Bacterial and Fungal Diseases

This lesson covers the bacterial and fungal diseases named on the AQA GCSE Biology specification. You need to know Salmonella and gonorrhoea as examples of bacterial diseases, and rose black spot as an example of a fungal disease. For each, you must learn the pathogen type, how it spreads, symptoms, and treatment or prevention.

---

How Bacteria Cause Disease

Bacteria are single-celled prokaryotic organisms. Most bacteria are harmless, but pathogenic bacteria cause disease by:

1. Reproducing rapidly inside the body — bacteria undergo binary fission (splitting in two) and can double in number every 20 minutes under ideal conditions
2. Producing toxins — these are poisonous chemicals that damage cells and tissues, causing the symptoms of disease

It is the toxins, not the bacteria themselves, that usually make you feel ill. For example, the toxins produced by Salmonella bacteria cause vomiting and diarrhoea.

graph TD
    A[Bacterium enters the body] --> B[Rapid reproduction by binary fission]
    B --> C[Large population of bacteria]
    C --> D[Toxins released]
    D --> E[Toxins damage cells and tissues]
    E --> F[Symptoms appear: fever, inflammation, pain]

---

Salmonella

Salmonella is a bacterial disease that causes food poisoning. It is one of the most common causes of foodborne illness worldwide.

Feature	Detail
Pathogen	Salmonella bacteria (several species)
Transmission	Eating contaminated food, especially undercooked poultry, eggs, or food prepared in unhygienic conditions
Symptoms	Fever, abdominal cramps, vomiting, diarrhoea — caused by the toxins the bacteria produce
Onset	Symptoms usually appear 12-72 hours after infection
Duration	Most people recover within 4-7 days without treatment
Treatment	Rest, fluids to prevent dehydration; severe cases may need hospital treatment
Prevention	Thorough cooking of food (especially chicken), proper food storage, hand washing, kitchen hygiene

Salmonella and Poultry

In the UK, poultry (especially chickens) are vaccinated against Salmonella to reduce the risk of the bacteria entering the food chain. This is why Salmonella cases have decreased significantly since the vaccination programme was introduced.

Key food safety rules to prevent Salmonella:

• Cook poultry to an internal temperature of at least 75 degrees Celsius
• Do not wash raw chicken (this can splash bacteria onto surfaces)
• Store raw meat separately from ready-to-eat food
• Wash hands thoroughly after handling raw meat
• Refrigerate food promptly to prevent bacterial growth

Exam Tip: When writing about Salmonella, emphasise that the symptoms are caused by toxins produced by the bacteria, not by the bacteria directly damaging cells. This is a key distinction that earns marks.

---

Gonorrhoea

Gonorrhoea is a sexually transmitted infection (STI) caused by bacteria. It is one of the most common STIs in the UK and is becoming increasingly difficult to treat.

Feature	Detail
Pathogen	Neisseria gonorrhoeae bacteria
Transmission	Sexual contact (unprotected sex) — the bacteria are passed through body fluids
Symptoms	Thick yellow or green discharge from the penis or vagina, pain when urinating
Complications	If untreated, can cause infertility in both men and women; can spread to other parts of the body
Treatment	Originally treated with the antibiotic penicillin, but many strains are now antibiotic-resistant
Current treatment	A combination of two antibiotics is now used due to resistance
Prevention	Using barrier contraception (condoms) during sexual contact

Antibiotic Resistance in Gonorrhoea

Gonorrhoea is a major concern for public health because many strains of the bacterium have developed antibiotic resistance. This means that the antibiotics that once killed the bacteria are no longer effective.

The development of antibiotic-resistant gonorrhoea has occurred because:

1. Overuse and misuse of antibiotics has created selective pressure
2. Bacteria with mutations that provide resistance survive and reproduce
3. The resistant strain becomes more common in the population
4. Eventually the antibiotic no longer works against most strains

This is a powerful example of natural selection in action, and it links directly to the evolution topic in AQA GCSE Biology.

Exam Tip: Gonorrhoea is a favourite exam topic because it links to antibiotic resistance and natural selection. Be ready to explain why the overuse of antibiotics has led to resistant strains of bacteria.

---

Rose Black Spot

Rose black spot is a fungal disease that affects rose plants. It is the only fungal disease named on the AQA GCSE Biology specification.

Feature	Detail
Pathogen	Diplocarpon rosae fungus
Host	Rose plants
Transmission	Spores spread by water (rain splash) and wind
Symptoms	Purple or black spots on the leaves; leaves turn yellow and drop early
Effect on plant	Loss of leaves reduces the area available for photosynthesis, weakening the plant
Treatment	Fungicides (chemical sprays) can be used; infected leaves should be removed and destroyed (not composted)
Prevention	Using resistant rose varieties; removing fallen leaves; improving air circulation around plants

Why Rose Black Spot Weakens Plants

The mechanism by which rose black spot causes damage is straightforward:

1. Fungal spores land on rose leaves and germinate
2. The fungus grows into the leaf tissue, causing black/purple spots
3. The plant responds by dropping the infected leaves
4. With fewer leaves, the plant has less surface area for photosynthesis
5. Less photosynthesis means less glucose is produced
6. The plant grows more slowly, produces fewer flowers, and may eventually die if the infection is severe

Exam Tip: When explaining how rose black spot affects a plant, always link the loss of leaves to reduced photosynthesis and therefore reduced growth. Examiners want to see this chain of reasoning.

---

Comparing Bacterial and Fungal Diseases