Page 119 - LECTURE NOTES

5.4. Methods for Laboratory Diagnosis of Bacteria

5.4.1. Specimen collection and Examination of Bacteria:

- Specimens are collected with a blade or by swabbing the involved areas of the

skin using a sterile dry cotton wool.
- If the tissue is deeply ulcerated, or if pustules and blisters are present, aspirate a

specimen using a sterile needle & syringe.

- The purulent discharges or exudates are spread as thinly as possible on a glass
slide for Gram staining.

- After collecting the specimen with the swab, insert the swab in to a sterile
tube for culturing.

- For actinomycetes, pus is collected from closed lesions by aspiration with a
sterile needle and syringe. Material is collected from draining sinuses by holding

a sterile test tube at the edge of the lesion & allowing the pus & granules to run in

to tube. Granules are aggregates of inflammatory cells, debris, proteinatious
material & delicate branching filaments. Pus & other exudates are examined for

the presence of granules.

a. Examine the specimen microscopically

Gram smear

1. Make an evenly spread smear of the specimen on a slide

2. Allow the smear to air-dry in safe place
3. Stain the smear with the gram technique

4. Examine the smear for pus cells and bacteria. Mostly skin infection causing
bacteria can be differentiated by their Gram reaction due to difference in their cell

wall structure.

- Gram positive Cocci that could be S. aureus
- Gram positive streptococci pyogens or pneumonia

- Gram negative rods that could be P.aeruginosa, proteus species, E.coli, or other
Coliforms

- Gram variable rods lying in chains that could be B.anthracis.

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