| The bug bible
The bug bible
Many types of Bacteria are naturally present in food while others find their way into food from contaminated water, soil, air, animals, equipment and people. While some may spoil food, the majority of bacteria usually do not cause harm to humans.
Understanding bacterial ecology in combination with recognizing many of our bad hygiene practices can give us the tools for the prevention of foodborne disease.
Only a few micro-organisms cause foodborne illness.
To ensure that you are practising food safety, take a look at the following information on the micro-organisms most commonly implicated in foodborne illness. And remember that food safety applies to ALL foods - the important point is to treat all foods with care.
Predominant Micro-organisms implicated in Foodborne Disease
Campylobacter jejuni
Campylobacter was not recognised as a cause of foodborne disease until the 1970s, and is now thought to be the main cause of foodborne disease in Australia. It is naturally present in the intestines of a number of different domestic animals and birds. It can be frequently isolated from the surface of poultry and also on the surface of cuts of raw meats. It has also been isolated from raw milk
The organism is sensitive to heat. Adequate cooking, reheating and control of cross contamination from raw foods to cooked foods are the primary control mechanisms in the kitchen. There have been a number of case investigations where salads have been implicated as the source of the organism. Transfer of the organisms to the salad vegetables from surfaces or implements used for raw meats being the most likely cause.
Staphylococcus aureus
Staphylococcus aureus is important from both a medical and food perspective. S. aureus, also known as Golden staph, is resistant to some antibiotics and is responsible for many diseases in warm blooded animals. Illness is caused by the production of a toxin in the food as a result of bacterial growth. S aureus remains one of the most frequently reported causative agents of foodborne illness.
The most common source of the organism is the human body however, it can also be isolated from animals and poultry and hence can be present on raw meat . The human strains are more likely to be toxin producers. S. aureus can be part of the normal flora in the nose throat and on the skin of humans. They can also be present in skin eruptions such as boils acne and styes. Skin wounds, even minor ones, can harbour large numbers of S. aureus. Therefore the primary transfer of organisms is by the food handler.
Growth and toxin production can occur over a wide range of temperatures however, most cases occur from storage of susceptible foods at ambient temperatures for some period prior to consumption.
The organism is susceptible to heat but the toxin can show significant resistance so reheating foods in which the toxin has been allowed to develop will not necessarily inactivate the toxin.
This organism will be present in foods. Personal hygiene of the food handler is very important in preventing or minimizing transfer of the organism to foods. Handling of food should be kept to a minimum and tongs and implements used instead of hands. Infected wounds and skin eruptions should be covered.
Susceptible foods must not be stored in the temperature danger zone.
Salmonella spp.
Salmonella was first isolated by Salmon and Smith in 1885 from swine affected with hog cholera. Since then many different serotypes of Salmonella have been isolated from a variety of sources. All Salmonella serotypes are considered potential pathogens in animal species and man.
The primary source of salmonellae is the intestinal tract of a wide variety of animals including man. These include pets, birds and wild animals, including both warm and cold blooded animals.
Salmonellosis is a communicable disease readily transmitted from animals to man either directly or through contaminated products of animal or plant origin, however, transmission from animal sources is more likely.
The number of cells required to produce illness varies with the serotype. Generally relatively few Salmonella cells are needed therefore, growth in the food is not a prerequisite for a food to cause illness.
Salmonella can survive well in the environment and can be widely disseminated. Cross-contamination from raw to cooked foods or contaminated raw foods are the primary causes identified in the investigation of outbreaks.
In the home the following practices are critical control steps in preventing or minimising the risk of Salmonellosis.
· Obtain food from safe sources
- Store perishable foods either chilled or frozen.
· Protect shelf stable food from moisture.
· Wash vegetables and fruit well.
· Cook foods to at least an internal temperature of 66ºC.
· If cooked food is not eaten immediately cool rapidly to 5ºC.
· Prevent cross-contamination from raw to cooked foods.
· Do not store reconstituted foods like re-hydrated dried milk in the temperature danger zone.
· Ensure that cleaning equipment like sponges and dishcloths are disinfected.
Wipe up meat juices with disposable paper towel.
Listeria monocytogenes
Listeria monocytogenes has been known for at least 60 years, but it has only been linked with foodborne disease since the early 1980s. Since this time it has become recognised as an important food poisoning bacteria.
The organism is widespread in the environment and is carried by a number of domestic and wild animals. There are several species however, L. monocytogenes is the most common and is pathogenic for both animals and humans. Transmission can be by person to person by the faecal-oral route as well as from the environment and from aerosols. However, foodborne transfer is considered the most significant. The organism is very good at establishing itself on food contact surfaces and survives well in the refrigerator.
While normal healthy individuals can become infected by L. monocytogenes those most at risk are the elderly, the young and those who are immune-compromised or pregnant. The organism can cause serious illness in these groups of people.
The foods commonly implicated in foodborne outbreaks include cold deli meats, cold cooked chicken, soft cheeses, salads, smoked salmon and trout
The organism is not heat resistant however, it is very tolerant of cold and can grow at refrigeration temperatures lower than 1ºC although growth at these temperatures is slow.
Consumer education has concentrated primarily on the “at risk” groups and a number of health authorities have published guidelines for foods served in hospitals, aged care facilities and food consumed during pregnancy. These guidelines should also be adopted in the home for food prepared for at risk individuals
In the home good hygienic practices should be adopted and foods most at risk of containing Listeria should be stored for no more than the recommended shelf life.
Refrigerators should be kept in a clean and hygienic condition and the temperature kept as cold as possible. Foods should be placed in containers and covered. Cooked foods should always be stored above raw foods.
Cross-contamination from raw to cooked foods must be avoided.
Cleaning clothes must be disinfected and paper towel used where possible to wipe up residues from raw meats, fish and vegetables.
Escherichia coli
Many strains of E. coli are harmless and are found naturally in the gut of humans and animals. Traditionally its presence in foods has been an indication of faecal contamination however, a number of serotypes are now known to be pathogens. Humans are known to be the major if not the only source of some strains. Other strains can be present in the intestinal tract of cattle and sheep. They have also been detected in pigs, buffalo, goat, deer and various birds and dogs and cats.
These strains are described by their pathogenicity action:
EIEC Enteroinvasive E coli
ETEC Enterotoxigenic E coli
EPEC Enteropathogenic E coli
EHEC Enterohaemorrhagic E coli
The infective dose varies however, for some serotypes this appears to be very low.
Dairy products, vegetables, salads and contaminated water have all been implicated in human illness.
These organisms may enter the kitchen on raw meats and also from domestic pets. If animal manures are used for fertilization this can also result in the transfer of these organisms to vegetable and fruit tissues. Similarly contaminated water can become a vehicle for transfer of the organism from a faecal source to other foods.
Human and animal faecal material is the primary source and a point of entry of E. coli into the food chain. It is important to note that vegetables and fruits may be contaminated internally and externally. If the organisms attach to the internal tissues during development or through damaged tissue, it is difficult to remove these by washing.
E. coli are not heat resistant and will be destroyed by normal cooking temperatures. Mincing meat increases the surface area and also increases the bacterial load. Therefore, it is very important that mince meat is thoroughly cooked prior to consumption. Improperly cooked hamburgers have been implicated in a number of food poisoning outbreaks.
It is also important to keep foods properly chilled below 5ºC.
Many foods will support the growth of these strains of E. coli. So good hygienic practices should be adopted to minimize the possibility of cross-contamination.
· Obtain food from safe sources and ensure as far as possible that unprocessed animal manures are not used on fruits and vegetables.
· Store perishable foods either chilled or frozen.
· In the refrigerator store cooked foods above raw foods.
· Wash vegetables and fruit well and do not use vegetables and fruits showing obvious tissue damage.
· Cook mince meat to an internal temperature of 75ºC for 2 minutes.
· If cooked food is not eaten immediately cool rapidly to 5ºC.
· Prevent cross-contamination from raw to cooked foods.
· Ensure that cleaning equipment such as sponges and dishcloths are disinfected.
· Wipe up meat juices with disposable paper towel.
· Wash your hands with soap after visiting the toilet, handling raw food and storing raw foods.
Clostridium perfringens
As early as 1895, Clostridium perfringens was associated with human diarrhoea, although it wasn't until the 1940s that it was confirmed to be a cause of food poisoning. C. perfringens is widely found in soil and is a minor component of the flora of the intestinal tracts of humans and animals.
The more common form of illness caused by this organism is of short duration and is characterised by diarrhoea and severe stomach cramps caused by an enterotoxin released in the gut.
The infective dose is large and therefore growth must occur in the food prior to ingestion. In other words, low numbers of the organism are not likely to bring on any illness.
This organism is an anaerobe. It requires a reduced level of oxygen for growth and it produces a spore, an entity which allows the organism to survive most normal cooking temperatures. The organism grows well at warm temperatures with an optimum temperature of 430-45ºC.
At cold temperatures <10ºC, the organism does not grow and while the cell may die off, the spore will survive.
The cells and spores of this organism are likely to be present in many foods of animal origin while the spores can be present in spices, seeds and vegetables. Many cooking techniques such as braising will not destroy the spores. It is important that soups casseroles and large joints of meat are consumed immediately after cooking or rapidly cooled and stored below 10ºC.
These foods must not be allowed to cool at ambient temperatures because rapid growth of clostridia will occur in the 40-50ºC range.
C. perfringens is ubiquitous in the environment and will be present in the kitchen. Control is achieved by:
· personal hygiene - the organism is part of the normal human gut flora
· correct cooking temperatures
· rapid cooling and storage below 10ºC for bulk foods eg soups, casseroles etc
Clostridium botulinum
From a food safety view Clostridium botulinum is one of the most important of the pathogenic foodborne bacteria because of the severity of the symptoms caused by powerful neurotoxins. In the past it has been associated with inadequately canned protein foods. Today it is relatively rare and most likely associated with home preservation of foods. Although, there is increasing concern over convenience foods and foods containing fewer preservatives, minimal processing and a greater reliance on chilling for preservation. Some of these newer foods may fall within the growth range of C. botulinum.
The organism is globally distributed and is primarily a soil organism however, certain strains can be found in sediments associated with aquatic environments.
It is an anaerobe which means it requires reduced levels of oxygen for growth and it is a spore former.
Foodborne botulism is an intoxication almost always associated with ingestion of preformed botulinum neurotoxin.
Infant botulism results after ingestion of spores of C. botulinum which subsequently germinate, multiply and produce neurotoxin in the infant’s intestinal tract. Honey has been linked to infant botulism but surveys of commercial foods including honey have not identified any with a high incidence of spores.
Storage of low acid cooked foods at ambient temperatures for extended periods can create conditions suitable for the growth of this organism. Outbreaks have occurred from baked potato stored at ambient temperature for several days and spiced onions sautéed in margarine that were kept warm and used throughout the day. Garlic butter was also implicated in an outbreak in North America in 1985. The butter was made with an aqueous mixture of garlic in soybean oil. The pH of the garlic in oil was above 4.6 and the product had been stored at ambient temperature.
The growth of other organisms like moulds has been shown to reduce the acid level in some tomato products to a level where outgrowth of surviving spores of C. botulinum occurred. Home preservation of tomatoes and tomato products is common. The product should not be consumed if mould growth is evident.
Foods particularly non acidified foods should not be stored for extended periods at ambient or warm temperatures.
It is important to follow the recommendations of the food manufacturer with respect to storage temperature and shelf life.
Guidelines-Home Food preservation:
National Center for Food Preservation (USA)
Preservation of Vegetables - Food Science Australia
Yersinia enterocolitica
Yersinia enterocolitica has been recognized as an important cause of foodborne disease since the late 1980s. It is found in the general environment as well as the gut of various animals, and is associated more with the temperate regions of the world.
Many foods have been implicated as a vehicle or source of infection. While Y. enterocolitica has been isolated from a wide range of domestic and wild animals and birds as well as from environmental sources, pigs are directly or indirectly the main primary source of the pathogenic serotypes that cause infections in man.
Contaminated food or water is regarded as the main source of human infections and there is a close association with consumption of raw or undercooked pork.
Water vegetables and fresh herbs have been linked to sporadic cases.
Yersinia enterocolitica is a cold tolerant organism and will survive frozen for extended periods and grow at refrigeration temperatures. Growth is slowed below 5ºC. It is not heat resistant and is destroyed by pasteurization 72ºC for 18 seconds.
The organism will most likely come into the kitchen environment on pork and pork products. The following measures are important in controlling this organism in the kitchen environment:
· All pork should be adequately cooked prior to consumption.
· Raw pork should be covered and refrigerated below cooked foods. Any drip should be promptly cleaned up.
· Prevent cross-contamination from raw to cooked foods.
· Ensure that cleaning equipment like sponges and dishcloths are disinfected.
· Wipe up meat juices with disposable paper towels.
· Wash hands thoroughly frequently and particularly after handling raw meats.
Bacillus cereus and other Bacillus species
Bacillus cereus has been suspected as a cause of foodborne disease since early this century, although the role in foodborne disease was not established until the 1950s.
Bacillus cereus causes two types of illness – one characterized by diarrhoea and the other by vomiting. Illness is caused by enterotoxins. The diarrhoeal enterotoxin is heat sensitive (inactivated by 56ºC for 5 minutes). However the emetic toxin (induces vomiting) is very heat resistant and would not be inactivated by normal cooking or reheating temperatures. Not all strains of B. cereus found in foods are associated with illness
Bacillus species are commonly found in nature particularly in soils and in water. They find their way into plant foods from these sources. Raw materials of plant origin are a major source of Bacillus cereus in foods. B. cereus has been isolated from flour, rice, spices, lentils dried potatoes beans meat and chicken. Rice is a well recognized source of B. cereus with most samples containing low to moderate levels of the organism.
The presence of B. cereus in processed foods is often associated with the ability of the spores to survive heat processes. Many outbreaks of illness due to B. cereus have been associated with boiled and fried rice that has been cooked and stored for extended periods at warm temperatures before consumption. Custards puddings and mousses have also been implicated and powdered milk products reconstituted with warm water and stored at ambient temperatures have been shown to support the growth of B. cereus. Storage of foods below 5ºC have been shown to restrict growth of this organism.
The emetic form of the illness appears to be primarily associated with rice dishes.
Storage of foods for too long at ambient temperatures is almost always the cause of illness due to the growth of B. cereus and toxin production.
Prevention of illness in the home depends on correct storage of cooked foods. Growth of B. cereus must be prevented by storing food below 50C or above 60ºC. Bulk cooked foods should be divided into smaller portions to facilitate rapid cooling and minimize time in the danger zone.
More information: info@safefood.net.au |