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Enterobacteriaceae I. General Characteristics

  • Family Overview: The Enterobacteriaceae family encompasses a large assortment of bacteria characterized by small, gram-negative (G-) rods measuring about 1 mm by 2-3 mm.
  • Habitat: Members are prevalent in various environments, such as soil, water, and decaying organic matter. Many species are also normal inhabitants of the human and animal large intestine.
  • Spore Formation: They are non-spore forming organisms.
  • Oxygen Tolerance: Although they thrive best in aerobic conditions, they exhibit facultative anaerobic growth capabilities.

Key Properties

  1. Ferment Glucose: All members can ferment glucose, a critical metabolic pathway.
  2. Reduce Nitrates to Nitrites: They can transform nitrates into nitrites, an important biochemical reaction.
  3. Oxidase Negative: They do not produce the enzyme oxidase.
  4. Catalase Positive: They possess catalase, which decomposes hydrogen peroxide.
  5. Motility: Many may be motile due to flagella, with exceptions including Shigella and Klebsiella.

Gram Stain - Enterobacteriaceae

  • The Gram stain reaction classifies species of Enterobacteriaceae as gram-negative, affirming their structural characteristics based on peptidoglycan layer properties.

Organisms Contained within the Enterobacteriaceae Family

Coliforms (Lactose Fermenting Bacteria)

  • These bacteria are classified as normal flora and can be opportunistic pathogens. They include:
       1. Escherichia
       2. Klebsiella
       3. Enterobacter
       4. Hafnia
       5. Serratia
       6. Citrobacter

Non-Coliforms (Lactose Negative Bacteria)

  • Members that are typically opportunistic and can serve as normal gut flora:
       1. Proteus
       2. Morganella
       3. Providencia
       4. Edwardsiella

Pathogenic Groups within Enterobacteriaceae

True Pathogenic Enterics

  1. Salmonella sp.
  2. Shigella sp.
  3. Yersinia enterocolitica

True Pathogenic Non-Enterics

  1. Yersinia pestis

Epidemiology

  • Enteric pathogens, particularly those within Enterobacteriaceae, represent the leading contributors to diarrheal illnesses worldwide.
  • Enterobacteriaceae, alongside Pseudomonas, occupy a significant share, accounting for about 50% of all isolates in nosocomial infections.

Common Virulence Factors of Enterobacteriaceae

  1. Complex Surface Antigens:
       - H (Flagellar Antigen)
       - K (Capsular Antigen/Fimbrial Antigen)
       - O (Somatic or Cell Wall Antigen)
       - Not every species possesses H & K antigens, but all members have O antigen.
  2. Endotoxin: The presence of lipopolysaccharide (LPS) in the outer membrane serves as an endotoxin.

Laboratory Identification of Enterobacteriaceae

Media Used

  • MacConkey (MAC) Agar:
       - Contains bile salts and crystal violet inhibiting gram-positive bacteria; distinguishes lactose fermenters by changing color.
  • Eosin Methylene Blue (EMB) Agar:
       - Similar to MAC, promotes dye reactions indicating lactose fermentation with metallic sheen for rapid fermenters.
  • Hektoen Enteric Agar (HEA):
       - Effective for isolating pathogens and demonstrating lactose fermentation and H2S production.

Other Key Tests for Identification

  1. Adhesins: Features that assist in attachment to host cells.
  2. Siderophores: Molecules that sequester growth factors such as iron.
  3. Antigenic Phase Variation: Control of H and K antigen expression.

IMViC Tests

  • The IMViC test set is useful for identifying coliforms and consists of:
       - Indole Test: Tests for the ability to cleave the amino acid tryptophan.
       - Methyl Red Test (MR): Indicates acid fermentation.
       - Voges-Proskauer Test (VP): Assesses for acetoin post-glucose fermentation.
       - Citrate Test: Evaluates the capacity to utilize citrate as a carbon source.

Protocol for Isolating Enteric Genera

Media Characteristics

  • MacConkey Agar: Promotes the growth of lactose fermenters (red colonies for positive; uncolored for negative).
  • Levine's EMB Agar: Produces distinctive coloration in fermenters (dark nuclei and metallic sheen).
  • Hektoen Enteric Agar: Differentiate lactose fermenters (salmon pink/orange) from nonfermenters (green/blue-green); H2S production indicated by black centers.

Enterobacteriaceae Identifications Flow Chart

  • Specimens subjected to media (EMB, MAC, HE) for lactose/glucose and motility tests leading to differentiation among species through subsequent tests.

Escherichia coli

Pathogenic Strains

  1. Enterotoxigenic (ETEC): Targets small intestine leading to watery diarrhea.
  2. Enteroinvasive (EIEC): Affects the large intestine with associated cramping and watery diarrhea.
  3. Enteropathogenic (EPEC): Causes diarrhea primarily in infants, leading to dehydration without blood in stools.
  4. Enterohemorrhagic (EHEC): Notable serotype O157:H7 linked to severe hemorrhagic colitis.
  5. Enteroaggregative (EAEC): Leads to persistent diarrhea in infants; associated with biofilm formation on tissue.

Virulence Factors Specific to E. coli

  • Adhesins: Crop between multiple types (CF pili, AAF, Bfp, Intimin).
  • Exotoxins: Various including heat-stable (STa, STb) and heat-labile (LT1, LT2) toxins plus Shiga toxins (Stx1, Stx2).

Clinical Diseases Associated with E. coli

  • Septicemia: Originates typically from urinary or gastrointestinal tracts; high mortality rate in immunocompromised patients.
  • Urinary Tract Infections (UTIs): Commonly associated with strains producing specific adhesins.
  • Neonatal Meningitis: Major cause of CNS infections in infants, particularly associated with K1 capsular antigen strains.
  • Gastroenteritis (EPEC, ETEC, EHEC): EPEC linked to severe infant diarrhea; ETEC common in traveler’s diarrhea; EHEC risk is from undercooked meat products.

Clinical Manifestations of E. coli Infections

Symptoms and Disease Duration

  • Initial symptoms for gastroenteritis typically present as watery diarrhea; follow with vomiting and become severe in strains like EHEC with potential progression to hemorrhagic colitis and HUS.

Other Notable Enterobacteriaceae Members

  • Klebsiella pneumoniae: Often involved in nosocomial pneumonia, UTI.
  • Enterobacter: Associated with UTIs.
  • Citrobacter: Typically seen in UTI cases and bacteremia in debilitated individuals.
  • Proteus: Notable for swarming motility, primarily linked to UTIs and occasionally septicemia.
  • Morganella and Providencia: Associated with UTIs and wound infections.

Salmonella I. General Characteristics

  • More than 2500 serotypes primarily under Salmonella enterica; with individual serotypes often misclassified as distinct species.

Pathogenicity and Epidemiology

  • Survival within macrophages; infections common through foodborne vectors or animal feces contamination. Salmonella Typhi and Salmonella Paratyphi uniquely adapted to humans without other reservoirs.

Salmonella III. Virulence Factors

  1. Pathogenicity Islands (I & II): Large gene clusters controlling adhesion and immune evasion.
  2. Acid Tolerance Response (ATR): Protects against acidic environments during infection.

Salmonella IV. Clinical Diseases

  • Enteric Fever: Caused by S. Typhi; transmission through contaminated food/water with global incidence estimates reaching millions annually.
       - Symptoms progressive after ingestion, lasting several weeks.
  • Gastroenteritis: Primarily foodborne, requiring high doses, with diarrhea appearing shortly after infection.

Shigella I. General Characteristics

  • Comprising four species (S. dysenteriae, S. sonnei, S. flexneri, S. boydii), historically classified as E. coli biogroups, recognized for significant public health implications.

Clinical Presentation

  • Shigella disease typically arises from fecal-oral transmission requiring very few organisms to cause infection.

Yersinia enterocolitica

  • Associated with gastroenteritis, particularly in colder climates, linked to contaminated food or water.

Yersinia pestis I. General Characteristics

  • A gram-negative rod known for a complex life cycle involving various hosts, the bacterium is known for its virulence, causing outbreaks historically known as the plague.

Treatment & Prevention

  • Plague treatment typically involves antibiotics with interventions dictated by clinical symptoms (fever, malaise, etc.).