McFarland standards

Scale for turbidity measurement

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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 antimicrobial 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).[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

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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

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Absorbance @600nm | 0.063 | 0.123 | 0.242 | 0.431 | 0.653 | 0.867

## References

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1. ^ Cockerill, Franklin R.; et al. (2012). Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically; Approved Standard—Ninth Edition. CLSI. p. 12\. 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

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Techniques in clinical microbiology

Isolation
and culture|

| Isolation techniques|

* Asepsis
* Streak plate
* Selective media

|

Cultures by body site|

* Blood culture
* Genital culture
* Sputum culture
* Throat culture
* Urine culture
* Wound culture

Cultures by organism|

* Bacterial culture
* Fungal culture
* Viral culture

Identification
and testing|

| Manual testing: basic techniques|

* Colonial morphology
* Hemolysis
* Staining
* Gram stain
* Acid-fast stain
* Giemsa stain
* India ink stain
* Ziehl–Neelsen stain
* Wet prep
* Rapid tests
* Oxidase
* Catalase
* Indole
* PYR

|

Manual testing:
biochemical and immunologic tests|

* ALA test
* Amino acid decarboxylase test
* Bile solubility test
* CAMP test
* Citrate test
* Coagulase test
* DNAse test
* IMViC
* KOH test
* Methyl red test
* Nitrite test
* ONPG test
* Oxidative/fermentation glucose test
* Phenylalanine deaminase test
* Reverse CAMP test
* Salt tolerance test
* Sulfide indole motility test
* Triple sugar iron test
* Urease test
* rapid
* Voges–Proskauer test
* X and V factor test
* Bacitracin susceptibility test
* Optochin susceptibility test
* Novobiocin susceptibility test
* Lancefield grouping
* RPR test

Automated and point-of-care testing|

* Analytical profile index
* MALDI-TOF
* Polymerase chain reaction
* VITEK
* Rapid strep test
* Monospot test

Antibiotic susceptibility testing|

* Beta-lactamase test
* Disk diffusion test
* Etest
* McFarland standards
* Minimum inhibitory concentration

Equipment|

* Agar plate
* Growth medium
* Anaerobic jar
* Gas-pak
* Durham tube
* Biosafety cabinet
* Incubator
* Inoculation loop
* Inoculation needle

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