Microbiology (Edexcel International A Level (IAL) Biology): Exam Questions

Bacteria and fungi can both cause infections.

The diagram shows part of the structure of a fungus.

(i) How many of the organelles labelled in this diagram have membranes?

(1)

☐ A 1

☐ B 3

☐ C 5

☐ D 7

(ii) Explain why fungi are not classified as bacteria, plants or animals.

Use the information in the diagram and your knowledge of cell structure to support your answer.

(3)

Rabies is a disease caused by a virus.

The diagram shows the structure of the rabies virus.

(i) The rabies virus has an envelope.

Which of the following pairs of viruses have an envelope?

(1)

(ii) The structure of the rabies capsid is described as complex.

Which of the following has a complex capsid structure?

(1)

(iii) Rabies virus is an RNA virus.

How many of the following viruses are RNA viruses?

• Ebola virus

• human immunodeficiency virus (HIV)

• lambda phage (λ phage)

• tobacco mosaic virus (TMV)

(1)

The rabies virus replicates in a lytic cycle.

The RNA of the rabies virus is a negative RNA strand.

The diagram shows how the negative RNA strand of the rabies virus is used to make positive RNA and proteins.

(i) The diagram shows part of the base sequence in the negative RNA strand.

Complete the diagram to show the corresponding base sequence in the positive RNA strand.

(1)

(ii) Explain why a positive RNA strand has to be made.

(2)

The idea of positive and negative strands of RNA might seem unfamiliar, and this is not surprising because it is only linked to the edges of the spec for this board. The examiners are trying to get you to use your knowledge of base-pairing to get through this question. The virus carries a negative strand, i.e. a strand of RNA that is the complementary base sequence to the RNA sequences that would be required to replicate viral particles. In doing so, the virus makes it easier for the host cell to manufacture positive strands, which can then be directly translated into viral proteins/components, to allow easy viral proliferation. 

Lemurs are found in Madagascar. It is thought that they might carry rabies viruses.

The photograph shows a lemur.

Mathias Appel, CC0, via Wikimedia Commons

A person was bitten by a lemur.

This person did not receive any treatment for rabies until 18 days after being bitten.

Explain why doctors were worried that this person had left it too long for the treatment to be successful.

Antimicrobial substances can be tested using a Minimum Inhibitory Concentration (MIC) assay.

This assay determines the lowest concentration of an antimicrobial substance that prevents visible growth of bacteria.

A microdilution plate is used in these assays. It is made of plastic and contains small wells that the antimicrobial substance and the bacteria can be added to.

The diagram shows a microdilution plate.

An investigation tested eight antimicrobial substances on one type of bacteria, E. coli.

Appropriate controls were included in this investigation.

The diagram shows the steps involved.

All steps in this investigation had to be carried out using aseptic technique.

(i) State the meaning of the term aseptic technique.

(1)

(ii) Describe two aseptic techniques that could be used in this investigation.

(2)

(iii) Explain why using aseptic technique in this investigation is important.

(2)

Explain why the microdilution plate had to be incubated at an appropriate temperature for E. coli for 72 hours.

The diagram shows the results of the MIC assay from this investigation.

(i) Explain why an antimicrobial substance that affects E. coli only was included in this assay (row H).

(2)

(ii) Explain why there was one column that had no antimicrobial substance added to it (column 12).

(2)

(iii) The MIC for the antimicrobial substance used in row E was in column 4. Describe how a 1 in 2 dilution plating method would have been carried out to achieve the dilution in this well.

(2)

(iv) Calculate how many times more effective the antimicrobial substance used in row G is than the antimicrobial substance used in row E.

(2)

Researchers investigated the growth of Leuconostoc mesenteroides in a closed batch culture.

Explain the changes in the number of viable bacterial cells over the course of this investigation.

During one phase of growth, the number of bacteria doubled every 2.5 hours. At the start of this phase the culture contained 2.0 × 10⁴ cells per cm³.

Calculate the number of cells per cm³ after 10 hours of exponential growth.

To estimate the number of bacteria in the culture at peak growth, the researchers took a 1 cm³ sample and made serial dilutions. A 0.1 cm³ volume of the 10^-5 dilution was plated and incubated. After incubation, 57 colonies were counted.

Calculate the number of bacteria per cm³ in the original culture. Give your answer in standard form.

The researchers also used optical density (turbidity) to monitor bacterial growth continuously.

Suggest one advantage and one disadvantage of the turbidity method compared with the dilution plating method.