Bio 348: Gram Neg Bacteria

Introduction to Enterobacteriaceae

• Classification: Family of gram-negative rods, many of which are found in the gastrointestinal (GI) tract.
• Common examples: Escherichia coli (E. coli), Klebsiella, Citrobacter.
• Fermentation: These organisms ferment glucose, utilizing it as their energy source.

General Characteristics of Enteric Bacteria

• Morphology:

  • All are gram-negative rods.
  • Can grow both aerobically and anaerobically (facultatively anaerobic).

• Biochemical Properties:

  • Oxidase negative.
  • Capable of reducing nitrates.
  • Appearance on blood agar: Typically medium to large gray colonies.
  • Mobility: Many are motile.

• Clinical Relevance:

  • Majority are part of the normal flora of the GI tract, also found in animals.

Pathogenicity

Yersinia Pestis

• Pathogen causing bubonic plague (the black death).
• Transmission: Fleas on rats serve as the vector of infection.
  • Reference to historical pandemic leading to massive mortality.

MacConkey Agar

• Definition: A selective and differential medium used to isolate gram-negative rods.
• Functionality: Contains crystal violet and bile salts to inhibit gram-positive bacteria.
• Composition: Contains lactose; allows identification of lactose fermenters, indicated by a pink color change.
• Lactose Fermenters:
  • Example: E. coli exhibits dry flat colonies; Klebsiella appears wet and mucoid due to its capsule.

Identification of E. Coli O157:H7

• Differentiation: Sorbitol is used in place of lactose on selective media to differentiate from other E. coli strains.
  • E. coli O157:H7 does not ferment sorbitol.
• Clinical Manifestation: Can cause bloody diarrhea and hemolytic uremic syndrome (HUS).

Eosin Methylene Blue Agar (EMB)

• Description: A selective and differential medium similar to MacConkey.
• Components: Contains lactose and sucrose.
• Lactose fermenters: May exhibit a green metallic sheen (indicative of E. coli).
• Non-lactose fermenters appear dark purple; slow fermenters appear brown or pink.

Antigenic Structure of E. Coli

• O Antigens: Somatic antigens located on the outer membrane of the lipopolysaccharide layer.
  • Involved in septic shock (toxic shock) when released.
• H Antigens: Related to flagellar structure.
• K Antigens: Capsular antigens, with specific relevance to virulent strains.

Pathogen Groups of Enterobacteriaceae

Opportunistic Pathogens

• Nature: Normally part of flora; may cause disease in immunocompromised hosts.
  • Example: E. coli causing UTIs when it enters urethra.

Intestinal Pathogens

• Nature: Not part of normal flora with greater virulence.

  • Examples include Salmonella and Shigella.

• Similarity: E. coli and Shigella are closely related; E. coli can produce Shigella-like toxins leading to bloody diarrhea.

Clinical Significance of Enterobacteriaceae

E. Coli

• Predominantly identified pathogen in clinical labs.
• Associated infections: UTI, sepsis, wound infections, pneumonia, and meningitis in newborns.

Identification in Lab

• Blood and MacConkey agar characteristics:
  • Blood agar colonies: Flat gray, sometimes beta hemolytic.
  • MacConkey agar colonies: Pink (indicating lactose fermentation).

Variation and Mutations

• Antibiotic treatment can lead to morphological changes in E. coli colonies.
  • Non-lactose fermenters may appear due to genetic mutations.

Pathogenic Variants of E. Coli

Uropathogenic E. Coli (UPEC)

• Causes urinary tract infections.

Meningitis E. Coli (MNEC)

• Causes neonatal meningitis; positive for K1 antigen.

Enterotoxigenic E. Coli (ETEC)

• Associated with traveler's diarrhea; produces cholera-like toxins causing clear, watery diarrhea.

Enteroinvasive E. Coli (EIEC)

• Produces bloody diarrhea and Shigella-like toxin through a type three secretion system (T3SS).

Shiga toxin-producing E. Coli (STEC)

• Notable variant includes E. coli O157:H7, associated with severe illnesses including HUS.

Shigella

• Highly infectious; primarily causes bacillary dysentery (shigellosis).
• Transmission: Fecal-oral, through contaminated food or water. Human-specific pathogen.
• Symptoms: Watery diarrhea leading to inflammation and the presence of blood and pus in stool.

Salmonella

• Commonly associated with poultry; can cause gastroenteritis and systemic infections (salmonellosis).
• Reservoir: Intestinal tracts of humans and animals.

Salmonella Typhi

• Causes typhoid fever; chronic carriers can result in ongoing transmission (e.g., Typhoid Mary case).

Yersinia

• Yersinia pestis causes bubonic plague.
• Yersinia enterocolitica: associated with undercooked pork, can mimic appendicitis.