Fiosrach <fiosrach at aol.com> wrote in message
news:20020416091001.20493.00004193 at mb-ma.aol.com...
> Good morning, Des:
>> Thank you for addressing my question. In the report, the TVC is often
higher
> than the summation of all other columns and occasionally has a value when
all
> other columns are non-detect.
>> I would have thought that the "total viables" would mean just that, and
that
> the total could not be higher than the constituent components.
My first microbiology teacher often used to say to us "Death is a function
of the recovery medium". In the present context, one might derive a viable
count using a non-selective culture medium and call it a "total viable
count" on the assumption that all viable organisms in the sample can grow on
the non-selective medium. This is a big assumption and likely not a correct
one.
As to the constituent components not adding up to the "total viable count",
the same logic applies. Selective culture media used to obtain counts of
sub fractions of a population of microorganisms do not necessarily have a
sharp cutoff and necessarily only enumerate fractions of the population.
Without an all encompassing range of perfectly selective media, some
fractions of a population may be missed and others counted more than once.
In the present context, given that your intial query mentioned yeast counts,
mould counts and a TVC count, my guess would be that the TVC is referring to
a bacterial count done using something like trypic soy agar (TSA) and this
is why the count there does not necessarily reflect the sum of counts using
other culture media. A sampling protocol using TSA for bacteria and some
formulation of Malt Extract Agar (MEA) for fungi would not be surprising.
On the MEA, filamentous fungi and yeasts can be readily differentiated by
colony morphology. The TSA count could be reported, misleadingly, as a
total viable count but obviously bears no numerical relationship to the
fungal counts. Only a query to the lab about methodologies will tell you
for sure.
>> I have talked to some notable professionals in the field of aerobiology
(such
> as Chin Yang and Brian Shelton) and they too had never heard of the
acronym.
Microbiologists have used many acronyms over the years to label what is
intended to be an estimate of the total microbiological population of a
sample. I can't recall having seen TVC but have certainly seen TPC (total
plate count) as well as SPC (standard plate count). Both of these refer to
bacterial counts - no molds!
However, partly for the reasons noted above, it is recognised that to use
the word "total" is misleading. Hence, one is now more likely to see
acronyms such as ACC (aerobic colony count) or HPC (heterotrophic plate
count) which attempt to describe how the count was done without imputing
unrealistic outcomes. It is for the same reason that we now speak of colony
forming units rather than cells as we often cannot know whether the visible
colony on a solid culture medium is derived from a single cell, a clump of
cells or even a fragment of fungal mycelium.
Austin Reade
