biology

Miles and Misra method

Miles and Misra method 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 Miles and Misra method rather than just read about it. In short: The Miles and Misra Method (or surface viable count) is a technique used in Microbiology to determine the number of colony forming units in a bacterial suspension or homogenate. The technique was first described in 1938 by Miles, Misra and Irwin who at the time were working at the LSHTM.

Key takeaways

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

Reference excerpt

The Miles and Misra Method (or surface viable count) is a technique used in Microbiology to determine the number of colony forming units in a bacterial suspension or homogenate. The technique was first described in 1938 by Miles, Misra and Irwin who at the time were working at the LSHTM. The Miles and Misra method has been shown to be precise. Materials

A calibrated dropping pipette, or automatic pipette, delivering drops of 20μl. Petri dishes containing nutrient agar or other appropriate medium. Phosphate Buffered Saline (PBS) or other appropriate diluent. Bacterial suspension or homogenate. Method

The inoculum / suspension is serially diluted by adding 1x of suspension to 9x of diluent. When the quantity of bacteria is unknown, dilutions should be made to at least 10−8. Three plates are needed for each dilution series, for statistical reasons an average of at least 3 counts are needed. The surface of the plates need to be sufficiently dry to allow a 20μl drop to be absorbed in 15–20 minutes. Plates are divided into equal sectors (it is possible to use up to 8 per plate). The sectors are labelled with the dilutions. In each sector, 1 x 20 μl of the appropriate dilution is dropped onto the surface of the agar and the drop allowed to spread naturally. In the original description of the method a drop from a height of 2.5 cm spread over an area of 1.5-2.0 cm. It is important to avoid touching the surface of the agar with the pipette. The plates are left upright on the bench to dry before inversion and incubation at 37 °C for 18 – 24 hours (or appropriate incubation conditions considering the organism and agar used). Each sector is observed for growth, high concentrations will give a confluent growth over the area of the drop, or a large number of small/merged colonies. Colonies are counted in the sector where the highest number of full-size discrete colonies can be seen (usually sectors containing between 2-20 colonies are counted). The following equation is used to calculate the number of colony forming units (CFU) per ml from the original aliquot / sample: CFU per ml = (Average number of colonies for a dilution / 0.02) x dilution factor. Advantages

Faster than other methods. Produce less bacterial contamination of the working surface.

Worked examples

Example 1 — a first encounter with Miles and Misra method

Start with the simplest possible case. Write down what Miles and Misra method 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 Miles and Misra method 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 Miles and Misra method 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 Miles and Misra method

In research: Miles and Misra method 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 Miles and Misra method 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: Miles and Misra method is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cell culture techniques, Microbiology techniques, so understanding it makes those chapters shorter.
In everyday life: Look for Miles and Misra method 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 Miles and Misra method in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Miles and Misra method 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 Miles and Misra method out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Miles and Misra method in simple terms?

Why does Miles and Misra method 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 Miles and Misra method?

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 Miles and Misra method.