Biology - Common exam question - Bacterial Cultures

Biology

Thank you to the two people who have replied below with their revision requests. Bacterial cultures was common for biology, so we shall start with that. It's also an extremely common exam question topic, often worth up to 5 or 6 marks! Remember, that's 10% of an exam paper, so getting it right could drastically bump up your grade.

Now, for the question....

A student is given a tube containing a liquid nutrient medium. The medium contains one type of bacterium.

In this question you will be assessed on using good English, organising information clearly and using specialist terms where appropriate.

The student is told to grow some of the bacteria on agar jelly in a Petri dish.

Describe how the student should prepare an uncontaminated culture of the bacterium in the Petri dish.

You should explain the reasons for each of the steps you describe. (6 marks)

A whopping SIX marks! How do we go about answering this question?

My advice is the same for all six mark questions:

1. Underline the command word(s) - make sure you know what the question is asking you to do.
2. If there is a diagram, add relevant annotations to help come up with ideas and/or frame your answer.
3. Bullet point key ideas around/near the question. There is usually space under the question or in the margin.
4. Write your ideas in full sentences, taking care with spelling, punctuation and grammar. You should try and use paragraphs if space allows.

Back to the question.....

A student is given a tube containing a liquid nutrient medium. The medium contains one type of bacterium.

In this question you will be assessed on using good English, organising information clearly and using specialist terms where appropriate.

The student is told to grow some of the bacteria on agar jelly in a Petri dish.

Describe how the student should prepare an uncontaminated culture of the bacterium in the Petri dish.

You should explain the reasons for each of the steps you describe. (6 marks)

1. Straight away, I know what I have to do and so I know how to structure my answer. Describe - step-by-step processes, explain - give scientific reasons. So I say step 1...this is because. Easy. I've also underlined a key word; uncontaminated. This means that all my scientific reasons must be related to this key point.

Il'll bullet point (very briefly) key points I'll include in my answer:

• Petri dish has to be sterilised
• Flame loop - kill unwanted bacteria
• Dip loop into culture
• Loop spreads bacteria on culture
• Open lid minimal amount - to prevent unwanted bacteria getting in
• Flame loop again
• Seal lid onto dish to stop it coming off - prevents unwanted bacteria getting in
• Incubate dish - so bacteria we do want can grow

Before you write your answer out in full sentences, have one more read of the question, and check this against your bullet points.

A student is given a tube containing a liquid nutrient medium. The medium contains one type of bacterium.

In this question you will be assessed on using good English, organising information clearly and using specialist terms where appropriate.

The student is told to grow some of the bacteria on agar jelly in a Petri dish.

Describe how the student should prepare an uncontaminated culture of the bacterium in the Petri dish.

You should explain the reasons for each of the steps you describe. (6 marks)

I've noticed some of my points don't relate to growing an uncontaminated culture, so I won't bother including them as they will take up precious time and won't get me any more marks! I tick/cross the points to include.

• Flame loop - kill unwanted bacteria
• Dip loop into culture X
• Loop spreads bacteria on culture
• Open lid minimal amount - to prevent unwanted bacteria getting in
• Flame loop again X
• Seal lid onto dish to stop it coming off - prevents unwanted bacteria getting in
• Incubate dish - so bacteria we do want can grow

I'm happy, so now I'll write my answer out in full....

Before the bacteria can be grown on the agar dish, all equipment including the petri dish must be sterilised; this is so that any unwanted bacteria are killed before the plate is inoculated. 

A loop is used to spread the bacterial culture onto the agar dish, this too has to be sterilised before use so that unwanted bacteria do not grow on the agar dish. This is done by flaming the loop before it is put into the dish. When spreading the bacterial culture onto the dish, the lid should be opened a minimal amount to prevent any unwanted microbes from contaminating the culture.

Finally, before the plate in incubated so that the bacteria can grow, the lid is sealed using sticky tape. This prevents the lid from accidentally coming off and so stops any unwanted bacteria from getting into the agar dish.

One more quick check of my SPAG, then I can move onto the next question. 6/6 :)

Now one for you to try.

The diagram shows how a student transferred some sour milk from a bottle to a Petri
dish of nutrient agar.

List A gives four actions carried out by the student.
List B gives five possible effects of these actions.

Draw a straight line from each action in List A to its effect in List B. 

Draw only one line from each action. (4 marks)