AQA GCSE Biology

Revision Notes

3.1.3 Bacterial Diseases

Bacteria: Basics

  • Bacterial pathogens are cells that can infect plants and animals, causing disease
  • Not all bacteria are harmful (so not all bacteria are pathogens) – both the skin and large intestine, in particular, are home to hundreds of different bacterial species that play a vital role in maintaining our health
    • Bacteria on the skin act as competitors to harmful pathogens
    • Bacteria in the large intestine digest substances we can’t in food (such as cellulose) and provide essential nutrients (such sa Vitamin K)
  • Bacterial pathogens produce toxins cause damage to cells and tissues directly
  • In certain optimum conditions, some bacteria can reproduce rapidly (with the fastest being once every 20 minutes for some species of coli)
    • Optimum conditions usually involve warmth, moisture and a supply of nutrients

Bacteria Growth curve, downloadable IGCSE & GCSE Biology revision notes

The graph shows the exponential growth of bacteria due to binary fission


  • Salmonella food poisoning is spread by bacteria ingested in food, or on food prepared in unhygienic conditions
  • It is found in the gut of many different animals

Salmonella table

Salmonella table, downloadable IGCSE & GCSE Biology revision notes


  • Gonorrhoea is a sexually transmitted disease (STD) – these diseases are also called sexually transmitted infections (STI)
  • In 2018 it had the largest increase (26%) of infections in the UK (a total of 56,259 cases!). It was the cause of 13% of diagnosed STDs

Gonorrhoea table

Gonorrhoea table, downloadable IGCSE & GCSE Biology revision notes

Exam Tip

Know the symptoms and how the pathogen causing each disease is spread. Typically this topic appears as data analysis questions in the exam where you may be given a graph to analyse and interpret.

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

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