Question on a MacFarland Standards

Judy and Bob Dilworth dilworth at megsinet.net
Wed Mar 3 15:04:20 EST 1999

Just off the top of my head, I would say because the bacteria would multiply
and grow, getting more dense, and change your McFarland.  We buy commercially
prepared McFarland standards in cuvettes to calibrate our
mini-spectrophotometer that we use to standardize the inoculum for our Vitek
on a daily basis (an antomated instrument for susceptibility and
identification testing).  You need a standard that's not going to change.  The
commercially available standards are in tubes that fit into the spec. and are
good for one year.

Judy Dilworth, M.T. (ASCP)
Microbiology since 1974

"William J. Mason" wrote:

> I am currently teaching an honors microbiology lab and was posed with a
> question that I could not fully answer.
> The MacFarland standards are solutions of BaCl2 and H2SO4 that are added
> to a spec cuvette to create a turbid solution of measurable OD.  This tube
> is then correlated to the OD of a growing bacterial sample.  Once the ODs
> are the same an alloquat of the bacterial culture is removed, diluted, and
> CFU/ml are determined.  By increasing the concentration of the BaCl2, one
> can establish a set of ODs that correspond to a specific number of
> bacteria/ml (obviously for a specific bacteria).  The question was Why is
> it necessry to use the chemicals to create the standard.  I tried to
> explain that it is better to have a non-biological solution to create the
> standards, but this did not satisfy the student.  She left with an
> understanding of how to do it, but not a good and detailed reason as to
> WHY?  Does anyone have any practical but good advice on how to explain
> this?  Your help would be greatly appreciated.
> Jeff Mason
> University of Arkansas, Biological Sciences/Microbiology
> wmason at comp.uark.edu
> http://comp.uark.edu/~wmason

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