C . DIPTHERIA

Corynebacterium diphtheriae

Corynebacterium diphtheriae is a gram-positive, non-spore-forming, non-motile bacterium that causes diphtheria, a potentially life-threatening respiratory disease. The bacterium is transmitted via respiratory droplets and colonizes the mucous membranes of the respiratory tract, where it produces a powerful exotoxin responsible for the disease's clinical manifestations. C. diphtheriae can also cause skin infections and, less commonly, systemic infections.

Microscopic Appearance

• Gram-positive rods.

• Club-shaped or “palisade” arrangement (resembling Chinese letters or V and Y shapes).

• Non-motile.

• Metachromatic granules may be seen with special stains (e.g., Albert's stain), giving a characteristic beaded appearance.

• Better stained with special stains - Albert’s, Neisser’s and Ponder’s stain

Virulence Factors

• Diphtheria Toxin:

  • The major virulence factor, produced by toxigenic strains of C. diphtheriae that are lysogenized by a bacteriophage carrying the tox gene.

  • The toxin is an A-B type exotoxin:

    • B subunit binds to the host cell membrane, facilitating entry.

    • A subunit inactivates elongation factor-2 (EF-2) in the host cell, halting protein synthesis and leading to cell death.

  • Toxin production can lead to localized damage in the throat and systemic effects, including myocarditis and neuropathy.

• Adhesins: Surface proteins that mediate adherence to the epithelial cells of the respiratory tract.

• K Antigen (Capsule): Although C. diphtheriae is not typically encapsulated, some strains may have a capsule that helps resist phagocytosis.

Pathogenesis

• Colonization:

  • The bacteria colonize the mucous membranes of the upper respiratory tract, particularly the tonsils, pharynx, and larynx.

• Toxin Production:

  • The diphtheria toxin is produced and released, leading to local tissue necrosis and formation of a pseudomembrane, a hallmark of the disease.

  • The toxin can also enter the bloodstream, causing systemic effects, particularly on the heart and nervous system.

  • Bacilli are noninvasive - secrete the toxin - spreads via bloodstream to various organs

• Pseudomembrane Formation:

  • The pseudomembrane consists of dead epithelial cells, fibrin, leukocytes, and bacteria, and it can obstruct the airway, leading to breathing difficulties.

Clinical Manifestations

• Respiratory Diphtheria:

  • Pharyngeal or Tonsillar Diphtheria:

    • Symptoms include sore throat, fever, malaise, and the presence of a thick, grayish pseudomembrane on the tonsils, pharynx, or larynx.

    • Severe cases can lead to airway obstruction, dysphagia, and hoarseness.

    • Systemic toxin effects can cause myocarditis, presenting with arrhythmias, heart failure, or even sudden death.

  • Laryngeal Diphtheria:

    • Can lead to croup-like symptoms, including stridor and respiratory distress.

• Cutaneous Diphtheria:

  • Presents with non-healing ulcers covered by a grayish membrane.

  • More common in tropical climates and among those with poor hygiene.

• Systemic Complications:

  • Myocarditis: Occurs in severe cases and can manifest with heart block, arrhythmias, and heart failure.

  • Neuropathy: Cranial nerves are often affected first, leading to difficulty swallowing, paralysis of the soft palate, and hoarseness. Peripheral neuropathy can follow.

Lab Diagnosis

• Culture:

  • Throat swabs or samples from the pseudomembrane are cultured on selective media such as Löeffler's medium or tellurite agar (Tinsdale agar), where C. diphtheriae forms black or brown colonies due to tellurite reduction.

• Toxin Detection:

  • The Elek test, an immunodiffusion assay, can be used to detect the diphtheria toxin.

  • PCR can detect the presence of the tox gene responsible for toxin production.

• Microscopy:

  • Staining with methylene blue or Albert's stain can reveal metachromatic granules within the bacteria, which are diagnostic.

• Biochemical Tests:

  • C. diphtheriae is catalase-positive and urease-negative.

  • It ferments glucose and maltose but not lactose or sucrose.

Elek’s gel precipitation test

• Isolates 1 and 2: Precipitation bands crossed over – toxins are not-identical - strains are unrelated

• Isolate 2 and 3: Partial fusion of precipitation bands - strains are partially related to each other

• Isolates 3 and 4: Precipitation bands fused with each other - strains are completely related

• Isolate 5 : non-toxigenic strain (no precipitation band is formed).

Treatment

• Antitoxin:

  • Diphtheria antitoxin, derived from horses, is administered intramuscularly or intravenously to neutralize the circulating toxin. It is most effective when given early in the course of the disease.

• Antibiotics:

  • Penicillin or erythromycin is used to eradicate the bacteria and stop toxin production. Antibiotics do not neutralize the toxin but prevent further production and transmission.

  • For patients allergic to penicillin, erythromycin is the alternative.

• Airway Management:

  • In cases of airway obstruction due to the pseudomembrane, intubation or tracheostomy may be necessary.

• Supportive Care:

  • Cardiac monitoring and management of complications such as myocarditis and neuropathy are critical.

• Prevention:

  • Vaccination with the diphtheria toxoid (part of the DTP/DTaP vaccine) is the primary preventive measure. Boosters are recommended every 10 years.

  • Close contacts of diphtheria patients should receive prophylactic antibiotics and, if necessary, a booster dose of the vaccine.

Importent points

• Corynebacterium diphtheria is Gram positive bacilli

• Corynebacterium diphtheria contains metachromatic granules

• Primarily infects - throat and produces toxin (diphtheria toxin) - causes an exudative pharyngitis and membranous tonsillitis.

• Microscopy appearance of C.diptheria ; Gram positive bacilli appeared chilies letter form V or L form

• Loeffler’s serum slope is the enriched media for C.diptheria

• Diphtheria toxin is Phage coded: β-corynephage - carrying tox gene

• Diphtheria is toxemia but never a bacteremia

• Bacilli are noninvasive - secrete the toxin - spreads via bloodstream to various organs

• Toxin responsible for all types of manifestations - local (respiratory) and systemic complications

• C.diphtheria will cause : Respiratory Diphtheria, Cutaneous Diphtheria

• Respiratory Diphtheria : Tonsil and pharynx (faucial diphtheria) - most common sites

• Pseudomembrane colitis is the important clinical feature formed due to C.diphtherial infection

• Bull-neck appearance is seen in C.diphtherial infection

• Selective media for C.diptheria : Potassium tellurite agar

• Elek’s gel precipitation test : For detection of Diphtheria toxin

• Vaccine given for C,diphtheria : DPT vaccine