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Here we discuss the use of mold culture plates, settlement plates, and mold test kits based on cultures to collect mold test samples to screen buildings for harmful indoor mold. In this article series discuss the validity of nearly all of the popular mold testing methods currently in use, pointing out the strengths and weakness of each approach to mold sampling in the indoor environment, beginning with air sampling for airborne mold levels indoors.

This study presents a summary and critique of some popular methods used to examine indoor air quality to test for presence or absence of problematic levels of toxic or allergenic mold or other bioaerosols. We describe and critique specific "testing" or "sampling" methods used to "test" buildings for mold in the course of a building investigation. The appropriateness of testing at all is discussed on this and other pages at our website. Because mold test validity and mold test accuracy are often confused, readers should also see ACCURACY OF VARIOUS MOLD TEST METHODS. People who need to conduct mold inspection and testing indoors should see MOLD TEST PROCEDURES and TECHNICAL & LAB PROCEDURES.

Our MOLD INFORMATION CENTER includes more broad discussions of the overall approach to building investigation, as do many expert references cited at that web. For a more comprehensive collection information about mold test methods see INDOOR AIR QUALITY METHODS COMPARED. For more on "mold classes" (Cosmetic mold vs. allergenic mold vs. toxic or pathogenic mold) see MOLD CLASSES, HAZARD LEVELS and more references such as a Mold Action Guide are at the end of this document.

© Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

A Guide to Using Mold Cultures or Settlement Plates to "Test for Mold"

15th Annual North Carolina/South Carolina
Environmental Information Association Technical Conference
Myrtle Beach, SC
Daniel Friedman 23 September 2005, Updated 4/14/2009

Mold cultures involve the collection of particles by air sampling pump, by gravity settlement, or by lift from a surface using a swab or tape. Some sampling equipment (Anderson™ spore traps) can collect spores directly into a petri dish of culture medium, and are used for "viable spore sampling in air."

The sample by pump, gravity, tape or swab is in any case applied to one or more petrI dishes of culture media for incubation and subsequent examination of the growth product.

Mold Culturing is useful for genera speciation once you have collected a single or dominant sample whose importance (frequency in the building) you already know. As a "home test kit" for the presence of problematic mold in a building this is an unreliable method, as we describe below at "shortcomings."

Mold Cultures are useful for:

• Identifying the genera/species of a mold which was not readily named by (faster, cheaper) light microscopy
• Identifying a problem genera to the species level for medical diagnostic purposes - we .e. pass this (possibly accurate) data along to your doctor if you're sick
• Distinguishing apparently similar outdoor mold counts from indoor mold counts of "look-alike" spores that may really be different genera/species

Seven Serious Shortcomings of Using Cultures to Test Buildings for the Presence of Toxic Mold

While this is an important tool which has a place in our arsenal, mold culturing is questionable as a means to characterize a mold risk in a building, particularly if it reports the absence of a mold problem. The objections listed below mean that field investigators must collect samples with some care and interpret lab reports with some caution.

1. Roughly 90% of all molds on earth will not grow on any culture under any condition. Others are quite difficult to coax into growing on culture, even with careful methods. So if you buy a "home test kit" that uses a single culture plate, you're 90% wrong when you open the container. To be fair, it might be that many common indoor problem-molds will show up in certain cultures, but these numbers still hold.
   
   
2. The toxicity or allergenicity of a specific mold (genera/species/strain) may vary widely depending on what it's growing on. So even a "toxic" building mold might be low or non-toxic when growing on certain substances. Molds that grow on cultures may produce very different structures and have different medical characteristics than when growing in nature or in a building.
   
   
3. Cultures may name the wrong mold as "the problem": Cultures have a high risk of both missing the problematic spore and of indicating that some other spore is the dominant or problem in a building. For example, to speciate one of the more than 100 members of the Aspergillus genera requires culturing the sample on four different media, simultaneously, comparing subtle things like growth rate among morphologically similar species. We believe that virtually no lab uses that troublesome procedure outside of university research and medical laboratories.
   
   
4. Settlement plate cultures (such as "home test kits") rely on gravity, making any comparison of "spore counts" dead wrong - different spores are of different sizes and masses, and settle out of the air at different rates.
   
   This over-states the presence of big heavy spores (like Stachybotrys chartarum) and under-states the presence of small light spores (like Aspergillus versicolor) in a given sample.
   
   These small spores (2-3u) tend to stay airborne due to very slight indoor convection currents (e.g. heat from lighting and natural building stack effects).
   
   Our lab photo shows two different mold colonies growing on a culture plate where individual spores settled out of the air onto this surface.
   
   
   
   
5. Swab and tape samples for cultures may collect the wrong mold. Swab or tape samples used for culture for identification of what's on a surface have the same viable-non-viable question we have already raised. Everything depends on where you collected the swab or tape sample.
   
   Moving a tape or swab over as little as one inch on a surface, and certainly moving it a few feet, can collect a completely different mold genera and species!
   
   An "expert" should know what's probably representative of the building and should know where the important genera/species are likely to be growing.
   
   Many investigators are quick to sample the highly-visible "black" mold on a surface and under-sample very important but hard to see light colored molds often found higher on a wall, for example, where the surface was less wet.
   
   
6. Cultures are probably not really being done with full accuracy in some labs, especially for Aspergillus: Culturing on one or even two media risks that the important genera/species in the sample does not grow at all on the medium, that it grows in a different form and is identified differently than it appears in the building, or that it is overgrown by another genera/species present which likes the culture more than the target species.
   
   We have demonstrated this culture-media variation in a study we am pursuing about mold in tea. In a problem-tea sample cultured on the most commonly used culture media, MEA, the culture produced an overwhelming growth of Cladosporium sp., while a parallel culture (from the identical sample) made on DG-18 produced a single Cladosporium colony and grew an overwhelming collection of Aspergillus niger!
   
   
   
7. Non-viable spores, that don't grow on culture may still be toxic or allergenic particles which are a problem for some people exposed to them.
   
   The Penicillium sp. at left was growing nicely in this culture sample, but we have no idea what other molds may have been present but that simply would not grow on this culture media even if they settled on or were placed onto the culture plate.
   
   

While we enjoy growing mold cultures in our lab (it makes for nice, photogenic mold colonies), it is less often useful than direct microscopic examination of a field-collected surface or vacuum sample. Without the added step of mold culturing, from a good surface sample using adhesive tape, a trained microscopist can identify mold genera and mold species as well in many cases.

In many instances, knowing the mold genera is enough to decide on a course of cleanup action without further expense. For example, if we agree that there are no harmless Aspergillus species or Penicillium species that grow indoors, then for purposes of deciding on the need for remediation, only the size of the reservoir is important. P. notatum, used for making the drug Penicillin, has not to our knowledge been found growing on building materials.

For more discussion about mold cultures from settlement plates or swabs see "The Validity of Cultures at InspectAPedia.com/sickhouse/cultures.htm

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MOLD TESTING METHOD VALIDITY
Introduction
  Air samples
    Shortcomings of air sampling
    Mold in Air: Quantitative Analysis
  Tape sampling for mold
    Determination of mold genera
    Determination of mold species
    Shortcomings of tape sampling
  Vacuum samples
    Surface vacuuming
    Shortcomings of surface and carpet vacuuming
    Vacuuming building cavities
    Vacuuming exposed insulation
    Shortcomings of vacuuming insulation
  Cultures to "Test for Mold"
    Shortcomings of culturing
  Swab sampling
  Shortcomings of swab sampling
  PCR methods for Mold Identification
  Mold "Testing" vs. Mold "Problem Identification"
  Are Mold Test Kits Useful?
  Reasons to Test for or Identify Mold
    1. Save Money if it's Just Cosmetic Mold
    2. Mold Related Illness
    3. Mold Cleanup Data baseline

• Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. (727) 595-4211 mark@BestTampaInspector.com

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