There are four biovars of C. diphtheria.
They are gravis, intermedius, mitis, and belfanti.
In diagnosis procedures it is not necessary to differentiate those into biovars.
The major disease, diphtheria is caused by C. diphtheria.
It is an infection in tissues of upper respiratory tract (Nasal, nasopharyngeal and tonsillar diphtheria).
This specially occur in young children.
The infection is mainly caused by inhaling respiratory droplets.
Mainly in young children oedema of the neck can be seen.
C. diphtheria produces a toxin that causes systemic infections, mainly in peripheral nerves system and heart.
Moreover, in developed countries there are mass immunization procedures for the control of the disease diphtheria.
However, in many regions in the world it is an endemic disease.
Skin infections also can be seen in some countries. Furthermore, non- toxigenic infections are also caused by this organism.
They are endocarditis, meningitis, cerebral abscess and osteoarthritis.
Exotoxin producing C. diphtheria strains produce exotoxin which can be penetrated to the blood circulation through the damaged mucous membrane.
This toxin can be neutralized by antitoxin. If not toxaemia can be occurred with fatal cardiac and neural complications.
Acute inflammatory response caused by this toxin can be extend towards the larynx and can cause the blockage the air passage and finally can cause death from asphyxia.
Moreover, cutaneous diphtheria usually occurred when C. diphtheria infects the wounds.
Usually cutaneous diptheriae rarely leads to serious infections.
Transmission
The only natural host for C. diphtheria are humans.
Although there are two types as toxigenic and non-toxigenic organisms, both are inhabit in the upper respiratory tract and transmitted via airborne respiratory droplets.
The skin infection may occur through the lesions existing in the skin.
Mainly cutaneous diphtheria occurs in tropical countries.
Isolation of organism and demonstration of toxin production is done for the confirmation of organism.
The specimens useful for the diagnosis of diphtheria in throat are throat swabs, nasopharyngeal swabs and skin swabs are useful for the diagnosis of cutaneous diphtheria.
Microscopy
They are gram positive when stained with gram stains. But usually they stain weakly and unevenly.
In gram stained smears organisms can be seen as long, thin, and curved as well as some are short and ends are club shaped.
They are arranged in palisades (Chinese letter) or in V-shaped or L-shaped formations.
Pleomorphism can be seen. In Albert stained smears Volutin granules can be seen.
When stained the smears with toluidine blue stain, the organisms stain in pale blue and granules stained in dark red purple.
This organism is non-motile, non-spore forming and non-capsulated.
However, direct microscopy of smears is unreliable as other coryneforms are also morphologically similar to C. diphtheria.
Culture
The temperature range of growth of C. diphtheria is 20–40ºC.
This microorganism lives aerobically and facultatively anaerobic conditions.
Best growth can be seen on blood or serum containing media.
Growth is optimal at 35–37°C with or without enrichment of carbon dioxide.
Loeffler serum medium and Dorset egg medium
Granule formation is enhanced and C. diphtheria grow rapidly on this medium.
But diphtheroids also give growth on this medium.
So it is not a specific medium for C. diphtheria.
Tellurite blood agar
This is the primary medium which widely use to isolate C. diphtheriae from throat and nasopharyngeal swabs.
C. diphtheria produce grey or grey-black colonies by reducing tellurite.
After 24–48 h incubation 0.5-2 mm diameter colonies can be seen. Colony shape varied from strain to strain.
Gray colour, raised, translucent colonies can be seen.
Some strains show haemolysis on blood agar.
Tinsdale medium
Shows grey-black colonies after 24–28 h incubation.
Those grey-black colonies are raised, and surrounded by dark brown halo.
Sometimes commensal diphtheroids grow on this medium. But they can be differentiate as they do not give a brown colour halo.
Biochemical tests
diphtheria is catalase positive, nitrate positive, oxidase negative and urease negative.
C. diphtheria ferment maltose and lactose and produce acid.
Elek gel precipitation test is used for the identification of toxigenicity of C. diphtheria.
Moreover, tissue culture associated cytotoxicity assays are also used.
For the diagnosis, polymerase chain reaction (PCR) test are also used.
Here toxin producing genes are detected by PCR.
Treatment
The patients infected with C. diphtheria should be treated immediately with anti-toxins.
Because they produce fatal toxins. Laboratory diagnosis is done to confirm the organism.
Administration of antitoxin is the treatment of choice.
It should be given immediately.
Penicillin G and erythromycin are the recommended antibiotics for the treatment.
Clindamycin and vancomycin are also recommended.
But antibiotics are not a substitute for antitoxin.
Antibiotics useful to inhibit the growth of the organism.
Toxin production is reduced by antibiotics and reduce the incidence of chronic carriers.
When in disease, patients should be kept strictly isolated and platelet counts and electrocardiography should be monitored daily.
Prevention
There is a diphtheria toxoid for the immunization of children.
It is a combination of three toxoids.
Three doses are given to immunize children.
Nasopharyngeal carriage of the organism does not prevent by immunization.
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