0.5 McFarland Standard

Use bacterial suspension with 0.5 McFarland turbidity (CLSI recommends) for antimicrobial testing of new antimicrobial agents (CLSI . The Number of bacterial cells present in the McFarland standard vary based on the cell size.
0.5 McFarland standard is equal to bacterial density of  1.5 x 10⁸  CFU/ml.

Uses :

For  Antimicrobial susceptibility testing, the inoculum does not contain a concentration of bacteria approximates the 0.5 McFarland turbidity standard, antimicrobial susceptibility test results will be affected. 

Resistant organism may appear susceptible if too less bacteria are used in the inoculum.

Procedure: 

McFarland Standard is mixing of 1% solution of barium chloride and 1% of sulfuric acid.

0.05 ml of 1 % barium chloride mixing with 9.95 ml of sulfuric acid to make 0.5 McFarland standard.

This standard can compare with bacterial suspension (saline) in another tube. If the bacterial suspension turbidity is less than McFarland standard add more bacteria can be added. If the bacterial suspension is more turbid than Macfarland standard add diluent. 

McFarlands standards and bacterial suspensions can be measured the OD at 600nm.

Mc Farland Standards

McFarland Std. No.	0.5	1	2	3	4
1% barium chloride (1ml)	0.05	0.1	0.2	0.3	0.4
1% Sulfuric acid  (1ml)	9.95	9.9	9.8	9.7	9.6
Cell density Approx. (1.5×108 CFU/ml)	1.5	3	6	9	12
Absorbance at 600nm	0.08 to 0.1	0.257	0.451	0.582	0.669

McFarland Std.