Corynebacterium diphtheriae Overview

Corynebacterium diphtheriae

• Gram-positive, aerobic, nonmotile, rod-shaped bacteria
• Related to Actinomycetes
• Do not form spores or branch
• Exhibit club-shaped, V-shaped, or irregular arrangements
• Undergo snapping movements after cell division
• Arrangements resemble Chinese letters
• "Barred" appearance due to metachromatic granules (polyphosphate inclusions)

Corynebacterium genus

• Diverse group of bacteria
• Includes animal and plant pathogens
• Some are saprophytes
• Part of the normal human flora

Corynebacterium diphtheriae strains

• Three recognized strains: gravis, intermedius, mitis
• Listed in descending order of disease severity
• All produce the same toxin
• Capable of colonizing the throat
• Differences in virulence explained by toxin production rate and quantity, and growth rate
• Gravis strain: 60 minutes generation time
• Intermedius strain: ~100 minutes generation time
• Mitis strain: ~180 minutes generation time
• Faster growing strains produce larger colonies

Corynebacterium diphtheriae pathogenicity

• Spread via droplets, secretions, or direct contact
• In situ lysogenic conversion of nontoxigenic strains to toxigenic phenotype occurs
• Infection primarily among humans
• Toxigenic strains isolated from horses
• Asymptomatic nasopharyngeal carriage is common in endemic regions
• Diphtheritic lesions often covered with a pseudomembrane (fibrin, bacteria, and inflammatory cells)
• Diphtheria toxin cleaved into fragment A (catalytic domain) and fragment B (transmembrane and receptor binding domains)
• Fragment A catalyzes NAD+-dependent ADP-ribosylation of elongation factor 2, inhibiting protein synthesis
• Fragment B binds to cell surface receptor, delivering fragment A to the cytosol

Corynebacterium diphtheriae symptoms

• Thick, gray membrane covering the throat and tonsils (pseudomembrane)
• Sore throat and hoarseness
• Swollen glands (enlarged lymph nodes) in the neck ("bull's neck")
• Difficulty breathing or rapid breathing
• Nasal discharge
• Fever and chills
• Tiredness

Corynebacterium diphtheriae toxigenicity

• Two factors influence diphtheria toxin production:
  • Low extracellular iron concentrations
  • Presence of a lysogenic prophage
• Toxin gene on prophage chromosome
• Bacterial repressor protein controls expression.
• Repressor activated by iron.
• High toxin yields in lysogenic bacteria under iron deficiency.

Corynebacterium diphtheriae regulation of Diphtheria Toxin Expression

• Expression controlled by the diphtheria toxin repressor gene (dtxR)
• Iron (Fe2+) activates the DtxR protein
• DtxR protein dimers inhibit toxin gene expression by blocking RNA polymerase binding.

Diphtheria Toxin

• Blocks protein synthesis
• Protein is 63 kDa.
• Controlled by the toxin gene
• Lysogenic phage (Beta-corynephage)
• Expressed if [iron] is low
• Two components: A and B

Diphtheria toxin mechanism

• Receptor-mediated endocytosis

• PH change, formation of open monomer

• Insertion of T-domain into membrane

• Translocation of C-domain into cytoplasm.

• Catalysis (EF-2 + NAD+ → ADP-ribose-diphthamide-EF-2 + nicotinamide + H)

• A subunit inactivates elongation factor-2 (EF-2), halting protein synthesis

• Toxin fragment A catalyzes a reaction yielding nicotinamide and an inactive adenosine diphosphate ribose-EF-2 complex

Corynebacterium diphtheriae diagnosis

• Primary isolation via various media (Loeffler's serum, Tinsdale tellurite agar)
• Diphtheritic lesions often covered with pseudomembrane
• Corynebacterium diphtheriae colonies on tellurite agar are black, on blood agar colonies are white
• Elek immunodiffusion test for toxigenicity (double diffusion of diphtheria toxin and antitoxin)
• Detection of precipitin band within 18-48 hours

Corynebacterium urealyticum

• Causes urinary tract infections (UTIs)
• Rare but important
• Urease hydrolyzes urea, releasing NH4+
• Increases pH, causing alkaline urine, and contributing to renal stone formation

Corynebacterium jeikeium

• Opportunistic infections in immunocompromised patients (e.g., blood disorders, bone marrow transplants, intravenous catheters)
• Multiple antibiotic resistance (MDR) common
• Often found on skin of hospitalized patients

Listeria monocytogenes

• Causes meningitis and sepsis (newborns, immunosuppressed adults) and febrile gastroenteritis
• Small, gram-positive rod, arranged in V or L shapes, similar to corynebacteria
• Exhibits unusual tumbling movement
• Produces hemolysin
• Can tolerate a wide temperature range, including refrigeration (5°C)

Listeria monocytogenes pathogenesis

• Primarily in two settings:
  • Fetal transmission via placenta or delivery.
  • Immunocompromised adults, especially renal transplant patients.
• Worldwide distribution in animals, plants, and soil.
• Transmitted through contact with animals/feces, unpasteurized foods/milk, contaminated vegetables
• In the U.S. listeriosis often food-borne, associated with unpasteurized cheese.
• Invade mononuclear phagocytic cells.
• Intracellular growth.
• Cell-mediated immunity more important than humoral to fight Listeria.
• Suppression of cell-mediated immunity increases risk.

Listeria monocytogenes pathogenicity mechanisms

• "Actin rockets" enable cell-to-cell movement
• Listeriolysin O (LLO) is a cytolysin/hemolysin similar to streptolysin O - it degrades cell membranes

Listeria monocytogenes laboratory diagnosis

• Primarily diagnosed through Gram stain and culture
• Gram-positive rods
• Small, grey, translucent colonieson blood agar
• Surrounded by a small zone of indistinct beta hemolysis.
• Incubation may take 48 hours
• Confirmed by presence of "motile" organisms to differentiate from nonmotile corynebacteria

Explore the characteristics and classifications of Corynebacterium diphtheriae, a Gram-positive bacterium known for its unique cellular arrangements and virulence factors. This quiz covers strains, growth rates, and their role in human flora, providing insight into this significant pathogen.