FOOD POlSONING
Most microorganisms present in or on food are harmless, but there are a number which can cause severe symptoms and even death, if present in human food in sufficient numbers.
The symptoms arising from the presence of harmful bacteria in food are familiar to most people and include headache, vomiting, diarrhoea and abdominal pain. In severe cases, loss of fluid through vomiting and diarrhoea may cause the body to become dehydrated. Very young, elderly or already ill people are most at risk.
Symptoms may arise through direct mechanical damage caused by pathogens themselves to, for example, cells lining the gut. Frequently, however, they are caused by toxins (harmful chemicals) which the pathogens produce. These toxins may already
be present in food on which the bacteria have been multiplying or they may be produced by bacteria as they live and reproduce in the gut.
(Students will not be expected to distinguish between exotoxins, released by the organism and endotoxins which form part of its structure).
(Students will not be expected to distinguish between exotoxins, released by the organism and endotoxins which form part of its structure).
Some of these toxins are amongst the most powerful known poisons. One, produced by a rare contaminant of food, Clostridium botulinum, is readily destroyed by thorough cooking t but other toxins are heat stable (e.g. that produced by Staphylococcus aureus). This means that it is possible for contaminated food which has been preserved, for example by irradiation with gamma rays, to contain the toxin, even though the
microorganisms themselves have been killed and the food appears fresh and safe.
Background
Strictly, a distinction is made between
In food poisoning, food has been contaminated by a microorganism which has then been able to reproduce, because of inappropriate conditions of food storage, using the food as a source of nutrients. People consuming the food are affected by toxins released into the food. Contamination by organisms such as Staphylococcus aureus, Bacillus cereus, Clostridium perfringens and Clostridium botulinum comes into this category.
Food-borne disease, on the other hand, arises from contamination of food with a pathogen which causes problems only because it reproduces inside the body.
These organisms may or may not reproduce significantly on the food itself. Thus the problem is really one of infection rather than of poisoning. Viruses, as well as fungi and bacteria, may be responsible for this type of disease. One example of an organism causing food-borne disease is the gut bacterium Escherichia coli which may be present through faecal or other contamination. The E.coli strain 0157 which caused 20 deaths and 200 other cases to be treated during an outbreak in Lanarkshire, Scotland, in late 1996 is particularly dangerous because so few bacterial cells, possibly as few as 20, need be ingested to cause severe symptoms. Salmonella outbreaks also are really infections, rather than instances of food poisoning.
There is no syllabus requirement for students to recognise these two separate aspects; it will be totally acceptable for them to adopt the general and widespread usage of the term 'food poisoning' in relation to Salmonella infection.
Students should be aware that humans can be harmed by a variety of different organisms which may contaminate food. However, the rest of this section focuses specifically on the problems caused by some of the species of Salmonella.
Background
Some teachers may wish to consider examples of bacteria other than Salmonella which can cause problems when present in food.
Campylobacter spp. These organisms are most liable to occur in meats, sausages, eggs, poultry, pies, milk and milk products. The number of reported cases of enteritis caused by Campylobacter exceeds those of Salmonella.
Listeria monocytogenes A soil organism which may occur in raw vegetables, including prepared salads. Other sources include pre-cooked meat and poultry, cook-chilled ready meals, unpasteurised milk, soft cheeses and soft ice cream, also pâté. It can cause severe problems, including miscarriage and meningitis. The organism is notable for its ability to reproduce at the temperature of domestic refrigerators.
Bacillus cereus A spore - forming organism which occurs in re-heated rice, cold cooked meats, cream, custard, sauces.
Staphylococcus aureus May be present in a wide variety of foods, but cold cooked meats, cream, custard, and sauces are commonly affected. This organism is readily transferred from the skin, wounds and boils, but especially from the nose of food handlers. It releases a heat-stable toxin.
Clostridium spp. These organisms form spores. They can occur in meats of all kinds. Clostridium botulinum, a soil bacterium, is an extremely serious occasional contaminant of inadequately processed canned meat in particular, though it can also be present in vegetables.
Shigella spp. The cause of bacterial dysentery. As with Salmonella, problems arise due to multiplication of the organisms in the intestine. Frequently the spread is via infected water.