While I was talking away about FYP with my sister, she mentioned that she was trying to do antimicrobial susceptibility test, on which she used a standard McFarland to measure the inoculum turbidity. Previously I thought inoculum for antimicrobial tests are standardized by CFU count. So now, what is a McFarland Standard; and how do we make it?
Well, different species (different inoculum) exhibit different inoculum density even in similar CFUs. This might be hard to measure the inhibition zone for lighter-density species. Therefore, it must be measured in a way that the density is similar, easing the data collection later.
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A McFarland standard is an optical standard of 0.5 made from Barium Sulfate. An equivalent optical density standard is the latex particle suspension. In this post I will put the McFarland standard procedure.
1) Prepare a Barium Chloride solution 0f 0.048 mol/L using a 1.175% w/v from a stock solution of BaCl2.H2O. Prepare also a 0.18 mol/L Sulfuric Acid from (1% v/v) stock.
2) Mix 0.5 ml of the barium chloride into 99.5 ml the sulfuric acid, with constant stirring.
3) Verify the turbidity standard with a spectrophotometer with 1-cm light path and matched cuvette at 625nm. The absorbance should be 0.008 to 0.10.
These suspensions must be kept in screw cap tubes of similar sizes with the one used in growing/diluting bacteria. No need to store it in freezer, it can do well in a cool and dark place. Before use though, just vigorously agitate this standard with vortexer. If using latex particle, just invert it gently.
1) Prepare a Barium Chloride solution 0f 0.048 mol/L using a 1.175% w/v from a stock solution of BaCl2.H2O. Prepare also a 0.18 mol/L Sulfuric Acid from (1% v/v) stock.
2) Mix 0.5 ml of the barium chloride into 99.5 ml the sulfuric acid, with constant stirring.
3) Verify the turbidity standard with a spectrophotometer with 1-cm light path and matched cuvette at 625nm. The absorbance should be 0.008 to 0.10.
These suspensions must be kept in screw cap tubes of similar sizes with the one used in growing/diluting bacteria. No need to store it in freezer, it can do well in a cool and dark place. Before use though, just vigorously agitate this standard with vortexer. If using latex particle, just invert it gently.
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A standard is past its prime if large particles appear. It needs to be checked once a month as the density may change.
you have a typo it should be 0.08 not 0.008
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