Preparing McFarland Turbidity Standards
There are various ways of estimating the population of microbes in suspension cultures. One of the easiest method is through visual comparison with a stable and known standard--the McFarland Turbidity Standard.
McFarland Turbidity Standards are a set of tubes with increasing concentration of barium sulfate suspension. The cloudiness or turbidity created by barium sulfate's white precipitates is used as a point of comparison of bacterial suspensions to known bacteria's turbidity. Unlike culture broths which may change appearance through time, barium sulfate solutions are un-reactive, non-changing and stable for a span of at least 6 months. Hence, it can serve as a long-term ruler for turbidity. Known strains of bacteria have already been measured using McFarland standards and had their population determined relative to it. With such knowledge, comparison with those known bacteria is possible even with samples in other laboratories.
Here are steps in creating your own McFarland Turbidity Standards:
- Make a 1% solution (w/v) of anhydrous barium chloride (BaCl2).
- Make a 1% solution (v/v) of sulfuric acid (H2SO4).
- Mix these two solutions using these ratio to obtain desired McFarland scale:
McFarland Scale No._____Amt. of 1% BaCl (ml)_____Amt. of 1% H2SO4 (ml)
5 _________________________0.5 ___________________9.5
6 _________________________0.6 ___________________9.4
7 _________________________0.7 ___________________9.3
8 _________________________0.8 ___________________9.2
9 _________________________0.9 ___________________9.1
Tightly seal these tubes and store them at room temperature in the dark. These would remain stable for at least 6 months.
To use, invert the tube several times to evenly resuspend the barium sulfate precipitates. Comparison is best done using a background with thick, horizontal, black-and-white stripes.