Corynebacterium and related Aerobic non-Spore forming Gram Positive Rods

Dan Freeman

Introduction

• Non–spore-forming gram-positive bacilli are a diverse group of aerobic and anaerobic bacteria.

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• Many species are members of the normal flora of skin and mucous membranes of humans e.g. Corynebacterium & Propionibacterium

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• Corynebacterium species and related bacteria tend to be clubbed or irregularly shaped.

Corynebacterium diphtheriae

• Corynebacterium diphtheriae is the causative agent of diphtheria. They are Gram-positive, pleomorphic, often club-shaped rods that do not form spores.

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• Selective indicator media containing tellurite are used to grow them.

Pathogenesis and Clinical Picture

• Local infection: usually affects the tonsils, pharynx, nose, and conjunctiva. The pathogens multiply and produce a powerful extracellular toxin, resulting in local cell damage.

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• There is collection of a grayish-white exudate - the “diphtherial pseudomembrane” consisting of fibrin, dead granulocytes, and necrotic epithelial cells on the tonsils.

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• This coating adheres quite strongly to the mucosa.

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• Systemic intoxication: affects mainly the liver, kidneys, adrenal glands, cardiac muscle, and cranial nerves resulting in parenchymal degeneration.

Diagnosis and Therapy

• Laboratory diagnosis is based on detection and identification of the pathogen in cultures from local infection foci. They can be grown on enriched selective media containing tellurite K.

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• Toxin detection is necessary for a lab diagnosis of diphtheria due to the occurrence of toxin-negative strains.

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• Antitoxic serum therapy is the primary treatment and must commence as soon as possible if diphtheria is suspected.

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• Penicillin or Erythromycin is administered as supplementary treatment.

Epidemiology and Prevention

• Diphtheria occurs only in humans.

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• Thanks to extensive diphtheria toxoid vaccination programs, it is now rare.

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• The disease is usually transmitted by droplet infection. The incubation period is 2-5 days.

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• Protective immunization with diphtheria toxoid is the most important preventive measure.

Actinomycetoma

• Mycetoma (Madura foot) is a localized, slowly progressive, chronic infection characterized by subcutaneous abscesses, usually on the feet or hands. It is destructive and often painless. It is caused mainly by soil fungi Madurella sp., Aspergillus sp. etc.

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• An actinomycetoma is a mycetoma caused by filamentous branching bacteria. The actinomycetoma granule is composed of tissue elements and gram-positive bacilli and bacillary chains or filaments.

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• The most common causes of actinomycetoma are Nocardia asteroides, Nocardia brasiliensis, Streptomyces somaliensis, and Actinomadura madurae.

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• Actinomycetomas respond well to various combinations of streptomycin, trimethoprim–sulfamethoxazole, and dapsone if therapy is begun early before extensive damage has occurred.

Madura Foot

Listeria monocytogenes

• The only listeriae that cause human disease are L. monocytogenes and the rare species L. ivanovii.

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• Human infections may result if large doses of pathogens enter the GIT with food.

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Morphology and Culture

• The small Gram-positive rods show peritrichous flagellation. Culturing is most successful under aerobic conditions on blood agar.

Clinical Picture & Control

• Listeriae are classic opportunists - the course of most infections is clinically silent.

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• In immunocompromised patients, disease manifests as a primary sepsis and/or meningoencephalitis. More rarely, listeriae cause endocarditis. Listeriosis during pregnancy may result in spontaneous abortion.

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• Penicillins (amoxicillin) and cotrimoxazole, sometimes in combination with aminoglycosides, are used in therapy.

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• Preventive measures include proper processing and storage of food products in keeping with relevant hygienic principles.

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