Shigella is a significant pathogen responsible for shigellosis, particularly in children and areas with poor sanitation. Understanding its characteristics, pathogenic mechanisms, and effective laboratory diagnosis is crucial for managing infections caused by Shigella. Continuous surveillance and antibiotic resistance research are necessary to combat this public health challenge effectively.
General Character
- Genus: Shigella
- Species: Shigella dysenteriae, Shigella flexneri, Shigella boydii, Shigella sonnei
- Family: Enterobacteriaceae
- Gram Staining: Shigella is a Gram-negative bacterium, appearing pink due to its thin peptidoglycan layer and outer membrane.
- Shape and Arrangement:
- Shape: Rod-shaped (bacilli).
- Arrangement: Typically found as single cells, but can form pairs.
- Oxygen Requirements: Shigella species are facultative anaerobes, capable of growing in both aerobic and anaerobic conditions.
Morphology
- Cell Wall Structure:
- It comprises a thin peptidoglycan layer surrounded by an outer membrane containing lipopolysaccharides (LPS), contributing to its virulence.
- Flagella: Shigella species are non-motile and do not possess flagella.
Cultural Characteristics
- Growth Media:
- MacConkey Agar: Selective for Gram-negative bacteria; Shigella appears as colorless colonies due to non-lactose fermentation.
- XLD Agar (Xylose Lysine Deoxycholate): Useful for differentiating Shigella, which produces red colonies without black centres.
- Hektoen Enteric Agar: Differentiates Shigella from other enteric pathogens; colonies appear green or colorless.
- Colony Appearance:
- On selective media, colonies are typically pale or colorless, indicating non-lactose fermentation.
- Temperature and pH Range:
- The optimal growth temperature is around 37°C, with a pH range of 6.0 to 7.5 being favourable.
Biochemical Reactions
- Catalase Test: Shigella is catalase-positive, producing bubbles when added hydrogen peroxide.
- Oxidase Test: Shigella is oxidase-negative, distinguishing it from other Gram-negative bacteria.
- Lactose Fermentation: Shigella does not ferment lactose.
- Indole Production: Some species, like S. flexneri, are indole-positive, while others are negative.
- Methyl Red Test: Positive, indicating mixed acid fermentation.
- Voges-Proskauer Test: Negative, indicating the absence of acetoin production.
Pathogenicity
- Virulence Factors:
- Type III Secretion System (T3SS): Injects virulence factors into host epithelial cells, facilitating invasion.
- Shiga Toxin: Produced primarily by S. dysenteriae, it can cause severe damage to the intestinal mucosa and lead to hemolytic uremic syndrome (HUS).
- Adhesins: Promote attachment to intestinal epithelial cells.
- Clinical Infections:
- Shigellosis: Characterized by diarrhoea, which may be bloody, abdominal pain, fever, and tenesmus (a feeling of incomplete defecation). The disease is often self-limiting but can be severe, especially in young children and immunocompromised individuals.
- Hemolytic Uremic Syndrome (HUS): A serious complication associated with Shiga toxin-producing strains.
Laboratory Diagnosis
- Specimen Collection: Clinical specimens may include stool samples.
- Microscopic Examination:
- Gram staining reveals Gram-negative bacilli.
- Culture Techniques:
- Inoculation on selective media like MacConkey or Hektoen agar, followed by incubation.
- Biochemical Testing:
- Confirmatory tests for lactose fermentation, indole production, and other biochemical reactions to identify Shigella species.
- Serotyping: O antigen serotyping can identify specific Shigella serovars.
Antibiotic Resistance
- Emergence of Resistance: Shigella species have shown increasing resistance to multiple antibiotics, especially in regions with high endemicity.
- Multidrug-Resistant Strains: Resistance to common antibiotics, including ampicillin and trimethoprim-sulfamethoxazole, is a significant concern.
- Treatment Options: Antibiotic susceptibility testing is crucial; treatment may include azithromycin or fluoroquinolones for severe cases.
Prevention
- Hygiene Practices: Proper handwashing and sanitation prevent Shigella transmission, particularly in crowded living conditions.
- Food Safety: Safe food handling and preparation practices can help reduce the risk of infection.
- Vaccination: Currently, no vaccines for Shigella are available, but research is ongoing.