McFarland standards
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In
Original McFarland standards were made by mixing specified amounts of barium chloride and sulfuric acid together. Mixing the two compounds forms a barium sulfate precipitate, which causes turbidity in the solution. A 0.5 McFarland standard is prepared by mixing 0.05 mL of 1.175% barium chloride dihydrate (BaCl2•2H2O), with 9.95 mL of 1% sulfuric acid (H2SO4).[1]
Now there are McFarland standards prepared from suspensions of latex particles, which lengthens the shelf life and stability of the suspensions. The standard can be compared visually to a suspension of bacteria in sterile saline or nutrient broth. If the bacterial suspension is too turbid, it can be diluted with more diluent. If the suspension is not turbid enough, more bacteria can be added.
McFarland nephelometer standards:{2}
McFarland Standard No. | 0.5 | 1 | 2 | 3 | 4 |
1.0% barium chloride (ml) | 0.05 | 0.1 | 0.2 | 0.3 | 0.4 |
1.0% sulfuric acid (ml) | 9.95 | 9.9 | 9.8 | 9.7 | 9.6 |
Approx. cell density (1X10^8 CFU/mL) | 1.5 | 3.0 | 6.0 | 9.0 | 12.0 |
% transmittance* | 74.3 | 55.6 | 35.6 | 26.4 | 21.5 |
Absorbance* | 0.08 to 0.1 | 0.257 | 0.451 | 0.582 | 0.669 |
*at wavelength of 600 nm
McFarland latex standards from Hardy Diagnostics (2014-12-10), measured at the UCSF DeRisi Lab:
McFarland Standard No. | 0.5 | 1 | 2 | 4 | 6 | 8 |
Absorbance @600nm | 0.063 | 0.123 | 0.242 | 0.431 | 0.653 | 0.867 |
References
- ISBN 978-1-56238-784-6.
- THE NEPHELOMETER: AN INSTRUMENT FOR ESTIMATING THE NUMBER OF BACTERIA IN SUSPENSIONS USED FOR CALCULATING THE OPSONIC INDEX AND FOR VACCINES. JOSEPH McFARLAND, M.D. JAMA. 1907; XLIX(14):1176-1178.
Mcfarland Standards-http://www.dalynn.com/dyn/ck_assets/files/tech/TM53.pdf