Bacterial diseases: Salmonella food poisoning is spread by bacteria ingested in food, or on food prepared in unhygienic conditions. In the UK, poultry are vaccinated against salmonella to control the spread. Fever, abdominal cramps, vomiting and diarrhoea are caused by the bacteria and the toxins they secrete.

The idea

Salmonella bacteria cause food poisoning when ingested in contaminated food or food prepared in unhygienic conditions. The bacteria and the toxins they secrete trigger fever, abdominal cramps, vomiting and diarrhoea. In the UK, poultry are vaccinated against salmonella to reduce the spread of infection to humans. For example, a person may eat undercooked chicken that carries salmonella; the bacteria multiply in the gut and release toxins that irritate the digestive system, producing the symptoms. Vaccinating poultry interrupts this transmission route by reducing the number of infected birds entering the food chain. Understanding this cycle helps explain how hygiene, cooking and vaccination work together to control bacterial disease.

What earns the marks

• Salmonella is a bacterium that causes food poisoning.
• Transmission occurs by ingesting contaminated food or food prepared in unhygienic conditions.
• Symptoms include fever, abdominal cramps, vomiting and diarrhoea.
• Symptoms are caused by the bacteria and the toxins they secrete.
• In the UK, poultry are vaccinated against salmonella to control the spread.
• Vaccination reduces the number of infected poultry, lowering the risk of contaminated food reaching consumers.

Common mistakes and how to put them right

• Thinking that salmonella is a virus.

  Put it right by stating it is a bacterium and that bacterial diseases can be treated with antibiotics, although antibiotics are not always used for mild food poisoning.

• Believing that symptoms are caused only by the bacteria themselves.

  Put it right by explaining that the bacteria secrete toxins, and both the bacteria and toxins cause fever, cramps, vomiting and diarrhoea.

• Confusing vaccination of poultry with treatment of humans.

  Put it right by clarifying that poultry are vaccinated to prevent them carrying and spreading salmonella, which reduces human infection.

• Assuming that food poisoning is always caused by poor personal hygiene.

  Put it right by noting that contaminated food, undercooking and cross-contamination can also spread salmonella.

Exam tips

• When asked how salmonella is spread, name both routes: ingestion of contaminated food and food prepared in unhygienic conditions.
• If asked why poultry are vaccinated, link vaccination to reducing the spread of salmonella to humans through the food chain.
• When describing symptoms, include fever, abdominal cramps, vomiting and diarrhoea, and state that toxins secreted by the bacteria contribute to these symptoms.
• Use the term 'toxins' accurately; do not write 'poisons' unless you also explain that toxins are substances produced by the bacteria.