Bacteria as pathogensEdexcel A-Level Biology B: Subtopic test

1. 1
   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.

   (a)

   Which term describes the toxin found in the patient's blood?

   [1 mark]

   • AExotoxin
   • BEndotoxin
   • CAntigen
   • DAntibody

   (b)

   What is the chemical nature of a bacterial exotoxin such as the one made by S. aureus?

   [1 mark]

   • AA lipopolysaccharide
   • BA polysaccharide capsule
   • CA protein
   • DA phospholipid

   (c)

   The bacteria were found only in the wound. Explain why the patient has symptoms in tissues far from the wound.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   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)

   Which component of the Salmonella cell wall acts as an endotoxin?

   [1 mark]

   • ACellulose
   • BPlasmid DNA
   • CRibosomes
   • DLipopolysaccharide

   (b)

   Which statement correctly contrasts endotoxins with exotoxins?

   [1 mark]

   • AEndotoxins are secreted by living cells; exotoxins are released only when cells die
   • BExotoxins are secreted by living bacteria; endotoxins are released when bacterial cells break down
   • CBoth are produced only by Gram-positive bacteria
   • DEndotoxins are enzymes and exotoxins are lipids

   (c)

   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]

   Total for question 2: 4 marks

3. 3
   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.

   (a)

   Explain how M. tuberculosis causes the lung damage seen in this patient.

   [3 marks]

   (b)

   Compare how S. aureus and M. tuberculosis cause their pathogenic effects, and suggest why the man's symptoms developed gradually over eight weeks.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A public health laboratory is writing a training guide on the ways in which bacteria cause disease. It uses three examples: Staphylococcus aureus in an infected wound, Salmonella in contaminated food and Mycobacterium tuberculosis in the lungs.

   (a)

   Describe and explain how each of the three bacteria causes harm to the host, distinguishing between the types of pathogenic effect.

   [6 marks]

   (b)

   A trainee claims that the harm caused by a bacterial pathogen depends only on how many bacteria are present. Evaluate this claim using the three examples.

   [6 marks]

   Total for question 4: 12 marks

End of questions