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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


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


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


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


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


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


  • 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


  • 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


  • 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


  • 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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