Bacteria as pathogensEdexcel A-Level Biology B: Revision notes
Section 1
Bacteria as agents of infection
Pathogens are organisms that cause disease. Bacteria are prokaryotes with no nucleus, and only a minority are pathogenic. A bacterial infection begins when bacteria enter the body, reach a suitable site and multiply there.
Bacteria harm the host in two main ways:
- by invading and destroying host tissue
- by producing toxins, which are poisons that damage host cells
The specification uses three examples to illustrate the different effects: Staphylococcus (exotoxins), Salmonella (endotoxins) and Mycobacterium tuberculosis (invasion of host tissue).
Section 2
Exotoxins: Staphylococcus
Exotoxins are toxins, mostly proteins, that are secreted by living bacteria into their surroundings. Because they pass into the blood, they can damage cells at sites far from the infection.
Staphylococcus aureus is an example. It can infect wounds and make exotoxins that cause food poisoning and toxic shock, with effects such as fever, rash and a fall in blood pressure. The bacteria may stay in the wound while the toxin spreads in the blood.
Link exotoxin to living bacteria that secrete it. This is the opposite of an endotoxin.
Section 3
Endotoxins: Salmonella
Endotoxins are part of the cell wall of Gram-negative bacteria. They are lipopolysaccharides, and they are released when the bacterial cell dies and breaks down (lyses), not secreted by living cells.
Salmonella is an example. Its endotoxin contributes to fever, vomiting and diarrhoea in food poisoning. Because the toxin is released when cells break down, killing many bacteria at once can briefly increase the amount of free endotoxin.
Do not say endotoxins are secreted. They are part of the cell wall and are released on death.
Section 4
Invasion of host tissue: Mycobacterium tuberculosis
Some bacteria damage the host mainly by invading and destroying tissue. Mycobacterium tuberculosis, which causes tuberculosis (TB), is breathed in as droplets and invades the lungs. It enters host cells such as macrophages and multiplies inside them.
The host cells and nearby tissue are destroyed and the lungs form small lesions (tubercles). In serious disease the tissue breaks down to form cavities. This causes a persistent cough, blood in the sputum, weight loss and night sweats. TB does not rely on a powerful toxin, and it develops slowly because the damage builds up over weeks.
Section 5
Comparing the three mechanisms
- Exotoxin (Staphylococcus): secreted by living cells, mainly protein, and can damage tissues far from the infection.
- Endotoxin (Salmonella): part of the cell wall, a lipopolysaccharide, released only when cells die and break open.
- Invasion (M. tuberculosis): bacteria multiply in the tissue and destroy it directly, so damage is at the site of infection.
A pathogen can use more than one mechanism, so in an answer name the mechanism and give the evidence for it.
Must Know
- Bacteria harm the host by invading and destroying tissue and by producing toxins
- Exotoxins are secreted by living bacteria, for example Staphylococcus
- Endotoxins are lipopolysaccharides in the cell wall, released when cells break down, for example Salmonella
- M. tuberculosis invades lung tissue and destroys it, without relying on a toxin
- Exotoxins can act far from the site of infection
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Bacteria as pathogens
- A patient develops a red, swollen surgical wound. Swabs taken from the wound grow Staphylococcus aureus, and the bacteria are found only in the wound. Over the next day the patient develops a high fever, a rash and a fall in blood pressure. Blood tests find a toxin made by S. aureus circulating in the blood, but no bacterial cells.The bacteria were found only in the wound. Explain why the patient has symptoms in tissues far from the wound.2 marks
- Several students became ill 12 hours after a barbecue, with fever, vomiting and diarrhoea. Salmonella was cultured from undercooked chicken that they had eaten. A doctor explained that part of their symptoms was caused by a component of the Salmonella cell wall that is released when the bacteria die and break open.A student suggests that killing large numbers of Salmonella in the gut could briefly make the symptoms worse. Explain why this is plausible.2 marks
- A 34-year-old man has had a cough for eight weeks, with blood in his sputum, night sweats and weight loss. A chest scan shows several small rounded lesions in his lungs and areas where lung tissue has been destroyed, leaving cavities. Mycobacterium tuberculosis is cultured from his sputum. Unlike Staphylococcus aureus, M. tuberculosis does not make a powerful toxin.Explain how M. tuberculosis causes the lung damage seen in this patient.3 marks
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).