Gram Positive Non-Spore Forming Rods

Gram Positive Non-Spore Forming Rods

Genus: Corynebacteria

  • Consists of 136 recognized species, with at least 30 associated with human diseases.
  • Corynebacterium diphtheriae:
    • Causal agent of diphtheria.
    • Most widely studied species.
  • Non-diphtherial corynebacteria are collectively referred to as diphtheroids.

Corynebacterium diphtheriae

Characteristics:
  • Gram-positive bacilli.
  • Non-spore forming.
  • Non-capsulated.
  • Non-motile.
  • Grows best under aerobic conditions.
  • Shows maximum pleomorphism on Gram staining.
  • Possesses metachromatic granules that store inorganic phosphate.
    • Appears as irregular swelling at one end, giving a "club-shaped" appearance.
  • Bacteria group together forming V and L shapes, called Chinese-character arrangement.
    • Caused by incomplete separation of daughter cells during division.
Host and Biotypes:
  • Humans are the only natural host and significant reservoir of infection.
  • Classified into three distinct biotypes based on growth characteristics, biochemical reactions, and disease severity:
    • mitis
    • intermedius
    • gravis
Virulence Factors:
  • Diphtheria toxin:
    • Key virulence factor.
    • Powerful exotoxin produced only by C. diphtheriae (tox+ strains) containing intracellular structural tox genes.
    • The gene is present in the prophage chromosome (corynephages tox+).
    • Consists of two functionally distinct fragments: A and B.
      • B: Binds to receptors of target cells.
      • A: Toxic activity.
      • Neither fragment is toxigenic on its own but acts together.
  • Toxin Mechanism:
    • Binds to cell receptors and enters via receptor-mediated endocytosis.
    • Fragment A is cleaved and released from the B subunit.
    • Fragment A inactivates host cell EF-2 (elongation factor) molecules, stopping protein synthesis, causing cell death and necrotic lesions.
Pathogenesis:
  • Transmitted by nasopharyngeal secretions, entering through the upper respiratory tract.
  • Infection starts with adherence of toxigenic strains in the nasopharyngeal cavity.
  • Incubation period: 2 to 5 days, with sore throat as the initial symptom.
  • Lesion formation:
    • Occurs on tonsils and oropharynx.
    • Organisms multiply rapidly, leading to local cell necrosis.
    • A pseudomembrane forms, composed of cell debris, exudative inflammatory response (red blood cells, necrosed cells, bacteria, fibrin, lymphocytes).
    • The membrane adheres tenaciously; removal causes bleeding.
  • Toxaemia:
    • Occurs due to toxin absorption from the membrane site in the nasopharyngeal cavity.
Clinical Manifestations:
  • Low-grade fever
  • Sore throat
  • Suffocation (due to upper respiratory tract pseudomembrane formation)
  • Arrhythmia
  • Difficulty of vision and swallowing
  • Difficulty in upper and lower extremities movement
Treatment:
  • Penicillin and Erythromycin:
    • Inhibit growth of diphtheria bacilli and arrest toxin production.
  • Administration of antitoxin.
  • DPT vaccine (diphtheria, pertussis, tetanus).

Genus: Listeria

  • Most important species: Listeria monocytogenes
General Characteristics:
  • Gram-positive rods
  • Non-sporulating
  • Non-capsulated
  • Motile by peritrichous flagella at 22-25°C; non-motile at 37°C.
  • Facultative anaerobe
  • Catalase positive
  • Can survive at refrigerator temperatures (4°C), low pH, and high salt conditions, making it a foodborne pathogen.
Virulence Factors:
  • Listreriolysin (hemolysin) toxin.
  • 13 known serovars based on O (somatic) and H (flagellar) antigens.
  • Serotype 4b causes most of the foodborne outbreaks.
Pathogenesis:
  • Transmitted to humans through ingestion of contaminated food.
  • Bacteria enter epithelial cells with the help of cell wall surface proteins called internalins A and B.
  • Organisms proliferate, then listeria are released, and the cycle begins again.
  • Symptoms of listeriosis:
    • Fever
    • Chills
    • Headache
    • Myalgia
    • Abdominal pain
    • Diarrhea
Intracellular Life Cycle:
  • Entry mediated by InlA and InlB
  • Vacuolar lysis mediated by LLO, PlcA, PlcB
  • Replication
  • Actin-based motility (ActA)
  • Cell-to-cell spread
Perinatal Listeriosis:
  • Early onset syndrome (granulomatosis infantiseptica):
    • Result of infection in utero when listeria crosses the placenta.
    • Death may occur before or after delivery.
  • Late-onset syndrome:
    • Causes meningitis between birth and the third week of life.
    • Newborn may become infected during or after delivery.
    • Often caused by serotype 4b.
Treatment:
  • Ampicillin
  • Gentamicin
  • Erythromycin
  • Intravenous trimethoprim-sulfamethoxazole.

Genus: Erysipelothrix

  • Most important species: Erysipelothrix rhusiopathiae
General Characteristics:
  • Appear singly or in short chains.
  • Non-motile
  • Non-sporulating
  • Non-capsulated
  • Catalase negative
  • Aerobic and facultative anaerobic
  • Grow better in 5-10% CO2CO_2
  • Causes disease in domestic swine, turkeys, ducks, and sheep.
  • Swine is a major reservoir.
Pathogenicity:
  • Most common human infection: erysipeloid.
    • Usually occurs on the fingers (seal finger and whale finger).
  • Persons at greatest risk: fishermen, fish handlers, abattoir workers, butchers, and others who have contact with animal products.
  • Incubation: 2–7 days.
  • Symptoms: Pain (can be severe) and swelling.
  • Lesion: Raised, well circumscribed, and violaceous in color; resembles cellulitis caused by Streptococcal skin infections.
  • Resolution: Can resolve without treatment after 3–4 weeks or more rapidly with antibiotic treatment.
  • Additional clinical forms: bacteremia with or without endocarditis.
Treatment:
  • Penicillin
  • Erythromycin
  • Ciprofloxacin
  • Clindamycin

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  • Gram Negative Diplococci