biology

Hektoen enteric agar

Hektoen enteric agar 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 Hektoen enteric agar rather than just read about it. In short: Hektoen enteric agar (HEK, HE or HEA) is a selective and differential agar primarily used to recover Salmonella and Shigella from patient specimens. HEA contains indicators of lactose fermentation and hydrogen sulfide production; as well as inhibitors to prevent the growth of Gram-positive bacteria.

Key takeaways

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

Reference excerpt

Hektoen enteric agar (HEK, HE or HEA) is a selective and differential agar primarily used to recover Salmonella and Shigella from patient specimens. HEA contains indicators of lactose fermentation and hydrogen sulfide production; as well as inhibitors to prevent the growth of Gram-positive bacteria. It is named after the Hektoen Institute in Chicago, where researchers developed the agar.

Use The definitive use of HEA is to discriminate between Shigella and Salmonella, although many other species may grow on these plates. However, while the other bacteria may be clinically relevant, the assay does not discriminate among them. Effectively, HEA uses a metabolic assay to divide colonies into "Salmonella and Shigella" and "everything else". Use of these plates assumes that the user is not interested in other enteric pathogens such as Klebsiella or Escherichia. The plates contain various sugar sources (lactose, sucrose, and salicin), none of which can be used by either Shigella or Salmonella. However, the medium also includes peptone which can be used as a carbon source. Since most bacteria can use the sugars in preference to peptone, these "uninteresting" bacteria acidify the medium and turn a pH indicator yellow or red. Peptone metabolism by Shigella and Salmonella alkalises the medium, turning a pH indicator blue. The presence of thiosulfate or ferric ammonium citrate in the medium produces a black precipitate in the presence of H2S, allowing Shigella – which does not produce H2S, and appears as green colonies – to be distinguished from Salmonella – which does produce hydrogen sulfide and appears as black colonies. Few sulfur-reducing bacteria exist other than Salmonella, which can be isolated from the intestines. Most of these are inhibited on HEA plates by the inclusion of bile salts, so encountering a black colony that is not Salmonella is unusual, although not unheard of. Those that are may be identified as red or yellow colonies with a black centre, indicating that they are fermenting sugar and probably not Salmonella. However, rare strains of Salmonella are capable of lactose fermentation, which will appear in the same way.

Illustrations

Worked examples

Example 1 — a first encounter with Hektoen enteric agar

Start with the simplest possible case. Write down what Hektoen enteric agar 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 Hektoen enteric 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 Hektoen enteric 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 Hektoen enteric agar

In research: Hektoen enteric agar 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 Hektoen enteric 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: Hektoen enteric agar is common in secondary-school and first-year university syllabi. It links to neighbouring topics Microbiological media, so understanding it makes those chapters shorter.
In everyday life: Look for Hektoen enteric 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 Hektoen enteric agar in 20 minutes

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

Frequently asked questions

What is Hektoen enteric agar in simple terms?

Why does Hektoen enteric agar 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 Hektoen enteric 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 Hektoen enteric agar.