Edexcel A-Level Biology: Microbiology and Pathogens

When a pathogenic bacterium enters the body for the first time, the specific (adaptive) immune response is activated. This response is slower than the non-specific response but is highly targeted, and it leaves the body with lasting protection.

Describe and explain how the cell-mediated and humoral responses act together to destroy a bacterial pathogen and to provide long-term immunity. In your answer you should refer to the roles of T cells, B cells, plasma cells and memory cells.

(6 marks)

A technician estimated the number of viable bacteria in a liquid culture using a serial dilution and the dilution plating (total viable count) method. The original culture was diluted in a series of tenfold steps. For each dilution, 0.1 cm³ of the diluted suspension was spread onto an agar plate and incubated, and the resulting colonies were counted.

Dilution factor	Volume plated / cm³	Number of colonies counted
$10^{-4}$10−4	0.1	too many to count
$10^{-5}$10−5	0.1	480
$10^{-6}$10−6	0.1	52
$10^{-7}$10−7	0.1	6

The technician chose to use the plate showing 52 colonies for the calculation.

(a) Calculate the number of viable bacterial cells per cm³ in the original culture. Show your working and give your answer in standard form with appropriate units. (3 marks)

(b) Explain why the technician chose the plate with 52 colonies, rather than the $10^{-5}$10−5 or the $10^{-7}$10−7 plate, and explain why this method gives a count of only the viable cells. (3 marks)

A bacterial lawn of one species was spread evenly over an agar plate. Five paper discs, each soaked in a different antibiotic (A–E) at the same concentration, were placed on the plate. After incubation, the diameter of the clear zone of inhibition around each disc was measured.

Antibiotic	Diameter of zone of inhibition / mm
A	22
B	0
C	14
D	30
E	18

(a) Calculate the area of the zone of inhibition for antibiotic D, using area $= \pi r^2$=πr2. Give your answer to 3 significant figures with appropriate units. (2 marks)

(b) Using the data, identify which antibiotic was most effective against this bacterium and explain your choice, and state what the result for antibiotic B indicates about this bacterium. (3 marks)

A wild tomato plant carries a gene that a cultivated variety lacks. When the wild plant is attacked by a fungal pathogen, cells immediately around the site of infection rapidly die, the cell walls near the wound become reinforced with extra deposits, and the plant releases chemicals that are toxic to fungi into the surrounding tissue. The cultivated variety, which lacks this gene, is killed by the same fungus.

A plant breeder wants to introduce this gene into the cultivated variety.

Suggest and explain how each of these three responses would help to limit the spread of the fungal pathogen through the plant. (5 marks)

A new vaccine is introduced against a bacterial disease that spreads directly from person to person. Public-health scientists estimate that herd immunity for this disease is reached once about 85 per cent of the population is immune.

In one town the vaccine is offered to everyone, but only 70 per cent of people take it up. A small number of unvaccinated babies, who are too young to be vaccinated, live in the town.

Explain why these unvaccinated babies are at greater risk of catching the disease in this town than they would be if vaccine uptake had reached 90 per cent. (4 marks)

When culturing microorganisms in a school laboratory, students must use aseptic technique to obtain a pure culture and to work safely.

Describe two precautions a student should take when transferring bacteria onto an agar plate using aseptic technique, and for each one state why it is important. (3 marks)