GROWING GOURMET - Anto2ni.it

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THE SIX VECTORS OF CONTAMINATION
tainers of media uninoculated. For instance,
the cultivator should always leave some culture
dishes uninoculated and unopened. These
"blanks" as I like to call them, give the cultivator
valuable insights as to which vector of contamination
is operating. At every step in the
cultivation process, "blanks" should be used
as controls.
The air in the growing rooms does not require
the degree of filtration needed for the laboratory.
Formushroom cultivators, cleaning the air by water
misting is practical and effective. (Rain is
nature's best method of cleansing the air.) This
cultivator's regimen calls for the spraying down
of each growing room twice a day. Starting from
the ceiling and broadcasting a spray of water back
and forth, the floor is eventually washed towards
the center gutter. The room feels clean after each
session. Each wash-down of a 1000 sq. ft. growing
room takes about 15 minutes. This regimen is
a significant factor in maintaining the quality of
the growing room environment.
3. The Media: Often the medium upon
which a culture is grown becomes the source
of contamination. Insufficient sterilization is
usually the cause. Standard sterilization time
for most liquid media is only 15-20 minutes at
15 psi or 250° F. (1210 C.). However, this exposure
time is far too brief for many of the
endospore-forming bacteria prevalent in the
additives currently employed by cultivators. I
recommend at least 40 minutes @ 15 psi for
malt extract or potato dextrose agars. If creating
soil extracts, the soil must be soaked for at
least 24 hours, and then the extracted water be
subjected to a minimum of 1 hour of sterilization.
Indeed, soil extracts are resplendent with
enormous numbers of contaminants. Because
of the large initial populations, do not be surprised
if some contaminants survive this
prolonged sterilization period. Should they
persist, then sterilizing the extracted water
first, and then re-sterilizing it with standard
malt sugar additives is recommended. Clearly,
sterilization is best achieved when the media
has a naturally low contamination content.
(See Preparation of Media in Chapter 12.)
A good practice for all laboratory managers
is to leave a few samples from each sterilization
cycle uninoculated. Not inoculating afew
petri dishes, grain jars, and sawdust/bran
bags and observing them for a period of two
weeks can provide valuable information about
the vectors of contamination. These quality
control tests can easily determine whether or
not the media is at fault or there has been a failure
in the inoculation process. Under ideal
conditions, the uninoculated units should remain
contamination-free. If they contaminate
within 48-72 hours, this is usually an indication
that the media or containers were
insufficiently sterilized. If the containers are
not hermetically sealed, and contaminants occur
near to the end of two weeks, then the
contamination is probably endemic to the
laboratory, particularly where these units are
being stored. Under ideal conditions, in a perfect
universe, no contamination should occur
no matter how long the uninoculated media is
stored.
Many researchers have reported that sawdust
needs only to be sterilized for two hours at
15 psi to achieve sterilization. (See Royse et al.
(1990), Stamets and Chilton (1983)). However,
this treatment schedule works only for
small batches. When loading an autoclave with
hundreds of tightly packed bags of supplemented
sawdust, sterilization for this short a
period will certainly lead to failure.
In the heat treatment of bulk substrates, absolute
sterilization is impractical. Here, sterilization
is more conceptual than achievable.
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