Bacterial Diseases (Salmonella, Gonorrhoea) and Protist Diseases (Malaria)

This lesson covers the named bacterial and protist diseases required by the AQA GCSE Combined Science Trilogy specification (8464): salmonella and gonorrhoea (bacterial), and malaria (protist). You need to know the pathogen, symptoms, transmission, and prevention or treatment of each.

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How Bacteria Cause Disease — Recap

Bacteria are prokaryotic cells that reproduce rapidly by binary fission. They cause disease by:

1. Reproducing rapidly inside the body, competing with host cells for nutrients and space
2. Producing toxins — poisonous chemicals that damage cells and tissues, causing symptoms such as fever, inflammation and vomiting

Exam Tip: Always say bacteria produce "toxins" rather than "poisons" or "chemicals" — the mark scheme specifically looks for the word toxins.

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1. Salmonella

Feature	Detail
Pathogen	Salmonella bacteria
Symptoms	Fever, abdominal cramps, vomiting, diarrhoea
Cause of symptoms	The bacteria produce toxins in the gut
Transmission	Eating contaminated food, particularly undercooked poultry, eggs, or food prepared in unhygienic conditions
Prevention	Thorough cooking of food (especially chicken), good kitchen hygiene, hand washing; in the UK, poultry are vaccinated against Salmonella
Treatment	Usually self-limiting (the body fights it off); severe cases may need rehydration or antibiotics

Food Hygiene and Salmonella Prevention

The following practices reduce the risk of salmonella infection:

Practice	Why It Helps
Cook poultry to at least 75 °C	Kills Salmonella bacteria
Wash hands after handling raw meat	Prevents transfer of bacteria to other surfaces
Use separate chopping boards	Prevents cross-contamination between raw and cooked foods
Store food at correct temperatures	Slows bacterial reproduction
Vaccinate poultry	Reduces Salmonella in the food chain at source

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2. Gonorrhoea

Feature	Detail
Pathogen	Neisseria gonorrhoeae (a bacterium)
Type of disease	Sexually transmitted infection (STI)
Symptoms	Thick yellow or green discharge from the genitals, pain when urinating; some people have no symptoms
Transmission	Sexual contact (unprotected sex)
Prevention	Use of barrier contraception (condoms)
Treatment	Originally treated with the antibiotic penicillin, but many strains of gonorrhoea have become antibiotic-resistant — doctors now use different antibiotics

Antibiotic Resistance in Gonorrhoea

Gonorrhoea is a key example of antibiotic resistance that AQA expects you to know:

• Overuse and misuse of antibiotics has led to natural selection of resistant strains
• Resistant bacteria survive treatment and reproduce, passing on the resistance gene
• This is why gonorrhoea has become increasingly difficult to treat
• It highlights the importance of completing antibiotic courses and not using antibiotics unnecessarily

graph TD
    A[Antibiotic used to treat gonorrhoea] --> B[Most bacteria killed]
    A --> C[Some bacteria have resistance mutation]
    C --> D[Resistant bacteria survive and reproduce]
    D --> E[Resistance gene passed to offspring]
    E --> F[Antibiotic-resistant strain spreads]
    F --> G[Original antibiotic no longer effective]

Exam Tip: Gonorrhoea is the only named STI on the AQA Trilogy specification. You must be able to explain why it has become difficult to treat (antibiotic resistance) and how its spread can be reduced (condom use).

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3. Malaria (Protist Disease)

Feature	Detail
Pathogen	Plasmodium — a protist (not a bacterium or virus)
Symptoms	Recurring episodes of fever, sweating, shaking, headache, and muscle pain; can be fatal
Transmission	Spread by a vector — the female Anopheles mosquito. The mosquito picks up the Plasmodium protist when it feeds on an infected person's blood and transmits it when it bites another person
Prevention	Mosquito nets (especially insecticide-treated nets), insecticides, draining stagnant water where mosquitoes breed, antimalarial drugs for travellers
Treatment	Antimalarial drugs (e.g. chloroquine); no widely available vaccine yet, although research is ongoing

How Malaria Spreads — The Vector Cycle

graph LR
    A[Infected person — Plasmodium in blood] --> B[Female Anopheles mosquito bites infected person]
    B --> C[Mosquito picks up Plasmodium]
    C --> D[Mosquito bites another person]
    D --> E[Plasmodium injected into new host's blood]
    E --> F[Plasmodium invades red blood cells and liver cells]
    F --> G[New host develops malaria symptoms]
    G --> A

Exam Tip: The mosquito is the vector, not the pathogen. The pathogen is Plasmodium. A vector is an organism that carries and transmits a pathogen without being affected by the disease itself.

Why Is Malaria Difficult to Control?

Challenge	Explanation
Mosquitoes breed in warm, stagnant water	Tropical countries have ideal conditions for mosquito populations
Insecticide resistance	Some mosquito populations have developed resistance to insecticides
No widely used vaccine	The Plasmodium life cycle is complex, making vaccine development difficult
Poverty	Many affected countries cannot afford large-scale prevention programmes

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Comparing the Three Diseases

Feature	Salmonella	Gonorrhoea	Malaria
Pathogen type	Bacterium	Bacterium	Protist
Main transmission	Contaminated food	Sexual contact	Mosquito vector
Key symptom	Vomiting/diarrhoea	Discharge/painful urination	Recurring fever
Treated with antibiotics?	Sometimes	Yes, but resistance is growing	No — antimalarial drugs used
Vaccine?	Poultry vaccinated	No	Limited availability

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Common Mistakes

Mistake	Correction
"Malaria is caused by a virus"	Malaria is caused by a protist (Plasmodium)
"The mosquito causes malaria"	The mosquito is the vector; Plasmodium is the pathogen
"All bacteria can be treated with antibiotics"	Some bacteria, like certain strains of gonorrhoea, are antibiotic-resistant
"Salmonella is spread by contact"	Salmonella is mainly spread through contaminated food

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Summary

• Salmonella is a bacterium spread through contaminated food; it produces toxins causing fever and diarrhoea; prevented by food hygiene and poultry vaccination
• Gonorrhoea is a sexually transmitted bacterium; many strains are now antibiotic-resistant; prevented by using condoms
• Malaria is caused by the protist Plasmodium, spread by female Anopheles mosquitoes (vectors); prevented by mosquito nets, insecticides, and draining stagnant water
• Bacteria cause disease mainly by producing toxins
• Antibiotic resistance is a growing problem, especially for gonorrhoea

Exam Tip: For 6-mark questions on disease, structure your answer around: (1) name the pathogen and its type, (2) describe symptoms, (3) explain transmission, (4) describe prevention or treatment. This covers all the marks.

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Extended Worked Examples

Worked Example 1 — Salmonella Growth in a Warm Kitchen

A raw chicken carcass is left on a worktop at 25 °C. The Salmonella population of 500 cells doubles every 30 minutes. Estimate the population after 4 hours.

Working: