biology

McFarland standards

McFarland standards is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand McFarland standards rather than just read about it. In short: 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.

Key takeaways

McFarland standards belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect McFarland standards to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of McFarland standards from memory before moving on to harder problems.

Reference excerpt

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). 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}

*at wavelength of 600 nm McFarland latex standards from Hardy Diagnostics (2014-12-10), measured at the UCSF DeRisi Lab:

References

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

Illustrations

Worked examples

Example 1 — a first encounter with McFarland standards

Start with the simplest possible case. Write down what McFarland standards claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to McFarland standards before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about McFarland standards ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of McFarland standards

In research: McFarland standards appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses McFarland standards in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: McFarland standards is common in secondary-school and first-year university syllabi. It links to neighbouring topics Microbiology equipment, Standards and measurement stubs, so understanding it makes those chapters shorter.
In everyday life: Look for McFarland standards outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study McFarland standards in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what McFarland standards means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain McFarland standards out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is McFarland standards in simple terms?

Why does McFarland standards matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study McFarland standards?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on McFarland standards.