Enterobacteriaceae: Salmonella Overview

Enterobacteriaceae: Salmonella

• Salmonella is a gram-negative rod in the Enterobacteriaceae family.

• Three species exist: S. enterica, S. bongori, and S. subterranea.

• Subspecies of S. enterica are determined by biochemical tests.

  • Enterica (subspecies I): warm-blooded animals
  • Salamae (subspecies II): cold-blooded animals
  • Arizonae (subspecies IIIa): cold-blooded animals
  • Diarizonae (subspecies IIIb): cold-blooded animals
  • Houtenae (subspecies IV): cold-blooded animals
  • Indica (subspecies VI): cold-blooded animals

• LPS O-antigen and flagella vary considerably within subspecies, resulting in over 2600 serotypes in S. enterica ssp. enterica alone.

• Specific nomenclature:

  • S. enterica subsp. diarizonae serotype 60:k:z is written as S. IIIb 60:k:z.
  • S. enterica susb. enterica serotype 6,7:c:1,5 is written as S. enterica ssp. enterica serotype Choleraesuis.

Salmonella Nomenclature (cont'd)

• Flagella variation is significant between Salmonella subspecies.
  • Flagella can be mono- or diphasic.
• Phase variation is a reversible genetic rearrangement, allowing the bacteria to switch between expressing flic (phase 1), fljB (phase 2), and flpA (phase 3), which are often encoded on plasmids and used by other bacterial pathogens.

Salmonella Nomenclature (cont'd) Table

• A table demonstrating serotypes, serogroup, somatic antigens (O), and flagella (H) antigens.
• Values from the table, which refer to the serotype, serogroup and antigens (O and H) are included here, representing the vast number of Salmonella serotypes.

Pathogenesis

• Pathogenesis depends on the host factors, the specific strain, and the infectious dose.
• The primary target for infection is M cells in the distal small intestine and upper large bowel.
• Three fundamental mechanisms: adhesion, invasion, and intracellular survival.

Pathogenesis (cont'd)

• Adhesion: Fimbriae (Pef, Agf, Lpf) bind to M cells.
• Invasion: SPI-1 effectors (SipA, SopB, SopE) trigger membrane ruffling, allowing the bacteria to enter epithelial cells utilizing the actin cytoskeleton rearrangement.
• Intracellular Survival: SPI-2 effectors enable survival inside macrophages. This is supported by formation of a Salmonella-containing vacuole (SCV), allowing the bacteria to replicate.

Pathogenesis (cont'd)

• The type III secretion system, encoded in the Salmonella pathogenicity island 2 (SPI-2), facilitates the intracellular growth of non-typhoidal Salmonella.
• It accomplishes this by interfering with the maturation of Salmonella-containing vacuoles.
• This system also protects Salmonella against the antimicrobial activity of reactive oxygen species (ROS).

Pathogenesis (cont'd)

• Intracellular survival strategies evade the immune system, mainly through the PhoP/PhoQ system regulation. This system is a two-component regulatory system (TCS), which allows bacteria to adapt to their environment.
• The system comprises of at least two proteins: the sensor kinase and a response regulator., involved in the survival of Salmonella inside macrophages.

Pathogenesis (cont'd)

• The PhoP/PhoQ system regulates survival of Salmonella in macrophages by reacting to factors like mildly acidic pH and periplasmic Mg2+ in the cytosol.
• This system affects bacterial virulence via effects on cell surface modifications, resistance to antimicrobial peptides and promoting growth in acid pH.

Host Immune Response to Salmonella

• Inflammatory responses are mediated by cytokines, such as interleukin-8 (IL-8).
• Neutrophil recruitment and epithelial damage are seen as part of the immune response to Salmonella infection.

Clinical Manifestations

• Gastroenteritis: The most common presentation, characterized by diarrhea and abdominal pain.
  • Typically in cases of non-typhoidal salmonellosis.
• Septicemia: Occurs in severe infection; cases often leading to a systemic spread.
  • Particularly associated with typhoidal salmonellosis.
• Host-specific infections: Salmonella can cause abortions in ruminants and horses.

Laboratory Diagnosis of Salmonella

• Samples: Fecal samples (enteric form) and blood cultures (systemic disease).
• Culture media:
  • MacConkey agar (lactos non-fermenting colonies)
  • XLD agar (black-centered colonies due to hydrogen sulfide production).
• Serotyping: Determining O and H antigens.
• Molecular methods (PCR) for detection and identification.

Treatment and Control

• Treatment:
  • Supportive care for gastroenteritis.
  • Antibiotics (antimicrobial susceptibility testing crucial).
• Control:
  • Good hygiene and biosecurity measures.
  • Vaccination (modified live vaccines).

New topic

• Edwardsiella tarda is associated with diarrhoeas and wound infections in some animals.
• Enterobacter aerogenes can cause coliform mastitis (Udder inflammation) in cows and sows, and related issues in different animals.
• Klebsiella pneumoniae is linked to coliform mastitis in cows, endometritis (inflammation of the uterus lining) in mares, pneumonia in calves and foals and urinary tract infections in dogs.
• Morganella morganii subspecies morganii causes ear and urinary tract infections in dogs and cats, plus other issues.
• Proteus mirabilis and P. vulgaris can cause urinary tract infections in dogs and horses and otitis externa, in dogs (inflammation of the outer ear).
• Serratia marcescens is associated with bovine mastitis and septicemia in chickens.

Explore the diverse species and subspecies of Salmonella, a member of the Enterobacteriaceae family. Learn about the important biochemical tests that determine subspecies and the significance of LPS O-antigen and flagella variation, resulting in numerous serotypes. This quiz will deepen your understanding of Salmonella nomenclature and classification.