Microbiology Lecture Notes on Gram-Negative Bacilli and Important Pathogens

Microbiology Lecture 26 - Overview

Key Concepts on Gram-Negative Bacilli

  • Overview of several important enteric bacteria:
    • Escherichia coli
    • Salmonella typhimurium
    • Shigella spp.
    • Haemophilus influenzae

Coliform Bacteria

  • Definition: Coliform bacteria are Gram-negative, enteric bacteria capable of lactose fermentation.
  • Transmission: Primarily spread via the fecal-oral route.
  • Common Examples: Include genera such as Escherichia, Klebsiella, Enterobacter, and Citrobacter.
    • Many coliforms are opportunistic pathogens.
  • Testing: Various tests have been developed to identify coliforms among other Gram-negative rods present in the intestines.

Lactose Fermentation Tests

  • Lactose non-fermenters appear clear on media.
  • Lactose fermenters appear purple-ish or pink.
  • Example: H₂S positive colonies indicate specific types of bacteria.

Pathogenicity and Immune Response

  • Bacterial antigens play a significant role in pathogenicity:
    • K Antigen: Capsule
    • O Antigen: Lipopolysaccharide (LPS)
    • H Antigen: Flagella
  • Notable strain: E. coli O157:H7

Transmission Routes for Coliforms

  • Spread via the Five F’s plus water:
    • Feces
    • Food
    • Fingers
    • Flies
    • Fomites (inanimate objects)
    • Water
  • Food Safety: Cook meat to an internal temperature of 165extoF165^ ext{o} F to kill potential pathogens.

Escherichia coli Overview

  • Characteristics:
    • Most prevalent enteric bacillus in the gut
    • A flagellated facultative anaerobe
    • Major pathogen and normal flora
  • Pathogenic strains vary in virulence and lead to various diseases including gastroenteritis and UTIs.

E. coli Disease Presentation

  • E. coli Gastroenteritis: Common via fecal-oral route.
  • Nosocomial UTIs: Can escalate to serious conditions like sepsis or meningitis.

Pathogenic Strains of E. coli

  • Enterotoxigenic E. coli (ETEC): Causes travel-related diarrhea due to two exotoxins.
  • Enterohemorrhagic E. coli (EHEC): Known for causing bloody diarrhea; associated with undercooked meat. Notable strain: O157:H7.
  • Enteroinvasive E. coli (EIEC): Causes bloody diarrhea without producing toxins; similar to Shigella dysentery.
  • Uropathogenic E. coli (UPEC): Leading cause of community-acquired UTIs.

Diagnosis and Treatment for E. coli Infections

  • Diagnosis through culture from sterile sites; bloody diarrhea often indicates a dysentery-causing organism.
  • Culture media:
    • MacConkey Agar: Pink colonies indicate lactose fermentation.
    • EMB Agar: Bright green colonies.
  • Treatment:
    • Generally supportive for gastroenteritis.
    • Cefotaxime for sepsis and meningitis, sulfa-trimethoprim for UTIs.

Salmonella typhimurium Overview

  • Description: Facultative anaerobic, flagellated, lactose-negative Gram-negative rod.
  • Obligate pathogen of the GI tract.

Salmonella typhimurium Symptoms and Presentation

  • Gastroenteritis: Symptoms include nausea, vomiting, and diarrhea through fecal-oral route.
  • More severe complications: Bacteremia or sepsis, particularly in vulnerable populations.

Typhoid Fever Presentation

  • Caused by S. typhi; incubation period varies from 5-21 days.
  • Symptoms: Nonspecific febrile illness, GI symptoms, abdominal pain, and rose spots rash.

Bioterrorism Case

  • Rajneesh Bioterrorism Attack: 751 people infected in Oregon, highlighting the potential for foodborne illness as a weapon.

Diagnosis and Treatment for Salmonella typhimurium

  • Diagnosis relies on isolation from blood, feces, or bone marrow; typically cultured on selective agar.
  • Treatment: Supportive therapy along with antibiotics like ciprofloxacin.

Shigella Overview

  • Characteristics: Non-lactose fermenting, non-motile Gram-negative rods causing dysentery.
  • Notable strains: S. dysenteriae, S. sonnei, S. flexneri, and S. boydii.

Shigella Presentation

  • Incubation leads to fever, cramps, and bloody diarrhea with mucus, especially severe in pediatric/geriatric patients.

Diagnosis and Treatment for Shigella

  • Culture on specific media shows colorless colonies.
  • Treatment if mild: Supportive therapy; severe cases include ciprofloxacin.

Haemophilus influenzae Overview

  • Description: Small Gram-negative rod, colonizes the upper respiratory tract, can be encapsulated.
  • Common diseases include acute bacterial meningitis, pneumonia.

Diagnosis and Treatment for Haemophilus influenzae

  • Growth on chocolate agar with necessary supplements (Factor X and V).
  • Treatment of choice: Ceftriaxone; vaccination has significantly reduced disease incidence.