SOP: Swab Counts on Working Surfaces, Floors and Walls
GIFSQ — Global Institute for Food Safety & Quality
Principle
Swab counts on working surfaces, floors and walls assess hygienic conditions. A sterile stencil limits the swabbed area to a known square, so that the count can be expressed per unit area of surface.
Apparatus required
- Card or cellophane (for cutting stencils)
- Cotton wool swabs (wound on wooden sticks or glass rods about 7–8 in. long)
- Test tubes (6 × 1 in.)
- Hot-air oven (150°C)
- Autoclave
- Sterile pipettes
- Petri dishes
Reagents
a. Sterile stencils — Cut card or cellophane into 10 cm squares, then cut a 5 cm square out of the middle of each. Sterilize the pieces in envelopes.
b. Sterile cotton wool swabs — Wrap non-absorbent cotton wool around wooden sticks or glass rods about 7–8 in. long until each swab is about 0.5 in. across. Put the swabs in pairs into 6 × 1 in. test tubes with about 1 in. of stick projecting from the tube, plug the tubes with non-absorbent cotton wool and heat them in a hot-air oven at 150°C for 90 min.
c. Sterile Ringer solution — To 1,000 ml of distilled water add 2.15 g of NaCl (AR), 0.075 g of potassium chloride (AR), 0.12 g of anhydrous calcium chloride (AR) and 0.5 g of sodium thiosulphate. Autoclave for 20 min at 15 psig. Use the solution as diluent at one-quarter of its original strength; it is ionically balanced.
d. Yeast extract agar (routine work) — The chapter’s ingredient table is partly illegible in the available copy: the legible routine-work figures are peptone 5 g, agar 10 g and water 1,000 ml. Heat and dissolve the ingredients, adjust the pH to 7.3 and filter if necessary. Bottle or tube the medium and autoclave at 10 psig for 10 min. Check the complete recipe against the printed book.
Sample preparation
None beyond sterilizing the stencils, swabs and media as above.
Procedure — Swab counts on working surfaces, floors and walls
Place a sterile card or cellophane square piece on the surface to be examined
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Swab the area within the 5 cm square with a cotton wool swab dipped in Ringer solution
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Transfer the swab to sterile Ringer solution as in the swab rinse method
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Carry out plate counts on the rinse as described for the swab rinse method (1 ml and 0.1 ml amounts in yeast extract agar)
Calculation
Count per 25 cm² = (Count per ml) × 10
Where,
- Count per ml = colonies per millilitre of the rinse or solvent
- 10 = factor for the 25 cm² (5 cm × 5 cm) swabbed area
Precautions
- Swab only the area within the 5 cm square of the stencil.
- Keep the stencils sterile until placed on the surface.
References
- De Man, J.C., Rogosa & Sharpe, J. Appl. Bact. 23, 130 (1960).
- Knox, R. & J. Walker, J. Hyg. Camb., 45, 187 (1947).
- (Author name illegible in the available copy — printed as “Gos) C.H.”), Microbiological Methods, 2nd edn., Butterworth & Co. Ltd., London, p. 347 (year printed as 1997).