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Bacteriology

BACTERIOLOGY

Sherko A Omer MB ChB, MSc., PhD

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Bacteriology

CORYNEBACTERIUM Corynebacteria are small, slender, pleomorphic, gram-positive rods of distinctive morphology that tend to stain unevenly.

They are nonmotile, unencapsulated, and do not form spores.

Corynebacterium is a large genus of diverse habitat.

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Bacteriology

Corynebacterium diphtheriae

Corynebacterium diphtheriaeDiphtheria bacteria are Gram-positive, pleomorphic, often club-shaped rods. The individual cells tend to group in V, Y, or palisade arrangements.

Neisser staining reveals the polar bodies, these arepolyphosphates stored at one end of the rod.

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Bacteriology

Corynebacterium diphtheriaePathogenesis and diseases

Diphtheria is caused by the local and systemic effects of a single exotoxin that inhibits eukaryotic protein synthesis.The toxin molecule is a heat-labile polypeptide that is composed of two fragments, A and B.

Fragment B binds to susceptible cell membranes and mediates the delivery of fragment A to its target.

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Bacteriology

Corynebacterium diphtheriaePathogenesis and diseasesInside the cell, fragment A separates from fragment B, and catalyzes a reaction between nicotine adenine dinucleotide (NAD+) and the eukaryotic polypeptide chain elongation factor, EF-2 and leads to irreversible blocking of protein synthesis (translation) in the target cells, which then die.

The toxin gene is always a prophage genome component

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Bacteriology

Corynebacterium diphtheriae

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Bacteriology

Corynebacterium diphtheriaePathogenesis and diseasesInfection may result in clinical disease which has two form“respiratory and cutaneous” or in an asymptomatic carrier state

Diphtheria consists of a strictly local infection, usually of the throat.

The infection produces a distinctive thick, grayish, adherent exudate (pseudomembrane) that is composed of cell debris from the mucosa and inflammatory products.

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Bacteriology

Corynebacterium diphtheriaePathogenesis and diseasesIt coats the throat, and may extend into the nasal passages or downward in the respiratory tract, where the exudate sometimes obstructs the airways, even leading to suffocation.As the disease progresses, generalized symptoms occur caused by production and absorption of toxin.

Although all human cells are sensitive to diphtheria toxin, the major clinical effects involve the heart and peripheral nerves.

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Bacteriology

Corynebacterium diphtheriaePathogenesis and diseasesCardiac conduction defects and myocarditis may lead to congestive heart failure and permanent heart damage.

Neuritis of cranial nerves and paralysis of muscle groups, such as those that control movement of the palate or the eye, are seen late in the disease.

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Bacteriology

Corynebacterium diphtheriae

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Bacteriology

Corynebacterium diphtheriaePathogenesis and diseasesIn Cutaneous diphtheria, a puncture wound or cut in the skin can result in introduction of C. diphtheriae into the subcutaneous tissue, leading to a chronic, nonhealing ulcer with a gray membrane.

Rarely, exotoxin production leads to tissue degeneration and death.

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Bacteriology

Corynebacterium diphtheriaeImmunityDiphtheria toxin is antigenic and stimulates the production of antibodies that neutralize the toxin's activity.

Formalin treatment of the toxin produces a toxoid that retains the antigenicity but not the toxicity of the molecule.This is the material used for immunization against the disease.

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Bacteriology

Corynebacterium diphtheriaeLaboratory identificationThe presumptive diagnosis and decision to treat for diphtheria must be based on initial clinical observation.

Diphtheria should be considered in patientswith pharyngitis, low-grade feverand cervical adenopathy.Erythema of the pharynx progressingto adherent gray pseudomembranesincreases suspicion of diphtheria.

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Bacteriology

Corynebacterium diphtheriaeLaboratory identificationHowever, a definitive diagnosis requires isolation of the organism, which must then be tested for virulence using an immunologic precipitin reaction to demonstrate toxin production. The method of choice is detection and identification of the pathogen in cultures from local infection fociThe swab should then be placed in semisolid transport media such as Amies.Smears stained with alkaline methylene blue or Gram stain show beaded rods in typical arrangement .

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Bacteriology

Corynebacterium diphtheriaeLaboratory identification

Inoculate a blood agar plate (to rule out hemolytic streptococci), Loeffler slant, and a tellurite plate (eg, cystine-tellurite agar or modified Tinsdale medium).

In 12–18 hours, the Loeffler slant may yield organisms of typical "diphtheria-like" morphology. In 36–48 hours, the colonies on tellurite medium are sufficiently definite for recognition of C diphtheriae..

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Bacteriology

Corynebacterium diphtheriae

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Bacteriology

Corynebacterium diphtheriaeLaboratory identification

The toxin may be detected by: Modified Elek method PCR methods have been described for detection of the diphtheria toxin gene (tox). ELISA Immunochromographic strip assay

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Bacteriology

Corynebacterium diphtheriaeTreatment and preventionTreatment of diphtheria requires prompt neutralization of toxin, followed by eradication of the organism.

A single dose of horse serum antitoxin inactivates any circulating toxin, although it does not affect toxin already bound to a cell-surface receptor.

Serum sickness caused by a reaction to the horse protein may cause complications in approximately ten percent of patients.

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Bacteriology

Corynebacterium diphtheriaeTreatment and preventionC. diphtheriae is sensitive to several antibiotics, such as erythromycin or penicillin. Antibiotic treatment slows the spread of infection and, by killing the organism, prevents further toxin production.

The cornerstone of diphtheria prevention is immunization with toxoid, usually administered in the DPT triple vaccine, together with tetanus toxoid and pertussis antigens.

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Bacteriology

Corynebacterium diphtheriaeTreatment and preventionThe initial series of injections should be started in infancy.

Booster injections of diphtheria toxoid (with tetanus toxoid) should be given at approximately ten-year intervals throughout life.

The control of an epidemic outbreak of diphtheria involves rigorous immunization and a search for healthy carriers among patient contacts

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Bacteriology

Corynebacterium diphtheriaeEpidemiologyThis disease is a local infection, usually of the throat. Therefore, the organism is primarily spread by respiratory droplets, usually by convalescent or asymptomatic carriers.

It is less frequently spread by direct contact with an infected individual or a contaminated fomite.

The incubation period is two to five days. Exposure prophylaxis involves isolation of infected persons until two cultures from specimens taken at least 24 hours apart are negative.

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Bacteriology

Corynebacterium diphtheriaeEpidemiologyThe development of effective vaccination protocols and widespread immunization beginning in early childhood has made the disease rare in developed countries. However, diphtheria is a serious disease throughout the world, particularly in those countries where the population has not been immunized.

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Bacteriology

DiphtheroidsDiphtheroidsSeveral other corynebacterium species that morphologically resemble the type species, C. diphtheriae, are common commensals of the nose, throat, nasopharynx, skin, urinary tract, and conjunctiva.

They are therefore called diphtheroids, and are generally unable to produce exotoxin, but a few cause disease in rare circumstances, such as in immunosuppressed individuals