I took some time during lunch yesterday to write a little commentary on
Methods and Specs. I figured I would submit it for your amusement.
Is There Fungus Among Us? If so, How Do We Count It? A Commentary on the
Not So Sensible World of Microbiological Nomenclature and Methodology used
in the USP .
Suppose an alien (or FDA inspector) were to walk into your lab and ask you
to explain the rationale behind the MLT procedure you are using and the
associated numerical specifications (I will only discuss numerical
specifications in this essay due to the massive deforestation/bandwith clog
that would result from a discussion of "objectionable organisms").
The Alien/Inspector begins:
"I see you have a specification for Total Aerobic Microbial Count, TAMC, of
No More Than 100 CFU/g. Tell me, how is this number calculated"
You Reply, with a high degree of confidence, "Well, we put a small amount of
product in a rich agar media called TSA, and then we incubate it at 32C for
"And all microorganims on your planet will form colonies under these
"Well, no. But it will pick up most of the fungi and bacteria that we think
are important to our industry"
"So it's really not a "total" aerobic count, it's more of a "the majority of
significant organisms" count?
"And this numerical limit of 100 cfu/g, I assume this number was derived
from data collected from numerous scientific studies"
"Well, no. It was derived from some guys who guessed it may be a good
number. But there are almost no reported cases of people getting sick from
contaminated pharmaceutical products, so the specification must be good."
"I see" says the so Alien, and continues with "And tell me about this
Fungal Count specification, how do you determine the number of fungi in a
"We use special media called SDA to determine the number of yeast and molds
in our samples and we incubate the samples at 24C for 5 days instead of 32 C
for 2 days"
"Ahhhh, and this SDA was specifically designed to enumerate fungi?"
"Well, no. It was actually developed as a highly selective media for the
qualitative recovery of fungi from samples which were heavily contaminated
"And your samples are heavily contaminated with bacteria?"
"Well, no. But if they were......"
"So why are you using a qualitative recovery media to generate quantitative
"Hmmmm, well, I assume that 24C is the optimum growth temperature for
"Well, no. C albicans and A. niger, the organisms we use to validate all of
our methods, grow much better on TSA than SDA and their optimum growth
temperature is closer to 37C. Actually most medically important fungi have
an optimum growth temp close to 37C, because this is close the body
temperature of us humans"
Confused, the Alien/Inspector continues, "I noticed that your Total Yeast
and Mold Count was derived directly from SDA plate counts"
"Yep, straight out of good ol' USP<51>"
"Will bacteria also grow on SDA"
"Sure, some of those little guys grow great on SDA at 24 C"
"So you could be counting bacteria and calling them fungi?"
"I also notice that the fungal specification is half the numerical value of
the Total Aerobic Microbial count"
"Yep, those fungi can be nasty little bugs"
"Nastier than bacteria"
"More infective than Bacteria?"
"More detrimental to the product than bacteria?"
"Then why the lower spec?"
"Well, just because....." does this Alien guy not understand anything!!
So the Alien/Inspector hops back into her spaceship, still wondering whether
or not she has encountered an intelligent life-form. You retire back to your
lab, thankful that the alien/inspector did not ask why you were testing for
Salmonella species in a product -type that would never support the growth of
these organisms, or why you use R2A media to recover waterborne /stressed
organisms, but TSA to recover organisms from your products (where most of
the contamination will probably come from water).
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