NAJRAN UNIVERSITYCollege of Applied Medical Sciences

Clinical Bacteriology 1

By

Dr. Ahmed Morad AsaadAssociate Professor of Microbiology

The genus CorynebacteriaIt 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 immunocompromised persones – C. ulcerans (diphtheria-like illness)

Listeria monocytogenes: 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

DiphtheriaHuman 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 URT2- Infection by droplet infection3- 1ry lesion is in throat or nasopharynx by formation of pseudomembrane4- The microbe multiply – produce toxin – into blood stream – distant organs

6- Toxin production is a lysogenic property7- All strains produce an identical toxin (Only one antigenic type of the toxin5- 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 carrierTo isolate the organism in pure form and test its ability to produce toxinIsolation 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 toxigenicIf 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