When bugs have you on the run
Box 1 | Common microbial agents of food-poisoning
Campylobacter
The bacterium Campylobacter was only identified as a food-borne pathogen in the 1970s but is now possibly the most common cause of gastroenteritis in Australia. Campylobacter is common in chickens, and cats and dogs may excrete Campylobacter if they have diarrhoea. Campylobacter infections have been attributed to the consumption of undercooked poultry, raw milk, untreated water and cross-contamination as a result of poor food-handling practices.
Clostridium
A distinguishing feature of the Clostridium bacterium is that it forms spores, usually when the bacterium is undergoing stress. The spores are very resistant to heat and drying out, and can germinate into complete bacterial cells when conditions are again suitable. High temperatures are required to kill the spores and prevent bacterial survival.
Clostridium bacteria are also obligate anaerobes – this means that they will only grow in the absence of oxygen. Anaerobic conditions can occur in foods such as stews and casseroles where oxygen doesn’t readily reach the bottom layers. Survival and multiplication of the bacteria is more likely if these foods have been kept warm for a long time, allowed to cool and then reheated.
Two clostridial bacteria are associated with food:
- Clostridium perfringens causes diarrhoea and abdominal pain;
- Clostridium botulinum causes the disease botulism, which is often fatal.
Escherichia coli
Escherichia coli is another common bacterium. It is not necessarily harmful: millions of these organisms reside in the intestines of all healthy people and perform useful digestive functions. Normal gut strains suppress the growth of harmful bacteria and synthesise vitamins. (The presence of E. coli is often used as an indicator of faecal pollution, especially in waterways.)
But some strains are deadly. Enterohaemorrhagic E. coli (EHEC) produces chemicals called Shiga toxins that are implicated in cases of diarrhoea, bloody diarrhoea and haemolytic-uraemic syndrome (HUS). This last condition is an illness affecting children less than 5 years old. It leads to death in up to 5 per cent of patients, while 5-10 per cent suffer long-term kidney damage. The strain of E. coli implicated in the mettwurst outbreak in Adelaide in 1995 was known as E. coli O111:H-, a different strain from the one affecting school children in Japan in 1996 (EHEC O157:H7).
Listeria
The bacterium Listeria monocytogenes occurs commonly in the environment, both in soil and water. Some strains are potentially fatal, causing a disease called listeriosis that can lead to septicemia (blood poisoning), meningitis, encephalitis and infection of the central nervous system. Listeria has formidable abilities: it can grow at temperatures as high as 50ºC and as low as 1ºC, and at salt concentrations as high as 10 per cent. Freezing does not appear to have any detrimental effect.
Listeriosis is uncommon in healthy people. It is a serious condition, sometimes fatal, in older people and people with deficient immune systems. It can result in miscarriage in pregnant women. Scientists estimate that less than a thousand Listeria bacteria can cause illness in susceptible people.
Salmonella
The bacterium Salmonella is a common cause of food-poisoning. There are more than 2000 different strains of Salmonella. Salmonella are found in the faeces of infected animals. A new species, Salmonella enteritidis, can infect the ovaries of chickens and contaminate the eggs inside the shells.
Symptoms of salmonellosis, the disease caused by Salmonella poisoning, include stomach pain, diarrhoea, chills, fever or headache. Most people feel better within 3-5 days, but data from the United States suggest that two people die out of every 1000 reported cases of salmonellosis. Typhoid, a disease no longer common in Australia, is caused by a strain of Salmonella.
Moulds
Moulds are fungi that commonly occur on damaged fruit, vegetables, stale breads and cakes. They play an essential role in the decomposition of most organic matter: check out the compost heap sometime and you'll see a vast array of moulds at work. While they may not cause dramatic symptoms of food-poisoning, many moulds produce toxic chemicals that can damage the liver and kidneys and suppress the immune system. Food mould experts generally suggest that if it's mouldy, throw it out.
Boxes
Box 2. Food preservation
Box 3. Chemicals in food
Box 4. Safe cooking
Related sites
Education pages 'The bug bible' gives information about bacterial pathogens (Food Safety Campaign Group, Australia)
The 'Bad bug book' (US Food and Drug Administration)
Is it safe to eat? (Scientific American, Explorations)
Microorganisms that contaminate food (Scientific American, Explorations)
Salmonella vaccine (Access Excellence)
Instant E. coli test (Access Excellence)
Page updated February 2010.