Infectious Disease: Question 4

Worked solution

Part (a): Pathogen and transmission

Tuberculosis is usually caused by Mycobacterium tuberculosis (or, less commonly, M. bovis), a bacterium. It is transmitted by airborne droplets: when an infected person coughs, sneezes or even talks, tiny droplets of moisture containing the bacteria are released into the air. Another person can become infected by inhaling these droplets, allowing the bacteria to reach and establish an infection in their lungs.

Part (b): Why overcrowding and poor ventilation help TB spread

In overcrowded housing, many people are in close and prolonged contact within the same enclosed space, which increases the chance that an infected person’s droplets will reach and be inhaled by someone nearby.

Poor ventilation compounds this: fresh air does not enter to dilute and disperse the droplets, so bacteria-containing droplets remain suspended in the air at a higher concentration and for a longer time. This means anyone sharing that air space inhales a larger dose of bacteria than they would in a well-ventilated space, increasing both the chance of infection and the number of people who become infected from a single source case.

Part (c): How the antibiotic kills the bacterium, and why it cannot affect the measles virus

How the antibiotic kills Mycobacterium: A bacterial cell’s cytoplasm is more concentrated (has a lower, more negative water potential) than its surroundings, so water constantly tends to move into the cell by osmosis. The rigid cell wall, held together by cross-linked strands of peptidoglycan (murein), resists this inward pressure and stops the cell bursting. If an antibiotic prevents the enzymes that cross-link peptidoglycan from working, this is especially damaging to bacteria that are actively growing and dividing, since they cannot build a properly cross-linked wall. Without this reinforced wall, the cell can no longer withstand the osmotic pressure: water continues to enter the cell, it swells, and it eventually bursts (lyses), killing the bacterium.

Why this has no effect on measles: Measles is caused by a virus, and viruses have no cell wall, indeed no cellular structure at all, only nucleic acid enclosed in a protein coat. There is therefore no peptidoglycan cell wall present for this antibiotic to act on. Furthermore, a virus does not build its own cell wall, carry out its own metabolism, or synthesise its own proteins independently; instead, it uses the host cell’s own ribosomes, enzymes and other machinery to make new virus particles. Since the antibiotic disrupts a specifically bacterial process (cross-linking of peptidoglycan) that simply does not occur in a virus, giving this antibiotic to a person with measles has no effect on the virus at all.

Final answers

• (a) Mycobacterium tuberculosis (a bacterium), transmitted by airborne droplets released by coughing, sneezing or talking and then inhaled by another person.
• (b) Overcrowding increases close contact with infected droplets, and poor ventilation stops these droplets dispersing, so a higher concentration of bacteria stays in the shared air for longer, increasing the dose inhaled.
• (c) The antibiotic stops peptidoglycan cross-linking, weakening the cell wall so that water enters the bacterium by osmosis and it bursts; it has no effect on the measles virus because a virus has no cell wall and instead uses host cell machinery to replicate, so there is nothing for the antibiotic to target.