Antibiotics and infection controlEdexcel International A Level Biology: Revision notes

Section 1

Bacteriostatic and bactericidal antibiotics

Antibiotics are chemicals that kill or inhibit bacteria. They do not work against viruses.

Bactericidal antibiotics kill bacteria, so the number of living bacteria falls
Bacteriostatic antibiotics stop bacteria multiplying, but do not kill them, so the number of living bacteria stays the same. The patient's immune system must remove the remaining bacteria

A bactericidal antibiotic is often preferred for patients with a weakened immune system. Broad-spectrum antibiotics act on many species of bacteria, whereas narrow-spectrum antibiotics act on only a few.

Key termsantibioticbactericidalbacteriostaticbroad-spectrum

Section 2

Antibiotic resistance

Antibiotic resistance evolves by natural selection. A random mutation gives some bacteria a resistance allele. When antibiotics are used, they act as a selection pressure: susceptible bacteria are killed, while resistant bacteria survive, reproduce and pass on the allele (and may pass it to other bacteria on plasmids). The allele frequency rises.

MRSA (methicillin-resistant Staphylococcus aureus) is a well-known resistant strain found in hospitals.

Key termsresistanceselection pressureMRSA

Common mistake

Bacteria do not 'become used to' antibiotics. Individual bacteria do not change; the population changes because resistant ones survive.

Section 3

Hospital-acquired infections

A hospital-acquired infection (HAI) is an infection caught in hospital. Patients are vulnerable because of open wounds, weak immune systems and invasive equipment such as catheters. Antibiotics are used heavily, which selects for resistant bacteria (e.g. MRSA), and broad-spectrum antibiotics can kill gut flora, allowing Clostridioides difficile to multiply. Pathogens spread on hands, equipment and surfaces.

Key termshospital-acquired infectionC. difficile

Section 4

Codes of practice

Prescription:

Prescribe antibiotics only when necessary, never for viral infections
Use narrow-spectrum antibiotics where possible, chosen after testing the bacterium
Patients must complete the full course

Infection control:

Hand washing and alcohol gel between patients
Gloves and aprons, isolating infected patients (barrier nursing)
Disinfecting surfaces and equipment, screening patients and staff

Key termscode of practice

Section 5

Core Practical 14: effect of antibiotics on bacteria

In this practical, a bacterial lawn is spread on agar and discs soaked in antibiotics are placed on it. After incubation, a clear zone of inhibition forms where bacteria cannot grow.

Use aseptic technique: disinfect surfaces, flame the loop, work near a Bunsen flame, and open the dish as little as possible
Use a control disc soaked in sterile water
Seal the lid with tape (not fully, to allow oxygen in), invert the dish, and incubate at 25 °C (not 37 °C), to avoid growing pathogens
Measure the diameter of each clear zone; a larger zone means a more effective antibiotic. Area = $\pi r^2$

Add the same concentration of each antibiotic so that the comparison is fair.

Key termszone of inhibitionaseptic techniquecontrol

Exam tip

Say why the temperature is 25 °C: it reduces the chance of growing human pathogens, which grow best near 37 °C.

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Antibiotics and infection control

A hospital laboratory compares two antibiotics on separate cultures of the same bacterium. When antibiotic X is added, the number of living bacteria stops increasing but does not fall. When antibiotic Y is added, the number of living bacteria falls rapidly.
Explain how a patient with a healthy immune system could still recover from an infection treated with antibiotic X.2 marks
A student spreads a culture of a harmless bacterium evenly over nutrient agar in a Petri dish. She places on the surface four paper discs, each 6 mm in diameter, soaked in antibiotic P, antibiotic Q, antibiotic R and sterile water. All antibiotic solutions have the same concentration. She tapes the lid on, inverts the dish and incubates it at 25 degrees Celsius for 48 hours. The clear zones around the discs measure 18 mm for P, 6 mm for Q, 24 mm for R and 6 mm for sterile water.
Calculate the area of the clear zone around antibiotic R. Use the formula area = pi r squared, and give your answer in mm squared.2 marks
On a hospital ward, six patients developed infections with MRSA, a strain of Staphylococcus aureus that is resistant to methicillin. Investigators found the bacterium on the hands of some staff and on bed rails.
Suggest three measures that could reduce the spread of MRSA on the ward, and explain how each would help.3 marks

See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).