McFarland standards are used as a reference to adjust the turbidity of bacterial suspensions, Formulas and forms for Biology
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McFarland standards are used as a reference to adjust the turbidity of bacterial suspensions, Formulas and forms for Biology

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In microbiology, McFarland standards are used as a reference to adjust the turbidity of bacterial suspensions so that the number of bacteria will be within a given range to standardize microbial testing.
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McFarland standards. No. 0.5, 1 and 2.

In microbiology, McFarland standards are used as a reference to adjust

the turbidity of bacterial suspensions so that the number of bacteria will be within a given range

to standardize microbial testing. An example of such testing is antibiotic susceptibility testing by

measurement of minimum inhibitory concentration which is routinely used in medical

microbiology and research. If a suspension used is too heavy or too dilute, an erroneous result

(either falsely resistant or falsely susceptible) for any given anti-microbial agent could occur.

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).

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

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 Standard No. 0.5 1 2 4 6 8

Absorbance @600nm 0.063 0.123 0.242 0.431 0.653 0.867

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