Salmonella is a significant pathogen responsible for various gastrointestinal and systemic infections. Understanding its characteristics, pathogenic mechanisms, and effective laboratory diagnosis is crucial for managing infections caused by Salmonella species. Continuous surveillance and research into antibiotic resistance are necessary to address the challenges posed by these bacteria effectively.
General Character
- Genus: Salmonella
- Species: Salmonella enterica (including various serovars, such as S. Typhi and S. Typhimurium)
- Family: Enterobacteriaceae
- Gram Staining: Salmonella 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 appear in pairs or chains.
- Oxygen Requirements: Salmonella species are facultative anaerobes, capable of growing in aerobic and anaerobic conditions.
Morphology
- Cell Wall Structure:
- It comprises a thin peptidoglycan layer surrounded by an outer membrane containing lipopolysaccharides (LPS), which can elicit strong immune responses.
- Flagella: Most Salmonella species are motile, possessing flagella that allows movement.
Cultural Characteristics
- Growth Media:
- MacConkey Agar: Selective for Gram-negative bacteria; Salmonella typically appears as colourless colonies due to non-lactose fermentation.
- XLD Agar (Xylose Lysine Deoxycholate): Differentiates Salmonella from other enteric pathogens; produces red colonies with black centres due to hydrogen sulfide production.
- Hektoen Enteric Agar: Similar to XLD, used to isolate Salmonella, which forms green colonies with black centres.
- Colony Appearance:
- On selective media, colonies are typically pale or colourless due to a lack of lactose fermentation and may exhibit black centres if hydrogen sulfide is produced.
- 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: Salmonella is catalase-positive, producing bubbles when added hydrogen peroxide.
- Oxidase Test: Salmonella is oxidase-negative, distinguishing it from other Gram-negative bacteria.
- Lactose Fermentation: Salmonella does not ferment lactose (except for some rare strains).
- Indole Production: Some serovars (e.g., S. Typhimurium) are indole-positive.
- 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 cells, aiding in invasion.
- Adhesins: Facilitate attachment to intestinal epithelial cells.
- Endotoxin: The lipopolysaccharide (LPS) triggers strong immune responses, contributing to symptoms.
- Clinical Infections:
- Gastroenteritis: Caused primarily by S. Typhimurium and S. Enteritidis; symptoms include diarrhoea, fever, and abdominal cramps.
- Typhoid Fever: Caused by S. Typhi; characterized by prolonged fever, abdominal pain, and systemic infection.
- Bacteremia: This can lead to sepsis, particularly in immunocompromised individuals.
- Enteric Fever: A systemic infection caused by S. Typhi and S. Paratyphi.
Laboratory Diagnosis
- Specimen Collection: Clinical specimens may include stool, blood, or urine samples, depending on the suspected infection.
- Microscopic Examination:
- Gram staining reveals Gram-negative bacilli.
- Culture Techniques:
- Inoculation on selective media like XLD or Hektoen agar, followed by incubation.
- Biochemical Testing:
- Confirmatory tests for lactose fermentation, indole production, and other biochemical reactions to identify Salmonella species.
- Serotyping: O and H antigen serotyping can identify specific Salmonella serovars.
Antibiotic Resistance
- Emergence of Resistance: Salmonella species have shown increasing resistance to multiple antibiotics, particularly in strains acquired through food sources.
- Multidrug-Resistant Strains: Concerns regarding resistance to first-line antibiotics, particularly in S. Typhi and non-Typhi serovars.
- Treatment Options: Antibiotic susceptibility testing is essential; treatment for typhoid fever often involves fluoroquinolones or third-generation cephalosporins.
Prevention
- Food Safety Practices: Proper food handling, cooking, and hygiene can prevent Salmonella infections.
- Vaccination:
- Typhoid vaccines (oral and injectable) are available for S. Typhi and are recommended for travellers to endemic areas.
- Surveillance: Monitoring food sources and outbreaks is essential for public health.