In-Depth Notes on Gram-Negative Bacteria (Enterobacteriaceae)

Gram-Negative Bacteria (Enterobacteriaceae)

Overview of Enterobacteriaceae

  • Large and diverse group of gram-negative bacteria known as "Enterics" or "Enterobacteria."
  • Includes normal flora in the gastrointestinal tract of humans and many animals.
  • Pathogenic examples include Salmonella, Shigella, and Yersinia.
  • Main Mode of Infection (MOI): Typically through ingestion, with fomites playing an important role.

Classification

  • Tribes/Groups:
  • Escherichiae
  • Klebsielliae
  • Proteae
  • Yersineae
  • Erwineae
  • Family: Enterobacteriaceae

General Characteristics

  • Structure includes:
  • Cell Wall: Composed of lipopolysaccharides (LPS) and a thin peptidoglycan layer.
  • Morphology: Medium-sized, gram-negative rods.
  • Metabolism: Aerobic or facultatively anaerobic (both respiratory and fermentative).
  • Flagella: Usually peritrichous except in Shigella and Klebsiella.
  • Capsulation: Typically non-capsulated, exceptions include Klebsiella and some E. coli strains.
  • Reproduction: Non-spore forming, non-fastidious, meaning they can grow on a wide range of media.

Lactose Fermenters

  • Identified on MacConkey Agar:
  • Escherichia coli: Pink colonies, metallic sheen on EMB agar, motile, no viscous colonies.
  • Enterobacter spp.: Raised non-viscous colonies.
  • Klebsiella spp.: Very viscous mucoid colonies, non-motile.
  • Slow Lactose Fermenters: (Opportunistic pathogens) Includes Citrobacter, Serratia, Providencia.
  • Non-Lactose Fermenters: (Colorless colonies on MacConkey, also opportunists):
  • Shigella: Non-motile
  • Salmonella: Motile
  • Proteus: Notable for swarming and urease production.

Escherichia coli

  • General:
  • Named after Theodor Escherich.
  • Contains 6 species, with E. coli being significant in animal health.

Morphology and Cultural Features

  • Produces large, smooth, circular colonies on Nutrient Agar.
  • On MacConkey Agar: Bright pink colonies due to lactose fermentation.
  • On Blood Agar: Shows beta-hemolytic colonies.
  • On EMB Agar: Exhibits metallic sheen colonies.

Biochemical Tests

  • Indole Test: Positive (produces indole).
  • Methyl Red Test: Positive (pH below 4.5).
  • Voges-Proskauer (VP) Reaction: Negative.
  • Citrate Test: Negative (no growth).

Pathogenic Mechanisms

  • Enterotoxigenic E. coli (ETEC): Causes diarrhea in livestock, particularly calves and pigs with virulence factors like enterotoxins.
  • Enteropathogenic E. coli (EPEC) and Enteroinvasive E. coli (EIEC): Disrupt epithelial cells in the intestines.
  • Enterohemorrhagic E. coli (EHEC): Produces verotoxins associated with severe disease in animals and humans.
  • Enteroaggregative E. coli (EAEC): Associated with diarrhea in calves; pathogenesis is still unclear.
  • Virulence mechanisms can cause severe diseases, especially in young animals due to their physiological conditions and immune status.

Salmonella spp.

  • Taxonomy:
  • Domain: Bacteria, Phylum: Pseudomonadota, Class: Gammaproteobacteria, Order: Enterobacterales, Family: Enterobacteriaceae.
  • Reservoir: Found in gastrointestinal tracts of a variety of animal species and in contaminated environments.

Pathogenesis

  • Initial adhesion to intestinal M cells using various adhesins through Type III secretion systems leading to invasion of the intestinal epithelium.
  • Immune evasion tactics ensure bacteria survive within macrophages.
  • Diseases associated with Salmonella include diarrhea, septicemia, gastroenteritis, and systemic infections in ruminants and pigs.

Host-Specific Diseases

  • Ruminants: Can exhibit septicemia, enteritis, and bronchopneumonia.
  • Swine: Associated with high mortality rates in acute infections. Chronic diseases can lead to severe enteric complications.
  • Clinical Signs: In adult livestock often represented by respiratory distress, diarrhea, and systemic shock.

Prevention and Control

  • Proper hygiene practices in feeding and housing environments, management of stress factors, and vaccination of susceptible animals.
  • Antibiotic use in livestock should be judicious to avoid resistance development.

Conclusion

  • Enterobacteriaceae comprise a significant part of veterinary microbiology. Understanding their pathogenic mechanisms helps in diagnosing and controlling diseases in livestock effectively.