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Swab clearance sampling for molds without identification

Equipment needed

  1. Sterile cotton swabs
  2. Plastic bags with zipper locks
  3. Sterile water (freshly boiled and cooled tap water)
  4. Culture plates (100 x 30 millimeter) - Available through Edge Biologicals @ 1-800-238-5004 - Check yellow pages for other labs. The plates will cost $1.25 - $3.00 per plate. Discounts are available for large quantities. Store plates in a refrigerator. Take plates and let them warm to room temperature 4 - 24 hours before using.

Culture plate media options

IMA - Inhibitory Mold Agar
MEA - Malt Extract Agar

Fast growing media (3 - 7 days) with potential for overgrowth. Promotes growth of water loving (hydrophilic fungi). Bacterial growth usually inhibited by an antibiotic. Growth of other organisms on the plate often prevents the identification of Stachybotrys sp.

Use on materials that are wet or recently (1 week) damp

MLpH - Mycological Low pH agar.

Moderate growth rate media (4-8 days) with some potential for overgrowth. Promotes growth of water loving (hydrophilic fungi). Slower growth allows for easier identification of Stachybotrys sp. Inhibits the growth of all but the acid tolerant bacteria.

Use on materials that are wet or recently (1 week) damp. Especially good on damp cellulose materials.

DG-18 - Dichloran Glycerol agar.

Slower growth rate media (5 - 9 days) with reduced potential for overgrowth. Promotes growth of fungi which need less water (xerophylic fungi). Slower growth allows for identification of a wider number of fungi. A chemical - dichloran is used to prevent overgrowth from certain fungi. The media does not provide enough moisture to promote the growth of Stachybotrys sp. The 18% Glucose content inhibits bacterial growth.

Use on materials that were wet previously but are now dry for more than 1 week. This media promotes the growth of most Aspergillus ssp. and Penicillium ssp.

Sampling protocol -  Illustration of procedure

  1. Dip sterile swab in sterile water if the surface is dry. Skip this step if the surface is wet.
  2. Wipe on defined area of surface (1 square inch)
  3. Wipe the swab on the surface of the culture plate while rotating the swab in the opposite direction. Use a the same standard pattern for each location. Consider using a 'o' because if it shows up on the plate, time and money will be needed to clean or remove the material sampled.
  4. Tape the plate shut. Mark the bottom of the plate with identification information. Place in a small zipper lock bag with the bottom of the agar plate facing up.
  5. Prepare control samples by placing an unused plate and a plate wiped with a swab dipped the sterile water in a small zipper lock bag.
  6. Place all of these bags into a larger zipper lock bag and incubate at room temperature (68 - 77 degrees Fahrenheit or 20 - 25 degrees Celsius).

Interpretation of results

Plate Appearance:
A defined '$' appears on the plate.
The surface sampled has heavy spore deposition or mold growth.
Suggested Action:
Re-clean or remove

Plate Appearance:   
A less defined '$' appears on the plate (organisms have similar color)
mold growth may be occurring.
Suggested Action:
Re-clean or remove

Plate Appearance:
A less defined '$' appears on the plate (organisms have different colors)
Moderate spore deposition. Mold growth less likely
Suggested Action:
Re-clean or possibly remove

Plate Appearance:
Plate becomes rapidly overgrown within 3-5 days and no '$' appears
Difficult to interpret. Growth may have been caused by one spore
Suggested Action:
Re-clean and Re-sample with DG-18 media

Plate Appearance:
A few organisms are on the plate, they appear to be of several different types and no '$' appears
Normal spore deposition on surface.
Suggested Action:
Cleaning adequate and Recleaning or removal is optional

Disclaimers - A single media will not pick up all fungi. Dead spores will not grow on media but can still cause allergic reactions in susceptible individuals. The preceding information represents rules of thumb which can be used to conduct a low cost assessment for cleaning effectiveness without extensive knowledge of mycology. People with respiratory problems or heart conditions may not be fit enough to wear a respirator. Consult with your physician if you have questions.

Safety tips

  1. Wear a respirator while sampling. Wash hands after sampling.
  2. Reminder: Keep the plates in the zipper bags and do not open.
  3. Disposal: Keep plates in plastic bags and place inside cardboard box. Place the box in the trash.

Related fungal links

HVAC ventilation cleaning procedure
Hidden Fungi Found in Buildings
Airborne Fungal Glossary
Managing water infiltration

Send questions and comments to: Neil Carlson, Industrial Hygienist, Department of Environmental Health and Safety, University of Minnesota.

Developed by Neil Carlson and James Lauer, University of Minnesota, Department of Environmental Health and Safety.