Detailed Study Notes on Gram-Positive and Gram-Negative Bacilli

LECTURE 5: GAN-POSITIVE BACILLI AND GRAM-NEGATIVE BACILLI

GRAM-POSITIVE BACILLI I

  • Spore-forming and Non-spore-forming Bacilli
    • Cocci: 2 types (staphylococci and streptococci)
    • Rods: 4 types (2 spore-formers and 2 non-spore-formers)
    • Unique Features: Bacilli are the only bacteria with a protein capsule made of poly-D-glutamic acid.
    • Capsule Function: Prevents phagocytosis.

Bacillus anthracis (Causes Anthrax)

  • Host Species: Primarily affects herbivores (cows, sheep).
  • Characteristics of Spores:
    • Very stable and resistant to drying, heat, ultraviolet light, and disinfectants.
    • Spores germinate and produce toxins.
  • Historical Use: Used in biological warfare (e.g., Japanese army in Manchuria, 1940).
  • Transmission: Human exposure typically occurs through direct contact with infected animals and soil.
  • Germination Factors: Activates with increased temperature (37ºC), carbon dioxide levels, and presence of serum proteins.
    • Activation: Spores only activate upon introduction into a host.
  • Plasmids:
    • pXO1: Encodes virulence factors.
    • pXO2: Encodes genes for the synthesis of the poly-glutamyl capsule.

Bacillus cereus (Causes Gastroenteritis)

  • Differences with B. anthracis:
    • Motile, non-encapsulated, resistant to penicillin.
    • Causes food poisoning which includes nausea, vomiting, and diarrhea when spores survive cooking.
  • Types of Enterotoxins Produced:
    1. Heat-labile toxin: Similar to cholera and LT from E. coli, symptoms include nausea and abdominal pain typically lasting 12-24 hours.
    2. Heat-stable toxin: Causes severe nausea and vomiting with limited diarrhea.
  • Antibiotic Note: Antibiotic therapy ineffective in food poisoning due to pre-formed toxins.

CLOSTRIDIUM SPECIES

  • Overview: Gram-positive, spore-forming, anaerobic rods.
  • Known for diseases such as:
    • Botulism
    • Tetanus
    • Gas gangrene
    • Pseudomembranous colitis

Clostridium botulinum

  • Effects: Produces lethal neurotoxin causing rapidly fatal food poisoning.
  • Mechanism: Blocks the release of acetylcholine, results in flaccid muscle paralysis.
  • Symptoms:
    • Bilateral cranial nerve palsies
    • Double vision
    • Difficulty swallowing
    • General muscle weakness (may lead to respiratory paralysis and death).
  • Sources of Infection: Eating improperly prepared canned vegetables or smoked fish.

Clostridium tetani

  • Source of Infection: Puncture wounds from contaminated objects.
  • Mechanism: Exotoxin tetanospasmin causes sustained muscle contraction (tetany) by blocking inhibitory neurotransmitter release.
  • Clinical Presentation: Includes severe muscle spasms and a grimacing expression (risus sardonicus).
  • Vaccination: Tetanus toxoid booster every 10 years is part of the DPT shot.

Clostridium perfringens

  • Cause: Gas gangrene
  • Mechanism: Bacteria grow in anaerobic conditions leading to gas production.
  • Symptoms:
    1. Cellulitis/Wound Infection: Characterized by gas formation beneath the skin (crepitus).
    2. Clostridial Myonecrosis: Severe muscle tissue damage due to the action of exotoxins.
  • Diagnosis: CT scans show gas pockets in tissue.

Clostridium difficile

  • Cause: Antibiotic-associated pseudomembranous enterocolitis.
  • Mechanism: Arises from overuse of antibiotics, which destroy normal flora.
  • Exotoxins Produced:
    • Toxin A: Causes diarrhea.
    • Toxin B: Cytotoxic to colonic cells.
  • Symptoms: Severe diarrhea and abdominal cramping.
  • Treatment: Discontinue initial antibiotic, administer metronidazole or vancomycin.

NON-SPORE-FORMING GRAM-POSITIVE RODS

Listeria monocytogenes

  • Characteristics: Can cross blood-brain, gastrointestinal, and feto-placental barriers; psychrophilic.
  • Symptoms: Vary from mild symptoms, meningitis to stillbirth in pregnant women.
  • At-risk Populations: Young, elderly, immunosuppressed individuals.
  • Treatment: Ampicillin or trimethoprim-sulfamethoxazole.

Corynebacterium diphtheriae

  • Pathogen for Diphtheria: Forms grayish pseudomembrane in pharynx and releases exotoxin into the bloodstream.
  • Mechanism of Action: Exotoxin damages heart and neural cells, inhibiting protein synthesis.
  • Treatment Steps:
    1. Administer antitoxin to inactivate circulating toxin.
    2. Treat with penicillin or erythromycin.
    3. Vaccination as part of DPT.
  • Pathogenicity Mechanism: Requires lysogenization by a temperate bacteriophage for exotoxin production.

GRAM-NEGATIVE BACILLI

General Properties:

  • Part of normal intestinal flora and cause gastrointestinal diseases.
  • Divided based on biochemical and antigenic properties.

SALMONELLAE

  • Properties: Lactose non-fermenters, motile, with animal reservoirs (except S. enterica serovar Typhi).
  • Nomenclature: Two species: S. enterica and S. bongori.
    • S. enterica has multiple serovars.
  • Infections in Humans:
    1. Enterocolitis: Caused by many Salmonella serotypes.
    2. Enteric Fever: Caused by S. enterica serovar Typhi and S. enterica serovar Paratyphi.

ENTEROCOLITIS (GASTROENTERITIS)

  • Pathogenesis: Severity influenced by:
    • Dose of ingested organisms (minimum of 10^5 usually required).
    • State of host, virulence of strain.
  • Incubation Period: Short (6-48 hours; usually 8-12 hours).
  • Symptoms: Nausea, vomiting, diarrhoea, abdominal pain, fever (38-39ºC).
  • Diagnosis: Stool culture.
  • Epidemiology: Contaminated food/water due to poor handling, especially animal products.
  • Treatment: Antimicrobials are not recommended for uncomplicated cases.

ENTERIC FEVER (TYPHOID AND PARATYPHOID)

  • Caused by: S. enterica serovar Typhi (typhoid fever) and S. enterica serovar Paratyphi (paratyphoid fever).
  • Pathogenesis: Severe generalized infection with multiplication in lymphoid tissue; can lead to intestinal ulceration.
  • Diagnosis: Isolation from blood and stool.
  • Epidemiology: Transmission through contaminated food and water; human carriers as reservoirs.
  • Prevention: Sanitary disposal, cleanliness, and vaccination (low efficacy).

SHIGELLAE

  • Characteristics: Generally non-lactose fermenters causing acute diarrhoea.
  • Pathogenesis: Invasion of the epithelial mucosa leads to severe illness.
  • Epidemiology: Often affects children under 10 in unsanitary conditions.
  • Prevention: Similar strategies to typhoid.

ESCHERICHIA COLI

  • Characteristics: Lactose fermenters; major agent of urinary tract infections.
  • Pathogenic Strains: Distinct pathotypes based on virulence:
    • Enteropathogenic E. coli (EPEC): Severe diarrhea in infants.
    • Enterotoxigenic E. coli (ETEC): Major cause of infantile diarrhea.
    • Enteroinvasive E. coli (EIEC): Bloody diarrhea similar to Shigella.
    • Enteroaggregative E. coli (EAEEC).
    • Enterohemorrhagic E. coli (EHEC): Causes hemorrhagic colitis and hemolytic uremic syndrome; e.g., E. coli O157:H7.

OTHER GRAM-NEGATIVE BACTERIA

  • Cronobacter spp.: Associated with hospital infections and contaminated powdered infant formula.
  • Vibrio cholerae: Causes cholera; produces enterotoxin causing massive diarrhea and dehydration.
  • Campylobacter spp.: Leading causes of gastroenteritis in humans.
  • Pseudomonas: Opportunistic pathogens found in moist environments affecting immunocompromised patients.

CONCLUSIONS

  • Knowledge of pathogenic bacteria structure, mechanisms, and treatment options is critical in clinical settings, particularly in diagnosing and managing infectious diseases effectively.