Yersinia

Yersinia species, particularly Y. pestis and Y. enterocolitica, are significant pathogens responsible for serious infections. Understanding their characteristics, pathogenic mechanisms, and effective laboratory diagnosis is crucial for managing infections caused by Yersinia. Ongoing surveillance and research into antibiotic resistance and preventive measures are essential to combat these public health threats effectively.

Table of contents

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 due to their thin peptidoglycan layer and 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.

Morphology

Cell Wall Structure:
- It comprises a thin peptidoglycan layer surrounded by an outer membrane containing lipopolysaccharides (LPS), contributing to virulence.
Flagella:
- Y. pestis is non-motile in its pathogenic form but has flagella in its environmental form.
- Y. enterocolitica is motile at 25°C but non-motile at 37°C.

Cultural Characteristics

Growth Media:
- MacConkey Agar: Yersinia appears as colorless colonies due to non-lactose fermentation.
- CIN Agar (Cefsulodin-Irgasan-Novobiocin): Selective for Yersinia enterocolitica; produces red colonies with a characteristic “bull’s-eye” appearance.
- Blood Agar: Supports growth and may show non-hemolytic colonies.
Colony Appearance:
- On selective media, Yersinia colonies are typically pale or colorless.
Temperature and pH Range:
- The optimal growth temperature is around 37°C for Y. pestis; Y. enterocolitica can grow at lower temperatures (up to 4°C).

Biochemical Reactions

Catalase Test: Yersinia species are catalase-positive.
Oxidase Test: Yersinia is oxidase-negative, distinguishing it from other Gram-negative bacteria.
Lactose Fermentation: Generally, Yersinia species do not ferment lactose (Y. enterocolitica can ferment lactose slowly).
Indole Production: Y. enterocolitica is often indole-positive.
Methyl Red Test: Positive for Y. enterocolitica; negative for Y. pestis.
Voges-Proskauer Test: Negative for both species.

Pathogenicity

Virulence Factors:
- Type III Secretion System (T3SS): Allows Yersinia to inject virulence factors into host cells, facilitating invasion and immune evasion.
- Yersinia outer proteins (Yops): Involved in inhibiting phagocytosis and disrupting immune responses.
- Capsule (Plasminogen activator): Enhances virulence in Y. pestis by preventing opsonization.
Clinical Infections:
- Yersinia pestis: Causes plague, which can manifest in several forms:
  - Bubonic Plague: Characterized by swollen lymph nodes (buboes), fever, and chills.
  - Pneumonic Plague: Involves the lungs, can lead to severe respiratory symptoms, and is highly contagious.
  - Septicemic Plague: This can occur as a complication of bubonic plague, leading to systemic infection.
- Yersinia enterocolitica: Causes yersiniosis, characterized by gastrointestinal symptoms such as diarrhoea (which may be bloody), abdominal pain, and fever. It is often associated with undercooked pork.

Laboratory Diagnosis

Specimen Collection: Clinical specimens may include blood, stool, or lymph node aspirates, depending on the infection type.
Microscopic Examination:
- Gram staining reveals Gram-negative bacilli.
Culture Techniques:
- Inoculation on selective media like CIN agar for Y. enterocolitica or blood agar for Y. pestis, followed by incubation.
Biochemical Testing:
- Confirmatory tests for lactose fermentation, indole production, and other biochemical reactions to identify Yersinia species.
Serotyping: Antigenic testing can be performed for further identification.

Antibiotic Resistance

Emergence of Resistance: Some strains, particularly Y. enterocolitica, may resist certain antibiotics.
Multidrug-Resistant Strains: Resistance to common antibiotics is a concern in some regions, particularly for Y. enterocolitica.
Treatment Options: Yersinia pestis is usually treated with aminoglycosides (like streptomycin) or tetracyclines, while Y. enterocolitica infections may be treated with fluoroquinolones or trimethoprim-sulfamethoxazole.

Prevention

Hygiene Practices: Good sanitation and hygiene prevent Yersinia infections.
Food Safety: Proper cooking and handling of food, particularly pork, are essential to prevent yersiniosis.
Vector Control: Reducing rodent populations and controlling fleas can help prevent the spread of Y. pestis.