Question on a MacFarland Standards

[Austin Reade]

Maybe the student is confused because the experiment is combining two methods 
for determining culture concentrations.

If you have a spec and don't mind using it, why would you bother to calibrate 
a set of MacFarland standards?  When we need to prepare suspensions of known 
cfu/mL , we do a standard curve for OD vs cfu for whatever combination 
of bacterium, culture conditions, diluents etc we are using, then subsequently 
use the OD to help us prepare suspensions of known cfu/mL.

My understanding is that the MacFarland standards are essentially a 
replacement for the spec whereby  you do a direct visual comparison between 
the unknown and the MacFarland standard.  You would still need to do a 
calibration ahead of time but you don't need to use a spec for that.  You just 
make suspensions which match MacFarland standards then do the counts on these 
suspensions.  After this you use the MacFarland standards as a yardstick in 
place of the spectrophotometer.
----------------------------------- * ----------------------------------
Austin Reade, Microbiologist      Phone:    (902) 424-8670
Innovacorp                        Fax:      (902) 424-4679
101 Research Drive, PO Box 790    Internet: areade at nsrfc.ns.ca
Dartmouth, NS, Canada  B2Y 3Z7