
C. Bipolar (safety-pin) staining
D. Spiral shape
Answer: C

5. Which stain best demonstrates bipolar staining of Y. pestis?

A. Ziehl–Neelsen stain
B. Gram stain
C. Wayson stain
D. Albert stain
Answer: C

6. Yersinia species are:

A. Obligate aerobes
B. Obligate anaerobes
C. Facultative anaerobes
D. Microaerophilic
Answer: C

7. Which species is motile at 22–25°C but non-motile at 37°C?

A. Y. pestis
B. Y. enterocolitica
C. Y. pseudotuberculosis
D. All of the above
Answer: B

8. Yersinia pestis is:

A. Motile
B. Spore forming
C. Non-motile
D. Acid-fast
Answer: C

9. The capsule of Y. pestis is composed of:

A. Polysaccharide
B. Lipid
C. Protein (F1 antigen)
D. Peptidoglycan
Answer: C

10. The major vector for plague transmission is:

A. Mosquito
B. Louse
C. Tick
D. Rat flea
Answer: D

11. The flea involved in plague transmission is:

A. Ctenocephalides felis
B. Xenopsylla cheopis
C. Pediculus humanus
D. Anopheles
Answer: B

12. The most common form of plague is:

A. Pneumonic
B. Septicemic
C. Bubonic
D. Cutaneous
Answer: C

13. Painful swollen lymph nodes in plague are called:

A. Nodes
B. Buboes
C. Abscesses
D. Granulomas
Answer: B

14. Yersinia enterocolitica primarily causes:

A. Pneumonia
B. Gastroenteritis
C. Meningitis
D. Endocarditis
Answer: B

15. Y. enterocolitica infection may clinically mimic:

A. Typhoid fever
B. Appendicitis
C. Cholera
D. Dysentery
Answer: B

16. Yersinia enterocolitica commonly grows at:

A. Only 37°C
B. Only 42°C
C. Refrigerator temperature
D. Only anaerobic conditions
Answer: C

17. Selective medium for Y. enterocolitica is:

A. TCBS agar
B. Lowenstein–Jensen medium
C. CIN agar
D. Thayer–Martin agar
Answer: C

18. Colonies on CIN agar show:

A. Green colonies
B. Swarming
C. Bull’s-eye appearance
D. Metallic sheen
Answer: C

19. On MacConkey agar, Yersinia forms:

A. Lactose fermenting pink colonies
B. Non-lactose fermenting pale colonies
C. Mucoid colonies
D. Black colonies
Answer: B

20. Yersinia pestis shows which broth growth?

A. Uniform turbidity
B. Pellicle formation
C. Stalactite growth
D. No growth
Answer: C

21. Oxidase test of Yersinia is:

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

22. Catalase reaction of Yersinia is:

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

23. Urease test is positive in:

A. Y. pestis
B. Y. enterocolitica
C. Y. pestis and Y. pseudotuberculosis
D. All Yersinia species
Answer: B

24. Indole test is usually positive in:

A. Y. pestis
B. Y. enterocolitica
C. Y. pseudotuberculosis
D. None
Answer: B

25. Glucose fermentation by Yersinia produces:

A. Acid and gas
B. Gas only
C. Acid only
D. No fermentation
Answer: C

26. Major virulence factor inhibiting phagocytosis is:

A. Capsule
B. Flagella
C. Yops
D. Spores
Answer: C

27. Type III secretion system is characteristic of:

A. Yersinia
B. Vibrio
C. Neisseria
D. Streptococcus
Answer: A

28. Endotoxin of Yersinia is:

A. Exotoxin
B. Polysaccharide capsule
C. Lipopolysaccharide
D. Protein toxin
Answer: C

29. Specimen for suspected bubonic plague is:

A. Stool
B. Urine
C. Bubo aspirate
D. CSF
Answer: C

30. Safety-pin appearance is best seen in:

A. Culture media
B. Gram stain only
C. Special stains
D. Electron microscopy
Answer: C

31. Laboratory handling of Y. pestis requires:

A. BSL-1
B. BSL-2
C. BSL-3
D. BSL-4
Answer: C

32. Antibiotic of choice for plague is:

A. Penicillin
B. Streptomycin
C. Erythromycin
D. Vancomycin
Answer: B

33. Yersinia is naturally resistant to:

A. Aminoglycosides
B. Fluoroquinolones
C. Penicillin
D. Tetracycline
Answer: C

34. Common food source for Y. enterocolitica is:

A. Poultry
B. Beef
C. Pork
D. Fish
Answer: C

35. Post-infectious complication of yersiniosis includes:

A. Rheumatic fever
B. Reactive arthritis
C. Glomerulonephritis
D. Myocarditis
Answer: B

36. Which antigen is specific for Y. pestis?

A. O antigen
B. H antigen
C. F1 antigen
D. Vi antigen
Answer: C

37. Plague vaccine currently:

A. Widely used
B. Highly effective
C. Not routinely recommended
D. Mandatory
Answer: C

38. Cold enrichment technique helps in isolating:

A. Y. pestis
B. Y. enterocolitica
C. Salmonella
D. Shigella
Answer: B

39. Yersinia species are:

A. Strict human pathogens
B. Zoonotic organisms
C. Plant pathogens
D. Normal skin flora
Answer: B

40. Pneumonic plague spreads mainly by:

A. Flea bite
B. Fomites
C. Droplet infection
D. Food
Answer: C

41. Mesenteric lymphadenitis is commonly caused by:

A. Y. pestis
B. Y. enterocolitica
C. E. coli
D. Vibrio cholerae
Answer: B

42. Phenylalanine deaminase test in Yersinia is:

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

43. H₂S production by Yersinia is:

A. Positive
B. Weak
C. Negative
D. Delayed
Answer: C

44. Reservoir of plague is mainly:

A. Humans
B. Birds
C. Rodents
D. Insects
Answer: C

45. The disease caused by Yersinia is called:

A. Yersinosis
B. Yersiniosis
C. Yersinitis
D. Yersinia fever
Answer: B

46. Bull’s-eye colonies are due to:

A. Capsule formation
B. Lactose fermentation
C. Mannitol fermentation on CIN agar
D. Hemolysis
Answer: C

47. Yersinia is non-lactose fermenter on:

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

48. Endemic plague areas include:

A. Antarctica
B. Africa and Asia
C. Australia only
D. Europe only
Answer: B

49. Chemoprophylaxis for plague contacts includes:

A. Penicillin
B. Doxycycline
C. Vancomycin
D. Rifampicin
Answer: B

50. Most important virulence mechanism of Yersinia is:

A. Spore formation
B. Acid-fast cell wall
C. Inhibition of phagocytosis
D. Capsule polysaccharide only
Answer: C