Question on a MacFarland Standards

K N and P J Harris ecoli at cix.compulink.co.uk
Sat Mar 6 07:25:18 EST 1999


> ==========
> bionet/microbiology #10146, from wmason at COMP.UARK.EDU, 1835 chars, 2 
Mar 1999 20:45:20 -0
> ----------
> Article: 11798 of bionet.microbiology
> From: wmason at COMP.UARK.EDU ("William J. Mason")
> Newsgroups: bionet.microbiology
> Subject: Question on a MacFarland Standards
> Date: 2 Mar 1999 20:45:20 -0800
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> 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
Fascinating,
I used these standards ages ago (and with the "black and white" lines 
system).

They certainly are simple to use and less expensive than 
sprectrophotometers or nephelometers.

A problem I have come across since is the need for VERY precise 
conditions on the production of the Ba sulphate suspension. Particle 
size is crucial as is the avoidance of any "inclusive co-precipitation" 
which will affect the "turbidity" of the suspension. The Ba sulphate 
precipitation is used as a quantitative chemical analysis for sulphate 
and anyone interested in the production of standards would be well 
advised to refer to a standard chemical text for the conditions for Ba 
sulphate precipitation. The other consideration is that any inorganic 
suspension is subject to crystaline change over time which is why Ba 
sulphate suspensions must be renewed after a time.

Peter Harris,
Soil Microbiologist,
(who has benefited for decades from the knowledge of chemists)
University of Reading,
UK





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