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NAJRAN UNIVERSITY College of Applied Medical Sciences. Clinical Bacteriology 1. By. Dr. Ahmed Morad Asaad Associate Professor of Microbiology. The genus Corynebacteria It includes: C. diphtheriae : Exotxin producer – causing diphtheria in man

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NAJRAN UNIVERSITY

College of Applied Medical Sciences

Clinical Bacteriology 1

By

Dr. Ahmed Morad Asaad

Associate Professor of Microbiology

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The genus Corynebacteria

It includes:

C. diphtheriae:Exotxin producer – causing diphtheria in man

Propionobacterium: Anaerobic – members of normal flora of skin & mucous membrane – involved in acne

Commensal species (diphtheroids): flora in mucous membrane of R.T and GIT – may cause diseases in immunocompromisedpersones – C. ulcerans (diphtheria-like illness)

Listeriamonocytogenes: mainly in animals – may cause sever diseases in man

Actinomyces: anaerobic – causing actinomycosis

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C. Diphtheriae (Diphtheria bacillus)

A powerful exotoxin producer (Viruelence factor)

Causative organism of Diphtheria

Characters:

1- Gram +ve bacilli (often club-shaped) – Chinese letter appearance

2- Contain intracellular metachromatic granules (Volutin granules)

3- Non-capsulated – non-motile

4- Catalase +ve – Oxidase +ve – Facultative anaerobe – Grow better under aerobic conditions

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Culture Characters:

1- Grow on Loffler’s serum

2- On blood agar: Colonies are small, grey, small areas of complete haemolysis

2- On blood tellurite agar: colonies are grey to black – 3 biotypes can be recognized on the basis of colony morphology and severity of disease

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Diphtheria

Human is the natural host. Both toxigenic and non-toxigenic organisms are in URT and transmitted by droplet infection. Conjunctival and skin diphtheria ocuur in tropics and spread by contact.

Pathogenesis:

1- Cases or carriers are harboring the organism in URT

2- Infection by droplet infection

3- 1ry lesion is in throat or nasopharynx by formation of pseudomembrane

4- The microbe multiply – produce toxin – into blood stream – distant organs

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6- Toxin production is a lysogenic property

7- All strains produce an identical toxin (Only one antigenic type of the toxin

5- Paralysis of soft palate – eye muscles – extremities – Its action is irreversible

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Lab Diagnosis:

  • To confirm clinical diagnosis
  • Specific treatment with antitoxin should NEVER wait for lab results
  • Diagnosis of the case
  • Specimen: Swab from the membrane
  • Direct smear stained with:
  • Gram’s stain: to show the organism morphology
  • Methylene blue: To show beaded appearance
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Diagnosis of the carrier

  • To isolate the organism in pure form and test its ability to produce toxin
  • Isolation of the organism
  • Specimen: Swab from the membrane
  • Direct smear stained with:
  • Gram’s stain: to show the organism morphology
  • Methylene blue: To show beaded appearance
  • Culture on selective media
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Virulence tests (Tests for toxigenicity)

  • In vivo tests:
  • The culture suspension is injected into 2 guinea pigs – one of them received 250 units of diphtheria antitoxin 2 hours before the test.
  • If the protected animal survives and the unprotected dies within 2 -3 days = The strain is toxigenic
  • If the 2 animals survive = The strain is non-toxigenic
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In vitro test (Elek’s test):

  • The isolated organism is streaked across a serum agar plate
  • A filter paper strip impregnated with antitoxin is placed on the surface of the medium at right angle to the organism
  • After 48 h, the plate is inspected for lines of precipitation
  • Presence of these lines = The strain is toxigenic
  • Tissue culture test (Agar overlay):
  • The strain is streaked into an agar overlay of a tissue culture
  • If the strain is toxigenic, the toxin will diffuse into the tissue and kill the cells
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