When bugs have you on the run

Key text

Counting the sick

Statistics on food-poisoning are hard to collect because many cases are mild and not reported. Nevertheless, it is a common complaint. In 2003, the Food Safety Information Council estimated that there are approximately 5 million Australians affected by food-poisoning every year. Although this is a very large number and we all must work toward reducing it, it indicates that only 0.02 per cent of meals caused some form of food-poisoning.

What is worrying is that food-poisoning can be fatal. Two elderly people died after eating contaminated ham and corned beef in Victoria in February 1997; another Victorian died from food-poisoning in a nursing home in 1997; a child died after eating contaminated mettwurst in Adelaide in 1995. This triggered the development of new national food safety standards. A 2005 report found that approximately 120 people die from foodborne illnesses in Australia every year.

What causes food-poisoning?

Microorganisms are the most common cause of food-poisoning. Disease-causing microorganisms occur commonly in the environment and are found in soil, water, air and on animals (including humans). For this reason it is likely that some microorganisms will be present on uncooked food. It is important to handle food in such a way that the microorganisms present do not have a chance to multiply and to prevent food from becoming contaminated with other microorganisms.

Many of these microorganisms are bacteria – single-celled organisms that can only be seen under a microscope. They include Salmonella, Listeria and Escherichia (Box 1: Common microbial agents of food-poisoning). Fungi, such as the moulds commonly seen on stale bread, can also cause illness, while viruses such as hepatitis A may also be food-borne.

Food-poisoning symptoms range from stomach cramps to diarrhoea, vomiting, headaches and fever. They vary depending on the pathogen causing them. Often the most dangerous effect of food-poisoning is dehydration, which can lead to death if untreated.

Who gets food-poisoning?

Healthy adults are usually less seriously affected by most forms of food-poisoning: the elderly, the young, and people already suffering from other diseases are most likely to succumb. In Australia, the annual death rate from intestinal infectious diseases – many caused by food-borne pathogens – is 1.4 per 100,000 amongst children less than a year old, 4.5 per 100,000 amongst people older than 75, and almost zero for people in the age range of 5-54 years.

The safe food tightrope

Every time we eat we have to rely on the many links in the food supply chain. The chain starts in the paddock and goes through various stages: washing, preparation, packaging, distribution and presentation. Everyone involved must take responsibility for their part. Most food-poisonings are the result of 'food abuse' during transport and final preparation, particularly in the home. When food-poisoning does result from food prepared in the home, it is rarely recognised or reported – we usually try to find a restaurant or a manufacturer to blame. We also blame the last meal we ate before the onset of symptoms, even though the incubation period can be as long as 10 days.

Food abuse can take many forms. The three most common occurrences that can disrupt the safe-food chain are temperature abuse, cross-contamination and inappropriate processing methods.

Temperature abuse

Food poisoning bacteria grow best at 35-37°C (our body temperature). For example, E. coli bacteria take 7-10 hours to double in number at a temperature of 10°C, but only 1½ hours at 20°C and just 15 minutes at 37°C. When the temperature is increased, such as during cooking, most pathogenic bacteria are killed. When the temperature is decreased, such as in refrigeration, the growth of the bacteria slows. Refrigeration is used extensively to extend the shelf life of food.

Temperature abuse usually takes one of two forms:

• A failure in the 'cold chain' can lead to conditions in which bacteria thrive, such as when a refrigerator in a shop breaks down or when a shopping bag full of fresh food is left in the car for an hour or two.

• The second form of temperature abuse can occur during food cooking and storage. Microorganisms can grow when food is cooked at temperatures too low to kill the bacteria and then stored above 4°C.

A food-poisoning outbreak at a youth camp near Dubbo in 1994 was caused by the bacterium Clostridium perfringens. Cold chicken served at lunch had been cooked the day before and allowed to cool at room temperature for more than 2 hours before it was refrigerated. Result: 230 camp participants sick with gastroenteritis, of whom 118 were treated in emergency departments and 13 admitted to hospital.

Temperature abuse was also the likely cause of an outbreak of E. coli infection among Japanese school children in 1996. As a result, 6000 children fell ill and two died.

Cross-contamination

Another form of food abuse, cross-contamination, can occur when bacteria from contaminated food or utensils are transferred onto other food. This is particularly perilous if the newly contaminated food is not cooked before eating. For example, a knife used to cut contaminated meat might then be used to chop lettuce. The meat is then cooked and the microorganisms killed, but because the lettuce is not cooked it can cause food-poisoning.

A spectacular example of the dangers of cross-contamination - and, coincidentally, of mass-distribution – occurred in the United States in 1994. Health investigators there found that cross-contamination of icecream pre-mix occurred because it was transported in tanker trailers that had previously been used to haul liquid eggs contaminated with Salmonella enteritidis. The contamination was not detected until the icecream had been distributed across the nation. Researchers estimated that 224,000 people in many different states contracted gastroenteritis as a result of eating the contaminated icecream.

Inappropriate processing methods and processing failure

Food-poisoning incidents may also occur when inappropriate processing methods are used or when food-processing systems fail. For example, mettwurst is fermented uncooked meat: this can hang for weeks in a butcher's window before being purchased and consumed. What stops it from going off?

The safe manufacture of sausages such as mettwurst relies on four factors:

• the addition of a 'starter culture' containing good bacteria that can out-compete spoilage organisms;
• fermentation, which lowers the pH to a level of acidity at which pathogens die or stop growing;
• the addition of salt, which effectively removes moisture, an essential element for bacterial growth;
• drying.

The contamination of mettwurst in Adelaide in 1995, which led to the death of a child, was caused by a breakdown in the processing system, although the precise nature of the breakdown was not identified.

Other reasons

Other reasons for the increase in food-poisoning have been proposed by health experts. The globalisation of the food supply means that harmful bugs common in one region may spread to other regions. Australia has so far remained free of some new and nasty bugs such as the strain of E. coli responsible for the school lunch contamination in Japan and Salmonella enteritidis, the bug responsible for the icecream fiasco in the United States. Nevertheless, experts consider it just a matter of time before such pathogens slip into the food supply chain here. The organism could be introduced into Australia either by infected travellers or by imported food.

Many consumers are expressing a preference for foods that are unprocessed or minimally processed and have fewer additives (eg, unpasteurised apple juice and preservative-free sausages). A reduction in the processing of some foods could be a factor in the increase in food poisoning.

Manufacturing processes such as canning, salting and acidification are designed to prolong shelf-life (Box 2: Food preservation). Refrigeration is increasingly used to prolong shelf-life in the face of increasing consumer preference for minimally processed foods. Maintenance of the cold chain has become even more critical.

Chemicals in food

Food poisoning from microorganisms is not the only food safety issue that concerns health professionals and thoughtful consumers. Some chemicals (eg, pesticide residues, food additives and antibiotics) may be added to food – either intentionally or unintentionally (Box 3: Chemicals in food).

Limiting food-poisoning

Industries in Australia and elsewhere are improving food safety through the introduction of what are known as hazard-analysis-critical-control-point (HACCP) systems. In these systems, the traditional reliance on testing the end-product for contamination is replaced by monitoring at key points along the chain. Scientists are also working to develop packaging and processing techniques that help keep food fresh and free from contamination.

Despite the advances of science, the basic rules for avoiding food contamination have been known for thousands of years, as people learned by trial and error to distinguish between safe and unsafe practice (Box 4: Safe cooking). If these rules are applied today at all points in the food supply chain, including how we use food in the home, our food should remain good enough to eat.

Box 1. Common microbial agents of food-poisoning

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.

Related sites

• Education pages (Food Safety Campaign Group, Australia)
  'The bug bible' gives information about bacterial pathogens.

• The 'Bad bug book' (Center for Food Safety and Applied Nutrition, 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)

Box 2. Food preservation

The techniques of food preservation can be separated into two groups: physical and chemical.

Physical methods of preservation such as canning and freezing rely on killing the microorganisms present, or at least stopping their growth for long enough to allow the food to be safely consumed. Other physical methods include drying, gamma irradiation, ultraviolet or high intensity white light, ultra high pressure and filtration.

Chemical preservatives work either as direct microbial poisons or by reducing the pH to a level of acidity that prevents the growth of microorganisms.

Acetic acid, better known as vinegar, has also been used as a food preservative since ancient times. Salted, pickled or dried foods were about the only nourishment sailors were offered on long sea voyages before the invention of modern refrigeration and preservation techniques.

Chemicals used today

Two commonly used preservative chemicals are:

• nitrates and nitrites that are used to preserve meats such as ham and bacon;
• sulphites that are commonly used to prevent the browning of fruits and vegetables after they've been peeled, and to prevent fungal spoilage.

As important and useful as they are, preservatives have developed a bad name in Western societies such as Australia. Salt is now widely shunned because of its effects on blood pressure. Nitrites and sulphites can both cause asthma, nausea, vomiting and headaches in some people.

For these reasons, consumers have started to demand foods containing lower levels of chemical preservatives. The potential drawback of this is the reduced length of time before conditions favour the rapid multiplication of food-poisoning agents like Salmonella and Listeria. A number of food-processing techniques have been developed to prolong the shelf-life of foods and permit a reduction in preservative levels.

Natural preservatives

Scientists are putting increased efforts into the discovery and purification of natural compounds for use as safe alternatives to chemical preservatives.

The new breed of protective compounds are small proteins. They are called bacteriocins and are starting to be used in a wide variety of foods. Anyone who has eaten yoghurt has been protected by bacteriocins without knowing it. Bacteriocins are produced by some good bacteria to kill competing organisms such as Listeria monocytogenes. The whole bacteria that produces the bacteriocin, or the purified bacteriocin itself, can be added to foods such as soft cheeses to reduce the risk of pathogen growth. An example of protection using bacteriocins is the use of nisin in crumpets to restrict the growth of Bacillus cereus.

Long-life treatments

Modern technology has produced several new processing techniques for prolonging the shelf-life of perishable foods.

UHT

Ultra high temperature treatment (UHT), for example, involves the rapid heating of food to about 140ºC. This temperature is maintained for a few seconds to kill bacteria. The product is then cooled rapidly and placed in sterile, airtight containers to prevent recontamination. This treatment is used commonly to produce 'long-life' milk and fruit juices. A disadvantage of the high temperature treatment is that heat-sensitive vitamins such as vitamin C are destroyed. In fruit juices the vitamins are added back after treatment.

Food irradiation

Food irradiation is another sterilising technique in which foods are bombarded by high-energy rays called gamma rays or by fast-moving electrons to kill bacteria, fungi and insects and, in some cases, to delay fruit ripening. A major benefit of irradiation is that it can occur after food is packaged and sealed.

Although irradiation is effective in killing contaminating microorganisms, it may mask the fact that the food had high levels of spoilage or insect infestation prior to treatment: the microorganisms or insects are killed but their carcasses, faeces or toxins remain.

As with all preservation techniques, irradiation should be used to prolong the life of appropriate food which is of high quality prior to treatment.

Modified atmosphere packaging

As the name implies, modified atmosphere packaging (MAP) alters the mix of gases – notably oxygen, nitrogen and carbon dioxide - in the atmosphere in which foods are stored. The altered storage atmosphere can limit the growth of spoilage bacteria and fungi and extend the shelf life of food. The technique is used for bread, cakes, fruit, vegetables, meat and seafood, which are stored in special plastic bags that help maintain the atmosphere for a certain length of time.

The technique is also applied to bulk foods in especially equipped storage containers. This is how fruits such as apples can be supplied to consumers throughout the year, long after the apple harvest takes place. Although modified atmosphere packaging (MAP) limits the growth of spoilage microorganisms, it does not slow the growth of some harmful bacteria. For this reason, MAP is usually used in conjunction with other preservation techniques such as refrigeration.

Related sites

• How food preservation works (How Stuff Works, USA)

• Food Science Australia
  • Preservation of vegetables in oil and vinegar
  • Small scale commercial preservation of acid foods
  • Active packaging

• What's the beef? Food irradiation (Scientific American, Explorations)

• New coatings improve food safety and quality (CSIRO media release)

Box 3. Chemicals in food

Pesticides

Since the 1960s, people have worried that pesticides – insecticides, weedicides and fungicides – used on farms and in food-processing premises may find their way into the food chain and adversely affect human health. Scientists remain uncertain about the human health effects of many of the pesticides used widely by Australian farmers. Some that are known to be harmful, such as DDT, are banned in Australia but continue to be used in some other countries.

Antibiotics

Concern has also been expressed that the excessive use of antibiotics in food-producing animals may encourage the spread of drug-resistant microorganisms. According to recent reports, about three-quarters of all antibiotics consumed in Australia are administered to animals. They are added to stock feed because they promote the growth of livestock. Good farming practice demands that antibiotics are not fed to livestock for some weeks prior to slaughter. This allows for sufficient time to minimize the amount of antibiotics in the meat we eat.

Additives

Many foods contain chemicals that have been added during processing. Australian food standards specifically state which additives may be used and the foods which may contain them. A few of the many different types of food additives are listed here:

• anti-oxidants help to prevent fats and oils and the foods containing them from going rancid;
• bleaching agents such as flour whitener;
• colours make foods appear more palatable;
• emulsifiers ensure that mixtures containing oil and water (eg, margarine) don't separate;
• enzymes prevent lollies from crystalising;
• flavours and flavour enhancers like monosodium glutamate (MSG);
• food acids balance sweetness in sweet foods and enhance the fruit flavour;
• free-flowing agents ensure that products such as salt flow freely;
• artificial sweeteners such as aspartame, otherwise known as Nutrasweet, and saccharine; and
• thickeners ensure an even consistency.

Some people may have an adverse reaction to additives. Some additives have been linked by scientists to hyperactivity in children, and some may provoke allergic reactions. The long-term effects of food additives on the body are often unknown and difficult to research. Increasingly, natural alternatives are being discovered that replace the artificial compounds.

Related site

• Antibiotic-resistant bacteria (Scientific American, Ask the experts)

• Chemicals in food (SaferHouses, UK)

Box 4. Safe cooking

Here are a few basic food-handling rules that have stood the test of time:

Clean it

• Always wash hands before starting food preparation, before handling new food, after finishing food preparation and before serving.
• Wash fruit and vegetables in cold running water.

• Prevent cross-contamination by never letting raw meat or poulty, or their juices, come into contact with other food. This can be done by using separate chopping boards, knives, other implements and benchtop wipers for different food types. A plastic or glass chopping board is best for meat.
• Immediately wash all implements used in the preparation of raw meat and poultry with detergent and hot water.
• Keep pets away from food and food preparation areas and equipment.

Cook it

• Most pathogenic bacteria don't survive cooking at high temperatures. Cook food to an internal temperature of 75°C.
• Cook rolled roasts, minced meats and poultry until there is no trace of pink in the juices.
• If reheating leftovers, cover and reheat thoroughly to 75°C to kill any bacteria that have survived in the food during cooling and refrigeration.
• Keeping dinners 'warm' is not a good idea. Keep the temperature above 60°C, or cool and refrigerate.

Cool it

• Refrigerate raw meat and poultry as soon as possible after bringing it from the shop.
• Refrigerate perishable food as soon as possible after cooking.
• Refrigerate or freeze cooked food in shallow pans rather than deep pots to allow quick cooling.
• Never thaw frozen meat and poultry at room temperature. Thaw it in the fridge, quickly under cold running water or in the microwave.
• Remember that refrigeration does not kill bacteria, it only slows down their growth.

Related site

• Food Safety Net (Food Safety Campaign Group, Australia)

Activities

• Behind the News (Australian Broadcasting Corporation)
  • Food safety – students identify safe handling practices, evaluate safe food handling in homes schools and the community. Activity sheets are available.

• Hand-washing laboratory activities (Access Excellence, US)
  These two activities determine whether hand washing removes microorganisms from the skin. Background information on hand washing is available.

• Antibiotics and antibiotic resistance (Newton's Apple, US)
  Activities and background information.

• National Center for Case Study Teaching in Science (Univeristy of Buffalo, USA)
  • The benign hamburger – introduces students to the concept of irradiation as a food preservation technique. Case teaching notes are available.
  • Irradiation: Is it consumer-friendly? – explores the response of students to food irradiation. Case teaching notes are available.

Further reading

Useful sites

Safe Food (Food Safety Campaign, Australia)

'Factsheets' explains how to practice food safety in your home. 'Food safety tips' provides information about preventing food borne illnesses, 'The bug bible' covers bacterial pathogens and 'Industry information' covers regulatory frameworks and quality assurance.
http://www.safefood.net.au/Safe+Food.htm

IFST: Institute of Food Science and Technology

This UK-based site covers many aspects of food science and technology. The following sections are especially relevant to food safety:

• Frequently asked questions: food safety
  http://www.ifst.org/site/cms/contentviewarticle.asp?article=225

• Frequently asked questions about: food additives
  http://www.ifst.org/site/cms/contentviewarticle.asp?article=226

• Frequently asked questions about: packaging
  http://www.ifst.org/site/cms/contentviewarticle.asp?article=227

Food Safety (NSW Health, Australia)

Provides a number of relevant articles such as 'Eating away from home. How to make sure food is safe' and 'How safe is your kitchen?'
http://www.mhcs.health.nsw.gov.au/mhcs/topics/Food_Safety.html

Glossary

bacteriocin. A small, naturally occurring protein produced by one species of bacterium that effectively protects it from competing organisms. Different bacteria produce different bacteriocins.

fermentation. The breakdown of organic molecules into simpler substances in the absence of oxygen. The process of fermentation can give rise to acids producing a lower pH.

gamma rays. The shortest wavelength of electromagnetic radiation. For more information see Gamma waves (NASA,USA).

pathogen. An organism capable of causing a disease.

pH. The pH scale is used to measure the strength of acids and bases (or alkalis). The acid strength in the human stomach is about pH 2. Alkalis such as caustic soda and basic household cleaners have a pH of about 12 to 14. Neutral is pH 7, (ie, neither acidic or alkaline). The scale is logarithmic, so pH 4 is ten times as acidic as pH 5 and pH 2 is ten times as acidic as pH 3, and so on. For more information see About soil pH (National Aeronautics and Space Administration, USA).