Infectious Disease: Question 9

Worked solution

Part (a): Why resistance has increased

Reason 1 (overuse and misuse in humans: Random mutations that confer antibiotic resistance arise in bacterial populations independently of whether an antibiotic is used; a mutation is not caused by the antibiotic itself. However, every time an antibiotic is used unnecessarily) for example prescribed for an infection that does not require it, or (mistakenly) for a viral illness that it cannot affect at all, it kills non-resistant bacteria in the body while leaving any bacteria that already carry a resistance allele to survive and reproduce with less competition. The more often and more widely antibiotics are used, the more often this selection pressure is applied, so the proportion of resistant bacteria in the overall bacterial population increases over time.

Reason 2. Use in agriculture: Antibiotics are also used on a large scale in livestock farming, for example to prevent disease spreading in crowded conditions or to encourage growth. This exposes very large numbers of bacteria in farm animals to antibiotics, selecting for resistant strains within these animal populations. These resistant bacteria can then reach humans, for example through contaminated food, water, or direct contact with animals or farm workers, adding to the pool of resistant bacteria that can cause human infections.

Part (b): Incomplete courses and spread within a ward

If a patient stops taking an antibiotic as soon as their symptoms improve, rather than completing the full prescribed course, some bacteria, including any that are less susceptible or partially resistant, may not yet have been eliminated from their body. These surviving bacteria can then multiply again, undisturbed by any more of the drug.

In a hospital ward, patients are in close contact with each other and share staff, equipment and surfaces, all of which can carry bacteria between people. This gives the surviving, harder-to-kill bacteria from the incompletely treated patient an increased opportunity to be transmitted to other patients on the ward, some of whom may already be more vulnerable to infection, spreading resistant bacteria through the ward.

Part (c): Consequences of resistant infections

• Treatment becomes harder: an infection caused by resistant bacteria may not respond to the first antibiotic tried, so doctors must use a different antibiotic, which can be more expensive, less effective, or cause more severe side effects than the original choice.
• Greater burden on the patient and health service: patients with resistant infections often remain ill for longer and may need a longer hospital stay, which increases healthcare costs and also increases the risk that the resistant bacteria will spread to other patients before the infection is brought under control.

Final answers

• (a) Resistance has increased because of overuse/misuse of antibiotics in humans (unnecessary use provides more selection pressure favouring pre-existing resistant bacteria) and because of large-scale antibiotic use in livestock farming (selecting for resistant bacteria that can spread from animals to humans).
• (b) Bacteria surviving an incomplete course can multiply and be transmitted between patients who share close contact, staff, equipment and surfaces on a ward, spreading resistance.
• (c) Resistant infections may need a different, more expensive or more toxic antibiotic, and can lead to a longer illness or hospital stay with a greater risk of spreading resistant bacteria to other patients.