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MacConkey agar

MacConkey agar is a chemistry 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 MacConkey agar rather than just read about it. In short: MacConkey agar is a selective and differential culture medium for bacteria. It is designed to selectively isolate gram-negative and enteric (normally found in the intestinal tract) bacteria and differentiate them based on lactose fermentation.

MacConkey agar — main illustration
MacConkey agar — illustration

Key takeaways

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

Reference excerpt

MacConkey agar is a selective and differential culture medium for bacteria. It is designed to selectively isolate gram-negative and enteric (normally found in the intestinal tract) bacteria and differentiate them based on lactose fermentation. Lactose fermenters turn red or pink on MacConkey agar, and nonfermenters do not change color. The media inhibits growth of gram-positive organisms with crystal violet and bile salts, allowing for the selection and isolation of gram-negative bacteria. The media detects lactose fermentation by enteric bacteria with the pH indicator neutral red.

Contents It contains bile salts (to inhibit most gram-positive bacteria), crystal violet dye (which also inhibits certain gram-positive bacteria), and neutral red dye (which turns pink if the microbes are fermenting lactose). Composition:

Peptone – 17 g Proteose peptone – 3 g Lactose – 10 g Bile salts – 1.5 g Sodium chloride – 5 g Neutral red – 0.03 g Crystal violet – 0.001 g Agar – 13.5 g Water – add to make 1 litre; adjust pH to 7.1 +/− 0.2 Sodium taurocholate There are many variations of MacConkey agar depending on the need. If the spreading or swarming of Proteus species is not required, sodium chloride is omitted. Crystal violet at a concentration of 0.0001% (0.001 g per litre) is included when needing to check if gram-positive bacteria are inhibited. MacConkey with sorbitol is used to isolate E. coli O157, an enteric pathogen.

History The medium was developed by Alfred Theodore MacConkey while working as a bacteriologist for the Royal Commission on Sewage Disposal.

Uses Using neutral red pH indicator, the agar distinguishes those gram-negative bacteria that can ferment the sugar lactose (Lac+) from those that cannot (Lac-). This medium is also known as an "indicator medium" and a "low selective medium". Presence of bile salts inhibits swarming by Proteus species.

Lac positive By utilizing the lactose available in the medium, Lac+ bacteria such as Escherichia coli, Enterobacter and Klebsiella will produce acid, which lowers the pH of the agar below 6.8 and results in the appearance of pink colonies. The bile salts precipitate in the immediate neighborhood of the colony, causing the medium surrounding the colony to become hazy.

Lac negative Organisms unable to ferment lactose will form normal-colored (i.e., un-dyed) colonies. The medium may also turn yellow. Examples of non-lactose fermenting bacteria include Salmonella, Proteus, and Shigella spp.

Slow Some organisms ferment lactose slowly or weakly, and are sometimes put in their own category. These include Serratia and Citrobacter.

Mucoid colonies Some organisms, especially Klebsiella and Enterobacter, produce mucoid colonies which appear very moist and sticky and slimy. This phenomenon happens because the organism is producing a capsule, which is predominantly made from the lactose sugar in the agar.

Variant A variant, sorbitol-MacConkey agar, (with the addition of additional selective agents) can assist in the isolation and differentiation of enterohemorrhagic E. coli serotype O157:H7, by the presence of colorless circular colonies that are non-sorbitol fermenting.

See also R2a agar MRS agar (culture medium designed to grow gram-positive bacteria and differentiate them for lactose fermentation).

References

Illustrations

MacConkey agar: With an active lactose non-fermenting bacterial culture
With an active lactose non-fermenting bacterial culture
MacConkey agar: Lactose
Lactose
MacConkey agar: The organism on the left is a lactose fermenter, as evidenced by the pink color. The organism on the right produced no color, so it does not appear to be a lactose fermenter.
The organism on the left is a lactose fermenter, as evidenced by the pink color. The organism on the right produced no color, so it does not appear to be a lactose fermenter.

Worked examples

Example 1 — a first encounter with MacConkey agar

Start with the simplest possible case. Write down what MacConkey agar claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, 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 MacConkey agar 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 MacConkey agar 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 MacConkey agar

In research
MacConkey agar appears in chemistry 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 MacConkey agar 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
MacConkey agar is common in secondary-school and first-year university syllabi. It links to neighbouring topics Biochemistry detection reactions, Cell culture media, Microbiological media, so understanding it makes those chapters shorter.
In everyday life
Look for MacConkey agar 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 MacConkey agar in 20 minutes

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

Frequently asked questions

What is MacConkey agar in simple terms?

MacConkey agar is a selective and differential culture medium for bacteria. It is designed to selectively isolate gram-negative and enteric (normally found in the intestinal tract) bacteria and differentiate them based on lactose fermentation.

Why does MacConkey agar matter?

Because it connects several chemistry 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 MacConkey agar?

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 MacConkey agar.

Tags

  • Biochemistry detection reactions
  • Cell culture media
  • Microbiological media

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