How is McFarland standard prepared?
How is McFarland standard prepared?
McFarland standards are prepared by adding barium chloride to sulfuric acid to obtain a barium sulfate precipitate. By adjusting the volumes of these two reagents, standards of various degrees of turbidity can be prepared to represent several different concentrations of bacteria.
How many CFU are in a 0.5 McFarland standard?
McFarland standards
| McFarland Standard No. | 0.5 | 4 |
|---|---|---|
| 1.0% barium chloride (ml) | 0.05 | 0.4 |
| 1.0% sulfuric acid (ml) | 9.95 | 9.6 |
| Approx. cell density (1X10^8 CFU/mL) | 1.5 | 12.0 |
| % transmittance* | 74.3 | 21.5 |
What is the purpose of the 0.5 McFarland standard?
The standard most commonly used in the clinical microbiology laboratory is the 0.5 McFarland Standard, which is prescribed for antimicrobial susceptibility testing and culture media performance testing.
How many CFU are in a 1 McFarland standard?
The optical density of each McFarland standard was measured and paired with an estimated bacterial cell density (number of CFU of E. coli) according to the following approximations: McFarland 0.5 = 1.5 × 108 CFU/ml, McFarland 1 = 3.0 × 108 CFU/ml and McFarland 2 = 6.0 × 108 CFU/ml [9].
How do you prepare McFarland standard for antimicrobial susceptibility testing?
Preparation of McFarland Standards
- Prepare a 1% solution of anhydrous barium chloride (BaCl2) and 1% solution of sulfuric acid (H2SO4)
- Combine and completely mix the barium chloride and sulfuric acid solutions to form a turbid suspension.
- Place the resulting mixture in a foil-covered screw-cap tube.
What is McFarland scale?
McFarland standard is a specific scale for the concentration of bacteria /ml.It is used to approximate the number of microbial cells in the liquid suspension.
What is the OD of 0.5 McFarland?
A McFarland standard of 0.5 should have an OD600 between about 0.08 and 0.1 but you will have to determine this relationship for your particular spectrophotometer.
What is 0.5 McFarland turbidity?
0.5 McFarland turbidity standard provides an optical density comparable to the density of a bacterial suspension with a 1.5 x 10^8 colony forming units (CFU/ml).
What is McFarland’s standards how is it useful to antibiotic sensitivity test?
In microbiology, McFarland standards are used as a reference to produce solutions that contain approximately similar numbers of bacteria for use in standardized microbial testing (Kirby Bauer). This is done by matching the turbidity (cloudiness) of McFarland standard with that of the test solution.
What is McFarland standard concentration?
The McFarland standard can be prepared of the varying concentration ranging from the 0.5 to 4 concentration and depending upon the concentration, the cell count density varies.
How do you calculate CFU?
- To find out the number of CFU/ ml in the original sample, the number of colony forming units on the countable plate is multiplied by 1/FDF. This takes into account all of the dilution of the original sample.
- 200 CFU x 1/1/4000 = 200 CFU x 4000 = 800000 CFU/ml = 8 x 10.
- CFU/ml in the original sample.