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What is the
difference between a viable and non-viable microbial sampling and
which type is better for identification and why?-Tom,
California
The term viable means that mold
spores/fragments are alive and are capable of growing on a culture
medium. Though spores/fragments are alive, they may or may not grow
on the culture medium used to collect the sample. As such, we often
use the term culturable for those spores/fragments that grow and form
colonies on the medium used for sample collection.
I typically collect samples on two
different media: malt extract agar (MEA) and dicloran glycerol agar
(DG-18). The former favors the growth of hydrophilic species such as
Stachybotrys; the latter favors the growth of xerophilic
species which include many of the Aspergillus group. DG-18 has
been shown to result in significantly higher counts (in two different
studies) than MEA, the most widely used mold sampling medium.
Culturable-viable sampling is the
method of choice for identifying mold colonies to both genus and
species. It can be used to distinguish between Penicillium and
Aspergillus, something total mold spore methods cannot do (a
fact that is important).
Total mold spore methods collect mold
spores/fragments on microscope slides or coverslips. Both viable and
non-viable spores are collected and can in theory be identified and
enumerated. Both viable and nonviable mold spores/fragments are
equally allergenic and therefore such counts should provide a more
realistic estimate of exposures and health-affecting potential.
Ratios between total mold spores/particles and viable mold
spore/particles vary from 1:1 to an order of magnitude or higher for
total mold spore sampling (depending on the nature of the
infestation).
Total mold spore sampling dominates
professional mold assessment practices. In reviewing such sampling
results, I am impressed by the fact that results reported by
laboratories are often less than those I typically observe with the
culture plate method using MEA and DG-18. This is particularly
striking in houses that have been previously “tested”. Not
surprisingly my total mold spore sampling results are often an order
magnitude higher than those previously reported. Why the differences
and why are they important? Differences are likely due to sample
counting practices which may result in significant undercounts when
total mold sampling is employed. Undercounts may unfortunately be
interpreted as a problem does not exist when one indeed exists.
Overcounts are also a concern lest it result in unnecessary
remediation.
I recommend using both culturable-viable
and total mold spore sampling in conducting building mold
assessments. Both methods have their advantages and limitations.
When culturable-viable test results are consistently higher than total
mold sampling results, the assessment professional should seriously
question the accuracy of total mold spore counting protocols employed
by the analytical laboratory conducting the analyses.
February 26, 2004
Indoor Environmental Quality (2000), Thad Godish Ph.D.,
C.I.H
Direct E-mail
00tjgodish@bsu.edu
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