Study Notes on Shigella

SHIGELLA

Introduction

  • Shigella is a genus named after Japanese microbiologist Kiyoshi Shiga, who isolated the first strain during a dysentery epidemic in Japan in 1896.

  • The first member was initially identified as Shigella shiga and is now referred to as S. dysenteriae.

Classification

  • Family: Enterobacteriaceae.

  • Characteristics of Shigella:

    1. Gram-negative: They do not retain the crystal violet stain used in Gram staining.

    2. Non-motile: Shigella bacteria do not have flagella for movement.

    3. Facultative anaerobe: Capable of anaerobic fermentation as well as aerobic respiration.

    4. Non-sporeforming: They do not form spores under adverse conditions.

    5. Non-capsulated: Shigella species do not produce a protective capsule.

    6. Non-lactose fermenting: Most species do not ferment lactose, with S. sonnei as an exception.

    7. Catalase positive: All except S. dysenteriae type 1 produce catalase; oxidase negative: Shigella is oxidase negative.

Cultural Characteristics

  • Growth Conditions:

    • Temperature range: 10-40°C, optimum at 37°C.

    • Colony Characteristics:

    1. On nutrient agar: colonies are 2 mm in diameter, circular, convex, smooth, and translucent.

    2. On MacConkey agar: colonies are colorless except for S. sonnei, which appears pink due to lactose fermentation.

    3. On Salmonella-Shigella agar: forms colorless colonies.

  • Selective Media:

    • Deoxycholate citrate agar and xylose lysin deoxycholate (XLD) agar are useful for isolating Shigella. Notably, Shigella colonies do not have a black center in these mediums, in contrast to Salmonella.

Viability

  • Death Point: The bacteria can be killed at 56°C for 1 hour or in 1% phenol for 30 minutes.

  • Survival:

    • Viable in water and ice for 1 to 6 months.

    • In feces, they die within a few hours due to the acidity caused by the growth of competing coliform bacteria.

Biochemical Reactions

  • Key Reactions:

    • Catalase: Positive except for S. dysenteriae type 1.

    • Methyl Red (MR): Positive.

    • Voges-Proskauer (VP): Negative.

    • Urease: Negative.

    • Citrate: Negative.

    • Oxidase: Negative.

    • Hydrogen Sulfide (H2S): Negative.

  • Notably, S. sonnei is characterized as a late lactose fermenter.

Classification Based on Biochemical and Antigenic Characteristics

  1. Subgroup A: S. dysenteriae - 15 serotypes.

  2. Subgroup B: S. flexneri - 8 serotypes.

  3. Subgroup C: S. boydii - 19 serotypes.

  4. Subgroup D: S. sonnei - only 1 serotype.

  • S. dysenteriae is noted as the most serious type, associated with bacillary dysentery due to Shiga toxin.

Virulence Factors

  • Plasmid Antigens: The plasmids facilitate the transfer of effectors from bacterial cytoplasm to the epithelial cell cytoplasm of the colon.

  • Invasiveness: Virulent strains of Shigella penetrate the mucosa and epithelial cells of the colon in a non-uniform manner, leading to intracellular multiplication and invasion of adjacent cells. The consequent inflammation contributes to cell death, influenced by the cytotoxic properties of Shiga toxin that disrupt protein synthesis.

  • Symptoms of Infection: Cellular death and the resultant phagocytic response by the host results in a bloody discharge of mucus and pus, as well as the characteristic shallow ulcers associated with the disease.

  • Toxins: Shigella produces a variety of toxins, particularly a Shiga toxin which is implicated in neurotoxic, cytotoxic, and enterotoxic activities, responsible for the watery diarrhea seen in infections.

Clinical Symptoms

  • The disease ranges from asymptomatic to severe bacillary dysentery

  • Two-stage disease: watery diarrhea changing to dysentery with frequent small stools with blood and mucus, tenesmus, cramps, fever

    Early stage:

    • Watery diarrhoea attributed to the enterotoxic activity of Shiga toxin

    • Fever attributed to neurotoxic activity of toxin

Process involves:

  • Ingestion

  • Non-invasive colonization and cell multiplication

  • Production of the enterotoxin by the pathogenic bacteria in the small intestine:

Second stage:

  • Adherence to tissue invasion of large intestine

  • Typical symptoms of dysentery

  • Cytotoxic activity of Shiga toxin increases severity

Pathogenesis

  • Source: Humans act as either cases or carriers.

  • Mode of Spread: Transmission occurs through contaminated fingers, food, flies, fomites, and also person-to-person contact.

  • Infective Dose: Only 10-100 viable bacilli are necessary to cause infection.

  • Concentration of Pathogen: During early/acute infection, there can be 10^3 to 10^9 viable bacilli per gram of stool.

Laboratory Diagnosis

  • Sample Collection: Fresh stool, mucus flakes, and rectal swabs are typically collected for testing.

  • Media for Enrichment: Selenite F broth (0.4%) serves as both enrichment and transport media, with incubation for 9-12 hours.

  • Culture Media: Various media are used including non-selective Bromocresol purple lactose agar, low selective MacConkey agar, and high selective deoxycholate citrate agar, alongside Salmonella-Shigella (SS) agar.

Epidemiology

  • Reservoir: Humans are the sole reservoir for Shigella.

  • Transmission: Primary route is the fecal-oral route.

  • Distribution by Geography: S. flexneri is more common in developing countries, while S. sonnei is predominant in developed countries.

Treatment and Control

  • Antibiotics: Treatment regimens typically involve:

    • Ciprofloxacin

    • Fluoroquinolone

    • Azithromycin

    • Pivmecillinam

    • Ceftriaxone

  • Control Measures:

    • Prevent infected individuals from handling food.

    • Ensure thorough washing of hands after changing infant diapers.

    • Disinfect surfaces handled by infected individuals.

    • Restrict infected children from communal swimming areas.

    • If traveling, consume only boiled or filtered water, peel fruits yourself, and eat only hot meals.

    • Follow proper food storage practices.

Conclusion

  • Understanding Shigella’s characteristics, pathogenesis, and control measures is crucial for prevention and treatment of dysentery caused by these bacteria.