Corynebacterium diphtheriae is the bacterium that causes diphtheria, a serious and potentially life-threatening disease. This Gram-positive bacterium is non-motile and typically appears as a club-shaped rod under a microscope. It primarily inhabits the human throat and skin, its natural reservoir.
The Bacterium and Its Toxin
The defining characteristic of Corynebacterium diphtheriae is its ability to produce diphtheria toxin, a powerful protein causing severe disease symptoms. Its production depends on the bacterium being infected by corynephage β, a specific bacteriophage carrying the toxin gene.
The diphtheria toxin is composed of two fragments: Fragment A, the catalytic domain, and fragment B, which binds to cell surface receptors and facilitates fragment A’s entry. Once inside the cell, fragment A inactivates eukaryotic elongation factor 2 (eEF-2), necessary for protein synthesis. This halts the cell’s protein production, leading to cell death and tissue necrosis.
How Diphtheria Spreads
Diphtheria primarily spreads from person to person through respiratory droplets. These droplets are released when an infected individual coughs, sneezes, or talks. Inhaling them can lead to infection.
Transmission can also occur through direct contact with discharge from infected skin lesions. Asymptomatic carriers, who carry the bacterium but show no symptoms, can also transmit Corynebacterium diphtheriae. This complicates disease control, as carriers may be unaware they are infectious.
Symptoms and Complications of Diphtheria
The primary symptom of respiratory diphtheria is the formation of a characteristic pseudomembrane. This tough, grayish membrane, composed of fibrin, bacterial and inflammatory cells, typically forms in the throat or nose. Its presence can obstruct the airway, leading to severe breathing difficulties and suffocation.
Other common symptoms include a sore throat, fever, swollen glands in the neck (often called “bull neck” due to swelling), and general weakness. The diphtheria toxin can also spread through the bloodstream to distant organs, causing serious complications. This can lead to heart damage (myocarditis), nerve damage (paralysis), and kidney problems.
Diagnosis and Treatment
Diphtheria diagnosis typically begins with a clinical assessment based on the observable symptoms, especially the presence of the pseudomembrane. This clinical suspicion is then confirmed through laboratory tests, primarily bacterial culture. Swabs are taken from the throat or nose and cultured to identify the presence of Corynebacterium diphtheriae.
Treatment for diphtheria requires immediate intervention. Diphtheria antitoxin (DAT) is administered promptly to neutralize the circulating toxin that has not yet bound to cells. This antitoxin is crucial because it can only neutralize free toxin, not toxin that has already adhered to host cells. Antibiotics, such as penicillin or erythromycin, are also prescribed to kill the bacteria and prevent further toxin production.
Preventing Diphtheria
Vaccination is the most effective method for preventing diphtheria. The diphtheria vaccine contains an inactivated form of the diphtheria toxin, called a toxoid. This toxoid stimulates the body’s immune system to produce antibodies that can neutralize the actual diphtheria toxin if a person is exposed to the bacterium.
The diphtheria vaccine is typically administered as part of combination vaccines. For infants and young children, it is often given as part of the DTaP vaccine, which also protects against tetanus and pertussis. Adolescents and adults receive Tdap or Td boosters, which provide continued protection against diphtheria and tetanus. Adhering to the recommended routine vaccination schedule and receiving booster shots as advised are important for maintaining long-term immunity against diphtheria. This sustained protection helps prevent outbreaks and reduces the severity of the disease in vaccinated individuals.