Microbiology
Summary
Corynebacterium diphtheriae is a gram-positive , non-spore-forming bacillus . It is characterized by a club shape in a Y or V formation and contains metachromatic granules that stain with aniline dyes. The exotoxin produced by Corynebacterium diphtheriae has two subunits, A and B, where A is the active subunit and B is the binding subunit. The exotoxin causes ADP-ribosylation of elongation factor-2 , inhibiting ribosome function and protein synthesis. This ultimately leads to cell death and the formation of pseudomembranes , a thick grayish exudate over the mucosal surface of the oropharynx .
Infection is transmitted by respiratory droplets and colonizes the oropharynx, which can lead to airway obstruction and severe lymphadenopathy. Systemic effects of the toxin can affect the heart and nervous system, leading to life-threatening myocarditis, arrhythmias, heart block, and local paralysis. Lab diagnosis of diphtheria involves culturing the bacteria on tellurite and Loeffler's media. To differentiate toxic and non-toxic strains, an Elek's test is used. Vaccination for diphtheria is performed using a toxoid vaccine consisting of inactivated exotoxin band of protein, typically given with tetanus toxoid and acellular pertussis vaccine. In unvaccinated individuals, passive immunization with anti-toxoid serves as the primary treatment.
Lesson Outline
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FAQs
The main virulence factor of C. diphtheriae is its exotoxin, which is responsible for the development of the disease. This potent toxin is encoded by the tox gene within the bacterial genome. When the bacteria infect the human respiratory system, they produce and release the exotoxin, which targets host cells and interferes with protein synthesis by inactivating the elongation factor-2 through ribosylation. This leads to cell death, inflammation, and formation of the characteristic pseudomembrane in the throat.
The pseudomembrane is a thick, grayish-white layer that forms in the throat during a diphtheria infection. It consists of dead cells, bacterial debris, fibrin, and other components that accumulate at the site of infection. The pseudomembrane can obstruct the airway, making it difficult for the patient to breathe and swallow, and can even be life-threatening if left untreated. It is a hallmark sign of diphtheria and helps clinicians to identify the disease.
Bull's neck is a term used to describe the severe neck swelling that can occur in diphtheria patients. This swelling is due to the spread of the infection to the cervical lymph nodes and the accumulation of fluid in the surrounding tissues. It is called bull's neck because the swollen appearance of the neck resembles that of a bull. This symptom is a sign of severe diphtheria and is usually associated with a higher risk of complications and poor prognosis.
In the laboratory, C. diphtheriae can be isolated from a patientÕs throat swab or other clinical samples. The bacteria are typically cultured on selective media like Loeffler's medium or tellurite agar, which encourages their growth while inhibiting other bacteria. Characteristic colony morphology and biochemical tests can provide preliminary identification. The definitive confirmation of C. diphtheriae involves the detection of the tox gene or the diphtheria toxin production, which can be assessed using the Elek's test, a specialized immunoprecipitation assay.
Elek's test is an in vitro immunoprecipitation method used to confirm the production of diphtheria toxin by the isolated Corynebacterium diphtheriae strains. The test involves the cultivation of the suspected C. diphtheriae isolate on a tellurite agar plate, together with antitoxin-impregnated filter paper strips. If the strain is toxigenic, the exotoxin produced during incubation reacts with the antitoxin, resulting in the formation of visible precipitation lines. Elek's test is essential for the diagnosis of diphtheria because it detects the presence of the main virulence factor (diphtheria toxin) and helps guide appropriate treatment strategies.
