Salmonella Bacteria: Microbiology, Virulence Factors, and Clinical Syndromes

Microbiological Profile of Salmonella

General Characteristics

• Gram-negative rods (1–3 µm × 0.5–1 µm).
• Motile via peritrichous flagella (except S. Gallinarum/Pullorum).
• Facultative anaerobes – grow with or without oxygen.
• Acid-resistant – survive gastric passage to infect intestines.
• Wide host range: Humans, livestock, poultry, reptiles, birds.

Taxonomy

Two species: Salmonella enterica (6 subspecies, >2,600 serovars) and Salmonella bongori. Serovars named by antigenic formula (O:H:K), e.g., S. Typhimurium, S. Enteritidis, S. Typhi.

Virulence Factors

• Type III Secretion Systems (T3SS): SPI-1 (invasion), SPI-2 (intracellular survival).
• Adhesins (fimbriae): Bind intestinal epithelium.
• Toxins: Enterotoxin (causes diarrhea), cytotoxins.
• Antigenic variation: Phase variation of flagellar (H) antigens evades immunity.
• Horizontal gene transfer: Acquires antibiotic resistance, pathogenicity islands.

Clinical Syndromes

• Gastroenteritis (non-typhoidal): Diarrhea, fever, cramps; self-limited in healthy adults.
• Typhoid/Paratyphoid fever (S. Typhi, S. Paratyphi): Systemic, prolonged fever, bacteremia; human-restricted.
• Invasive non-typhoidal salmonellosis: Bacteremia, focal infections (meningitis, osteomyelitis) in immunocompromised.

Public Health

Reportable disease. Outbreaks linked to eggs, poultry, produce, reptiles. Prevention: cook thoroughly, avoid cross-contamination, wash hands, pasteurization.