CORYNEBACTERIA & DIPHTHERIA - Corynebacteria are aerobic (facultatively anaerobic). - They are Gram positive bacilli (rods). - They are non-spore bearing - They are non-motile - They are catalase positive - They ferment various carbohydrates, producing lactic acid xx. The appearance of stained films resemble Chinese letters……………. xx. Only C. diphtheriae causes a major disease (diphtheria) which is now rare where immunization is effective. Other Corynebacteria are our normal flora: C. minutissimum causes superfical skin infection (erythrasma) that appears similar to the mycoses. xx. Pathogenesis & Virulence Corynebacteria provide a spectrum of pathogenicity from the aggressive primary pathogen C. diphtheriae, through the normal flora which occasionally become opportunistic pathogens, e.g

C. haemolyticum, C. xerosis, C. pseudodiptheriticum, and C. jeikeium. x. Virulence in C. diphtheriae is caused by the diphtheriae exotoxin, which interfers with protein synthesis; this is only produced when the tox gene is transduced to the Corynebacterium by a lysogenic bacteriophage (beta phage)**. Toxin is only produced when the concentration of iron in the medium is low.

All exotoxin is antigenically the same, so one (monovalent) antitoxin is sufficient for treatment. Dermonecrotic toxin act on vascular endothelial cells to increase vascular permeability. Confirmatory tests Laboratory tests can take a week, so the initial diagnosis of diphtheria must be clinical. x. Gram stain: throat swabs often negative……… x. Culture on blood agar is more reliable: three variants of C. diphtheriae occur, mitis (small black colonies); intermedius AND gravis (large, grey-black colonies). Tellurite is used in selective media. x. Biochemical tests: C. diphtheriae ferments maltose x. Toxin production: Elek’s immunodiffusion method. xx. Clinical Syndromes & management

C. Diphtheriae causes pharyngeal, nasopharyngeal & laryngeal diphtheria with a pseudo-membrane, cutaneous diptheria (Veldt sore, Barcoo rot). The other Corynebacteria may cause opportunistic infections. x. Diphtheria is completely preventable by immunisation usually as “triple antigen”. Antitoxin is essential for clinical diphtheria and must be given as early as possible without waiting for confirmatory tests. Penicillin is used in addition to kill the bacteria.

DIPHTHERIA: Diphtheria, caused by toxin-producing strain of C. diphtheriae, is a rare but important infection of the upper airways that causes respiratory obstruction and distant effects, particularly on myocardium and nervous tissue.

It is classified by the major site of infection, i.e nasal, pharyngeal and tonsilar, nasopharyngeal, laryngeal, bronchial or cutaneous diphtheria.

“Pseudo-diphtheria” means similar but milder infections with C. haemolyticum or C.ulcerans. The bacteria multiply locally without spreading…..

The exotoxin destroys epithelial cells and polymorphs causing a local ulcer. The toxin enters the blood stream causing fever, myocarditis within the first 2 weeks.

Clinical features:

Nasal diphtheria is usually mild, with purulent nasal discharge and few systemic symptoms. Pharyngeal diphtheria is the initial and commonest clinical presentation, with sudden onset of fever, malaise and pharyngitis, followed by a prominent pseudomembrane of the bacteria and necrotic tissue cells on the tonsil and posterior pharynx. This can spread upwards, with marked systemic symptoms. Respiratory obstruction and death may follow rapidly, or myocarditis or neurological complication

Cutaneous diphtheria: (Veldt sore, Barcoo rot) is now very rare. It forms a chronic indolent ulcer. Systemic symptoms rarely follow. Confirmatory test x. Treatment cannot wait on laboratory tests, but throat and nasopharyngeal swabs should be taken for later confirmation.

Management: Diphtheria is a life-threatening illness. As soon as the diagnosis is suspected the patient is isolated to reduce spread, and treatment with anti-toxin is started. The airway must be secured. Penicillin is the drug of choice. Erythromycin is a second choice. Control and prevention: Immunization with killed vaccine, usually of DPT triple antigen with pertusis and tetanus is highly effective for 10 years…….. Patient contacts need a booster if immunization was more than 10 years ago …… Erythromycin usually eradicates asymptomatic carriage. Corynebacteria & Diphtheria – Wrap-up x. Corynebacteria are Gram positive bacilli (rods) x. They are aerobic (facultatively anaerobic) x. They are non-spore bearing x. They are non-motile x. They are catalase positive. x.They ferment various carbohydrates, producing lactic acid x. The appearance of stained films (using Gram method) resemble chinese letters. xx. Diphtheria is a serious infection of the upper (and sometimes the lower) airways causing respiratory obstruction, and at times, myocarditis, peripheral neuritis and death. xx. It is caused by C. diphtheriae, with a potent exotoxin. xx. It is diagnosed clinically, and confirmed by culture, Gram stain, biochemical tests, and immunodiffusion for toxin production. xx. Treatment is antitoxin and penicillin, with respiratory and cardiac support. xx. Prevention is by immunization.