859
Sources of infection: food DIANE ROBERTS
In the more developed countries there are some foods which can be regarded as "safe", having been made safe by the application of well controlled decontamination processes,
such as pasteurisation and sterilisation. Such foods include
milk, ice cream, whole egg mix in bulk, and canned foods.
Drinking and food-manufacturing water supply is controlled by filtration and chlorination. Other foods, such as bread, flour, jams, honey, pickles, fruits, and fats are regarded as safe because their composition, processing, or both provide conditions under which bacteria cannot multiply. The various properties of foods, such as pH, water activity (aw), and salt or sugar content, can give an indication of whether microorganisms will grow. For example, most pathogenic bacteria will not grow in foods with a pH of less than 4-5, a low moisture content (aw < 0-86), or a high salt or sugar concentration. Many of these indices are inter-reactive and can be exploited in the preservation of foods. However, we are constantly being encouraged to eat fresh rather than preserved foods, and it is this wide range of foods that may introduce into our kitchens many of the organisms which can lead to gastrointestinal illness if these items are not handled and stored correctly. There are three main routes by which microorganisms reach our food-namely, raw foodstuffs and ingredients, the foodhandler, and the
eliminate these organisms but sporing organisms such as Cl perfringens will still survive. Red meats may also be contaminated but the prevalence of Salmonella and Campylobacter is probably lower than in poultry.6 Comminution of meat will spread organisms throughout the product, so minced-meat items are a greater hazard. TABLE I-PRINCIPAL FOOD SOURCES OF THE COMMON FOOD-POISONING ORGANISMS
environment.
Raw food and
ingredients
Meats *For
Foods of animal origin are the primary source of many of the bacteria responsible for foodbome infections and intoxications (table I). Organisms found in the live animal can be carried through to raw meats after slaughter, may persist through further processing, and ultimately may appear in the final retail product if insufficient attention is paid to hygiene and temperature control. Because there is mass rearing for food, a large proportion of animals reach the slaughterhouse excreting organisms, such as Salmonella and Campylobacter, in addition to the pathogens that form part of their normal faecal flora-namely Clostridium perfringens, Escherichia coli, Yersinia enterocolitica, and Listeria monocytogenes. Continuous line processing, as for example in poultry processing establishments, increases spread from carcass to carcass so that many contaminated carcasses are distributed for retail or manufacturing purposes. The rapid growth of the broiler industry has led to a more readily available and cheaper source of meat but also has increased infection in the birds and thus contamination of carcasses. 60-80% of retail chickens in the UK are contaminated with Salmonella’ and reports from other countries indicate levels which range from 5 to 73%.2 Up to 100% of birds may contain Campylobacter3.4 and 60% may also harbour L monocytogenes.5 Adequate heat treatment, which ensures centre temperatures of at least 70°C, will
example, small round structured viruses, parvovirus, hepatitis virus.
Eggs It has long been recognised that the shell of eggs may become contaminated with Salmonella from chicken faeces during laying and may be transferred to the bacteria-free contents during unhygienic breaking out. With the recent findings in the USA and the UK that S enteritidis infected flocks lay a small proportion of eggs which have already become infected while in the hen’s ovary,l’ there is a more serious problem with this food. Until the infection has been eradicated from laying flocks, raw egg must be regarded as a potentially hazardous food. Other European countries have also had increasing reports of egg-associated S enteritidis infection-in particular, Spain.’ The UK Government’s health warning against the use of raw eggs in foods which will not receive further heat treatment should be heeded.
Dairy products or
Milk that has been treated (by pasteurisation, sterilisation, ultra heat treatment) is a safe product, but raw milk from a
ADDRESS: PHLS Food Hygiene Laboratory, Central Public Health Laboratory, 61 Colindale Avenue, London NW95HT, UK (Dr D. Roberts, PhD)
860
TABLE II-FACTORS THAT CONTRIBUTED TO OUTBREAKS OF FOOD-POISONING IN ENGLAND AND WALES, 1970-82
*1479 outbreaks studied
healthy animal, which when freshly drawn contains very few bacteria, may acquire various pathogens from the milking animal or from the dairy environment and equipment. Outbreaks of milkbome infection, mainly of salmonellosis and campylobacteriosis, associated with the consumption of raw cows milk are still reported in England and Wales8 and North America.9 Raw milk is only rarely implicated in other European countries.9 The introduction of legislation in Scotland in 1983 which prohibits the sale of raw milk led to a large fall in the number of milkbome incidents. Occasional incidents of illness have been attributed to incorrectly pasteurised milk in some countries.9-12 Products prepared from untreated milk are more likely to be contaminated than are those prepared from milk that has been heat treated. Staphylococcus aureus, Bacillus cereus, Y enterocolitica, and L monocytogenes may also be found in raw milk. All but B cereus will be removed by heat treatment; this organism, which produces heat-resistant spores, is a common dairy spoilage organism. However, incidents of B cereus intoxication from milk are uncommon, possibly because, owing to spoilage by the organism or other spoilage bacteria, the product becomes organoleptically unacceptable before it reaches the toxic stage.
Many of the methods of further acidification and fermentation of milk into other products will remove or inhibit most enteric pathogens, but a few organisms may survive. There are also further opportunities for the re-introduction of organisms by the addition of other untreated ingredients or from the production environment. Hard cheeses, yoghurt, and butter can be regarded as safe because of low pH and/or lack of moisture, but care must be taken with mould-ripened soft cheeses; these cheeses have a higher pH and thus permit the growth of L monocytogenes. 5-15% of soft ripened cheeses on sale in many countries in Europe--eg, the UK, the Netherlands, Germany, and Switzerland-may contain this organism4,13,14 (usually in low numbers), irrespective of the heat treatment of the original milk. 13 The presence of the organism is most probably due to recontamination during the cheese making process or during handling at the distribution and retail stage. Fish and shellfish Fish and shellfish may become contaminated either from the environment from which they are harvested or from the
environment during further processing. If such creatures are taken from water that has been polluted with sewage, they may contain many faecal pathogens (table 1). Vibrio parahaemolyticus, a marine microorganism, is a common contaminant of raw fish and other seafoods, especially those from the Far East. It can be eliminated by heat treatment but poor production hygiene may lead to recontamination.
Shellfish are filter feeders and concentrate, in their bodies, organisms from the water in which they are breedingmicroorganisms such as Salmonella, E coli, V parahaemolyticus, clostridia, and viruses have been detected. Thus, consumption of raw shellfish--eg, oysters--or shellfish that have been inadequately heat treated can lead to bacterial and viral infections.15 These shellfish usually go through a depuration process in clean water whereby many organisms are washed out of the gills and bodies. This process is effective for bacteria but less so for viruses.
Fruits, vegetables, and cereals In the raw state these foods may be contaminated with any of the organisms present in the soil in which they are grown. Fruits that grow high above the ground are less likely to be contaminated than those that are in direct contact with the soil. The cleanliness of irrigation water also plays an important part in the extent of contamination of these products and in the pathogens which may be found. In countries where polluted water is used for irrigation and animal and human excreta for fertilisation, there is a risk of contamination with enteric bacterial pathogens, such as Salmonella (including S typhl), Shigella, and V cholerae, as well as viruses and parasites.16 This is not a problem in the UK and developed countries where these practices are not common; in these countries most of the potentially harmful contaminating organisms are Clostridium spp, Bacillus spp, and L monocytogenes. For fruits and vegetables that are eaten without heat treatment, the extent of contamination can be reduced if they are washed and rinsed with a sanitising agent. Cooking will eliminate all but the sporing organisms.
Dried foods The predominant flora of most dried foods are organisms of the Clostridium and Bacillus groups, the spores of which survive the dehydration process. With some methods of drying, in which the temperature is not bactericidal or in which there are opportunities for recontamination, other organisms may persist-eg, Salmonella and E coli.11 These foods are safe when dehydrated but as soon as they are rehydrated they must be treated as fresh foods. Since herbs and spices frequently carry heavy loads of sporing organisms,18 it is important to add such ingredients at the beginning of the food processing stage (eg, before heat
treatment).
Ready-to-eat foods In addition to
our basic food items we are seeing a wide of range prepared, ready-to-eat foods retailed in either the frozen or chilled state. Many of these foods will have received some form of heat treatment but they are unlikely to be sterile. Frozen foods will remain safe while frozen but chilled foods require greater care with respect to shelf-life
861
and temperature of storage. The organisms found in these foods are those that have survived the heating process—eg, Clostridium spp and Bacillus spp-and those that have gained entry during the further manipulation of the product. The latter includes a wide range of organisms but those of concern are Y enterocolitica and L monocytogenes because they can grow at refrigeration temperatures. Numerous studies have found L monocytogenes in chilled foods,19 including cooked chicken, complete meals, cooked meats, pâté,20 and salads. Thus, greater care must be taken with this group of foods with respect to hygiene of production, temperature control during storage and display, and shelf life of the product.
The foodhandler and food environment
Organisms may also be transferred to food by the foodhandler either directly or by cross-contamination through the use of hands, surfaces, utensils, and equipment which have not been adequately cleaned and disinfected between the preparation of different foods. The factors which contributed to outbreaks of food-poisoning in England and Wales between 1970 and 1982 are shown in table II.21 The infected foodhandler appears low on the list. Only in relation to S aureus food poisoning does the foodhandler have an important role. S aureus is frequently found in the nose and on the skin and can be readily transferred to foods by handling; subsequent storage of food at unsuitable temperatures will allow the organisms to multiply and produce their toxins. Thus, foods that are much handled during preparation and not reheated before final consumption are at greatest risk. Foodhandlers who continue to work with active symptoms of gastroenteritis are a hazard in the food preparation area because there is an increased risk of faecal organisms reaching food. Many people who constantly handle raw foods, particularly those of animal origin, often become symptom-free excretors and may carry the organisms on their hands. Provided they have good hand hygiene and formed stools, such individuals are not a major risk in food preparation.22 To prevent spread of organisms from man and the food environment, separate surfaces, equipment, and personnel should be used to deal with raw and cooked foods; there should be regular hand washing and good cleaning schedules that are regularly enforced. In large-scale food production, separate well-defined areas should be allocated to the preparation of raw and processed foods. Removal of food debris, which can attract insects and vermin, and thorough cleaning of all food preparation equipment (including the dismantling of items such as mixers and slicers so that all surfaces and crevices can be reached) followed by a final disinfection procedure and drying are essential to keep recontamination to a minimum. It is virtually impossible to eradicate all organisms from the food preparation environment but efforts must be made to keep levels as low as possible and to prevent outgrowth. Every person involved in the preparation of food must be made aware that very few of the items that enter the kitchen are sterile. Most foods will be contaminated to a greater or lesser extent according to the amount of heat processing and other manipulation. If the food is consumed immediately, the extent of contamination is usually low-insufficient to cause illness. Most of the factors which contribute towards making a relatively harmless food harmful can be grouped as either attributable to poor temperature control or to cross-
(table II). Thus, although improvement in agricultural procedures can lead to the reduction in the presence of some organisms in our foods (eg, Salmonella and Campylobacter), good hygienic food preparation and education of those involved in the preparation, processing, contamination
and service of food are the final lines of defence in the prevention of most types of foodbome illness.
REFERENCES 1. Public Health
Laboratory Service. Memorandum of evidence to the Agriculture Committee Inquiry on Salmonella in eggs. PHLS Microbiol Dig 1989; 6: 1-9. 2. D’Aoust J-Y. Salmonella. In: Doyle MP, ed. Foodborne bacterial pathogens. New York: Marcel Dekker, 1989: 327-445. 3. Hood AM, Pearson AD, Shahamat M. The extent of surface contamination of retailed chickens with Campylobacter jejuni serogroups. Epidemiol Infect 1988; 100: 17-25. 4. Lammerding AM, Garcia MM, Mann ED, et al. Prevalence of Salmonella and thermophilic Campylobacter in fresh pork, beef, veal and poultry in Canada. J Food Protect 1988; 51: 47-52. 5. Pini PN, Gilbert RJ. The occurrence in the UK of Listeria species in raw chickens and soft cheeses. Int J Food Microbiol 1988; 6: 317-26. 6. Bolton FJ, Dawkins HC, Hutchinson DN. Biotypes and serogroups of thermophilic campylobacters isolated from ovine, bovine, and porcine offal and other red meats. In: Pearson AD, Skirrow MB, Lior H, Rowe B, eds. Campylobacter III. London: Public Health Laboratory Service, 1985: 106. 7. Humphrey TJ, Baskerville A, Mawer S, Rowe B, Hopper S. Salmonella enteritidis phage type 4 from the contents of intact eggs: a study involving naturally infected hens. Epidemiol Infect 1989; 103: 415-23. 8. Barratt NJ. Milkbome diseases in England and Wales in the 1980s. J Soc Dairy Technol 1989; 42: 4-6. 9. Sharp JCM. Infections associated with milk and dairy products in Europe and North America, 1980-85. Bull WHO 1987; 65: 397-406. 10. Rampling A, Taylor CED, Warren RE. Safety of pasteurised milk. Lancet 1987; ii: 1209. 11. Jones PH, Willis AT, Robinson DA, Skirrow MB, Josephs DS.
Campylobacter enteritis associated with the consumption of free school milk. J Hyg (Camb) 1981; 87: 155-62. 12. Rampling A. The microbiology of milk and milk products. In: Linton AH, Dick HM, eds. Topley and Wilson’s principles of bacteriology, virology and immunity. 8th ed. Vol 1. London: Edward Arnold, 1990: 265-89. 13. Greenwood M, Roberts D, Burden P. The occurrence of Listeria spp in milk and dairy products: a national survey in England and Wales. Int J Food Microbiol (in press).
EH, Ryser ET. Occurrence of Listeria in foods: milk and dairy foods. In: Miller AJ, Smith JL, Somkuti GA, eds. Foodborne listeriosis. Amsterdam: Elsevier, 1990: 151-64.
14. Marth
15. Public Health Laboratory Service Working Party on Viral Gastroenteritis. Foodborne viral gastroenteritis. PHLS Microbiol Dig
1988; 5: 69-75. 16. Geldreich EE, Bordner RH. Fecal contamination of fruits and vegetables during cultivation and processing for market. A review. J Milk Food Technol 1971; 34: 184-95. 17. Rowe B, Begg NT, Hutchinson DN, et al. Salmonella ealing infections associated with consumption of infant dried milk. Lancet 1987; ii: 900-03.
D, Watson GN, Gilbert RJ. Contamination of food plants and plant products with bacteria of public health significance. In: Rhodes-Roberts M, Skinner FA, Bacteria and Plants. Society for Applied Bacteriology symposium series No 10. 1982: 169-95. Gilbert RJ, Hall SM, Taylor AG. Listenosis update. PHLS Microbiol Dig 1989; 6: 33-37. Morris IJ, Ribeiro CD. Listeria monocytogenes and pâté. Lancet 1989; ii:
18. Roberts
19. 20.
1285-86. 21. Roberts D. Factors contributing to outbreaks of foodborne infection and intoxication in England and Wales 1970-1982. Proceedings of the 2nd World Congress Foodbome Infections and Intoxications. Berlin: Institute of Veterinary Medicine, 1986; 157-59. 22. Cruickshank JG, Humphrey TJ. The carrier food handler and typhoid salmonellosis. Epidemiol Infect 1987; 98: 223-30.
non-
Sources of infection: food DIANE ROBERTS
In the more developed countries there are some foods which can be regarded as "safe", having been made safe by the application of well controlled decontamination processes,
such as pasteurisation and sterilisation. Such foods include
milk, ice cream, whole egg mix in bulk, and canned foods.
Drinking and food-manufacturing water supply is controlled by filtration and chlorination. Other foods, such as bread, flour, jams, honey, pickles, fruits, and fats are regarded as safe because their composition, processing, or both provide conditions under which bacteria cannot multiply. The various properties of foods, such as pH, water activity (aw), and salt or sugar content, can give an indication of whether microorganisms will grow. For example, most pathogenic bacteria will not grow in foods with a pH of less than 4-5, a low moisture content (aw < 0-86), or a high salt or sugar concentration. Many of these indices are inter-reactive and can be exploited in the preservation of foods. However, we are constantly being encouraged to eat fresh rather than preserved foods, and it is this wide range of foods that may introduce into our kitchens many of the organisms which can lead to gastrointestinal illness if these items are not handled and stored correctly. There are three main routes by which microorganisms reach our food-namely, raw foodstuffs and ingredients, the foodhandler, and the
eliminate these organisms but sporing organisms such as Cl perfringens will still survive. Red meats may also be contaminated but the prevalence of Salmonella and Campylobacter is probably lower than in poultry.6 Comminution of meat will spread organisms throughout the product, so minced-meat items are a greater hazard. TABLE I-PRINCIPAL FOOD SOURCES OF THE COMMON FOOD-POISONING ORGANISMS
environment.
Raw food and
ingredients
Meats *For
Foods of animal origin are the primary source of many of the bacteria responsible for foodbome infections and intoxications (table I). Organisms found in the live animal can be carried through to raw meats after slaughter, may persist through further processing, and ultimately may appear in the final retail product if insufficient attention is paid to hygiene and temperature control. Because there is mass rearing for food, a large proportion of animals reach the slaughterhouse excreting organisms, such as Salmonella and Campylobacter, in addition to the pathogens that form part of their normal faecal flora-namely Clostridium perfringens, Escherichia coli, Yersinia enterocolitica, and Listeria monocytogenes. Continuous line processing, as for example in poultry processing establishments, increases spread from carcass to carcass so that many contaminated carcasses are distributed for retail or manufacturing purposes. The rapid growth of the broiler industry has led to a more readily available and cheaper source of meat but also has increased infection in the birds and thus contamination of carcasses. 60-80% of retail chickens in the UK are contaminated with Salmonella’ and reports from other countries indicate levels which range from 5 to 73%.2 Up to 100% of birds may contain Campylobacter3.4 and 60% may also harbour L monocytogenes.5 Adequate heat treatment, which ensures centre temperatures of at least 70°C, will
example, small round structured viruses, parvovirus, hepatitis virus.
Eggs It has long been recognised that the shell of eggs may become contaminated with Salmonella from chicken faeces during laying and may be transferred to the bacteria-free contents during unhygienic breaking out. With the recent findings in the USA and the UK that S enteritidis infected flocks lay a small proportion of eggs which have already become infected while in the hen’s ovary,l’ there is a more serious problem with this food. Until the infection has been eradicated from laying flocks, raw egg must be regarded as a potentially hazardous food. Other European countries have also had increasing reports of egg-associated S enteritidis infection-in particular, Spain.’ The UK Government’s health warning against the use of raw eggs in foods which will not receive further heat treatment should be heeded.
Dairy products or
Milk that has been treated (by pasteurisation, sterilisation, ultra heat treatment) is a safe product, but raw milk from a
ADDRESS: PHLS Food Hygiene Laboratory, Central Public Health Laboratory, 61 Colindale Avenue, London NW95HT, UK (Dr D. Roberts, PhD)
860
TABLE II-FACTORS THAT CONTRIBUTED TO OUTBREAKS OF FOOD-POISONING IN ENGLAND AND WALES, 1970-82
*1479 outbreaks studied
healthy animal, which when freshly drawn contains very few bacteria, may acquire various pathogens from the milking animal or from the dairy environment and equipment. Outbreaks of milkbome infection, mainly of salmonellosis and campylobacteriosis, associated with the consumption of raw cows milk are still reported in England and Wales8 and North America.9 Raw milk is only rarely implicated in other European countries.9 The introduction of legislation in Scotland in 1983 which prohibits the sale of raw milk led to a large fall in the number of milkbome incidents. Occasional incidents of illness have been attributed to incorrectly pasteurised milk in some countries.9-12 Products prepared from untreated milk are more likely to be contaminated than are those prepared from milk that has been heat treated. Staphylococcus aureus, Bacillus cereus, Y enterocolitica, and L monocytogenes may also be found in raw milk. All but B cereus will be removed by heat treatment; this organism, which produces heat-resistant spores, is a common dairy spoilage organism. However, incidents of B cereus intoxication from milk are uncommon, possibly because, owing to spoilage by the organism or other spoilage bacteria, the product becomes organoleptically unacceptable before it reaches the toxic stage.
Many of the methods of further acidification and fermentation of milk into other products will remove or inhibit most enteric pathogens, but a few organisms may survive. There are also further opportunities for the re-introduction of organisms by the addition of other untreated ingredients or from the production environment. Hard cheeses, yoghurt, and butter can be regarded as safe because of low pH and/or lack of moisture, but care must be taken with mould-ripened soft cheeses; these cheeses have a higher pH and thus permit the growth of L monocytogenes. 5-15% of soft ripened cheeses on sale in many countries in Europe--eg, the UK, the Netherlands, Germany, and Switzerland-may contain this organism4,13,14 (usually in low numbers), irrespective of the heat treatment of the original milk. 13 The presence of the organism is most probably due to recontamination during the cheese making process or during handling at the distribution and retail stage. Fish and shellfish Fish and shellfish may become contaminated either from the environment from which they are harvested or from the
environment during further processing. If such creatures are taken from water that has been polluted with sewage, they may contain many faecal pathogens (table 1). Vibrio parahaemolyticus, a marine microorganism, is a common contaminant of raw fish and other seafoods, especially those from the Far East. It can be eliminated by heat treatment but poor production hygiene may lead to recontamination.
Shellfish are filter feeders and concentrate, in their bodies, organisms from the water in which they are breedingmicroorganisms such as Salmonella, E coli, V parahaemolyticus, clostridia, and viruses have been detected. Thus, consumption of raw shellfish--eg, oysters--or shellfish that have been inadequately heat treated can lead to bacterial and viral infections.15 These shellfish usually go through a depuration process in clean water whereby many organisms are washed out of the gills and bodies. This process is effective for bacteria but less so for viruses.
Fruits, vegetables, and cereals In the raw state these foods may be contaminated with any of the organisms present in the soil in which they are grown. Fruits that grow high above the ground are less likely to be contaminated than those that are in direct contact with the soil. The cleanliness of irrigation water also plays an important part in the extent of contamination of these products and in the pathogens which may be found. In countries where polluted water is used for irrigation and animal and human excreta for fertilisation, there is a risk of contamination with enteric bacterial pathogens, such as Salmonella (including S typhl), Shigella, and V cholerae, as well as viruses and parasites.16 This is not a problem in the UK and developed countries where these practices are not common; in these countries most of the potentially harmful contaminating organisms are Clostridium spp, Bacillus spp, and L monocytogenes. For fruits and vegetables that are eaten without heat treatment, the extent of contamination can be reduced if they are washed and rinsed with a sanitising agent. Cooking will eliminate all but the sporing organisms.
Dried foods The predominant flora of most dried foods are organisms of the Clostridium and Bacillus groups, the spores of which survive the dehydration process. With some methods of drying, in which the temperature is not bactericidal or in which there are opportunities for recontamination, other organisms may persist-eg, Salmonella and E coli.11 These foods are safe when dehydrated but as soon as they are rehydrated they must be treated as fresh foods. Since herbs and spices frequently carry heavy loads of sporing organisms,18 it is important to add such ingredients at the beginning of the food processing stage (eg, before heat
treatment).
Ready-to-eat foods In addition to
our basic food items we are seeing a wide of range prepared, ready-to-eat foods retailed in either the frozen or chilled state. Many of these foods will have received some form of heat treatment but they are unlikely to be sterile. Frozen foods will remain safe while frozen but chilled foods require greater care with respect to shelf-life
861
and temperature of storage. The organisms found in these foods are those that have survived the heating process—eg, Clostridium spp and Bacillus spp-and those that have gained entry during the further manipulation of the product. The latter includes a wide range of organisms but those of concern are Y enterocolitica and L monocytogenes because they can grow at refrigeration temperatures. Numerous studies have found L monocytogenes in chilled foods,19 including cooked chicken, complete meals, cooked meats, pâté,20 and salads. Thus, greater care must be taken with this group of foods with respect to hygiene of production, temperature control during storage and display, and shelf life of the product.
The foodhandler and food environment
Organisms may also be transferred to food by the foodhandler either directly or by cross-contamination through the use of hands, surfaces, utensils, and equipment which have not been adequately cleaned and disinfected between the preparation of different foods. The factors which contributed to outbreaks of food-poisoning in England and Wales between 1970 and 1982 are shown in table II.21 The infected foodhandler appears low on the list. Only in relation to S aureus food poisoning does the foodhandler have an important role. S aureus is frequently found in the nose and on the skin and can be readily transferred to foods by handling; subsequent storage of food at unsuitable temperatures will allow the organisms to multiply and produce their toxins. Thus, foods that are much handled during preparation and not reheated before final consumption are at greatest risk. Foodhandlers who continue to work with active symptoms of gastroenteritis are a hazard in the food preparation area because there is an increased risk of faecal organisms reaching food. Many people who constantly handle raw foods, particularly those of animal origin, often become symptom-free excretors and may carry the organisms on their hands. Provided they have good hand hygiene and formed stools, such individuals are not a major risk in food preparation.22 To prevent spread of organisms from man and the food environment, separate surfaces, equipment, and personnel should be used to deal with raw and cooked foods; there should be regular hand washing and good cleaning schedules that are regularly enforced. In large-scale food production, separate well-defined areas should be allocated to the preparation of raw and processed foods. Removal of food debris, which can attract insects and vermin, and thorough cleaning of all food preparation equipment (including the dismantling of items such as mixers and slicers so that all surfaces and crevices can be reached) followed by a final disinfection procedure and drying are essential to keep recontamination to a minimum. It is virtually impossible to eradicate all organisms from the food preparation environment but efforts must be made to keep levels as low as possible and to prevent outgrowth. Every person involved in the preparation of food must be made aware that very few of the items that enter the kitchen are sterile. Most foods will be contaminated to a greater or lesser extent according to the amount of heat processing and other manipulation. If the food is consumed immediately, the extent of contamination is usually low-insufficient to cause illness. Most of the factors which contribute towards making a relatively harmless food harmful can be grouped as either attributable to poor temperature control or to cross-
(table II). Thus, although improvement in agricultural procedures can lead to the reduction in the presence of some organisms in our foods (eg, Salmonella and Campylobacter), good hygienic food preparation and education of those involved in the preparation, processing, contamination
and service of food are the final lines of defence in the prevention of most types of foodbome illness.
REFERENCES 1. Public Health
Laboratory Service. Memorandum of evidence to the Agriculture Committee Inquiry on Salmonella in eggs. PHLS Microbiol Dig 1989; 6: 1-9. 2. D’Aoust J-Y. Salmonella. In: Doyle MP, ed. Foodborne bacterial pathogens. New York: Marcel Dekker, 1989: 327-445. 3. Hood AM, Pearson AD, Shahamat M. The extent of surface contamination of retailed chickens with Campylobacter jejuni serogroups. Epidemiol Infect 1988; 100: 17-25. 4. Lammerding AM, Garcia MM, Mann ED, et al. Prevalence of Salmonella and thermophilic Campylobacter in fresh pork, beef, veal and poultry in Canada. J Food Protect 1988; 51: 47-52. 5. Pini PN, Gilbert RJ. The occurrence in the UK of Listeria species in raw chickens and soft cheeses. Int J Food Microbiol 1988; 6: 317-26. 6. Bolton FJ, Dawkins HC, Hutchinson DN. Biotypes and serogroups of thermophilic campylobacters isolated from ovine, bovine, and porcine offal and other red meats. In: Pearson AD, Skirrow MB, Lior H, Rowe B, eds. Campylobacter III. London: Public Health Laboratory Service, 1985: 106. 7. Humphrey TJ, Baskerville A, Mawer S, Rowe B, Hopper S. Salmonella enteritidis phage type 4 from the contents of intact eggs: a study involving naturally infected hens. Epidemiol Infect 1989; 103: 415-23. 8. Barratt NJ. Milkbome diseases in England and Wales in the 1980s. J Soc Dairy Technol 1989; 42: 4-6. 9. Sharp JCM. Infections associated with milk and dairy products in Europe and North America, 1980-85. Bull WHO 1987; 65: 397-406. 10. Rampling A, Taylor CED, Warren RE. Safety of pasteurised milk. Lancet 1987; ii: 1209. 11. Jones PH, Willis AT, Robinson DA, Skirrow MB, Josephs DS.
Campylobacter enteritis associated with the consumption of free school milk. J Hyg (Camb) 1981; 87: 155-62. 12. Rampling A. The microbiology of milk and milk products. In: Linton AH, Dick HM, eds. Topley and Wilson’s principles of bacteriology, virology and immunity. 8th ed. Vol 1. London: Edward Arnold, 1990: 265-89. 13. Greenwood M, Roberts D, Burden P. The occurrence of Listeria spp in milk and dairy products: a national survey in England and Wales. Int J Food Microbiol (in press).
EH, Ryser ET. Occurrence of Listeria in foods: milk and dairy foods. In: Miller AJ, Smith JL, Somkuti GA, eds. Foodborne listeriosis. Amsterdam: Elsevier, 1990: 151-64.
14. Marth
15. Public Health Laboratory Service Working Party on Viral Gastroenteritis. Foodborne viral gastroenteritis. PHLS Microbiol Dig
1988; 5: 69-75. 16. Geldreich EE, Bordner RH. Fecal contamination of fruits and vegetables during cultivation and processing for market. A review. J Milk Food Technol 1971; 34: 184-95. 17. Rowe B, Begg NT, Hutchinson DN, et al. Salmonella ealing infections associated with consumption of infant dried milk. Lancet 1987; ii: 900-03.
D, Watson GN, Gilbert RJ. Contamination of food plants and plant products with bacteria of public health significance. In: Rhodes-Roberts M, Skinner FA, Bacteria and Plants. Society for Applied Bacteriology symposium series No 10. 1982: 169-95. Gilbert RJ, Hall SM, Taylor AG. Listenosis update. PHLS Microbiol Dig 1989; 6: 33-37. Morris IJ, Ribeiro CD. Listeria monocytogenes and pâté. Lancet 1989; ii:
18. Roberts
19. 20.
1285-86. 21. Roberts D. Factors contributing to outbreaks of foodborne infection and intoxication in England and Wales 1970-1982. Proceedings of the 2nd World Congress Foodbome Infections and Intoxications. Berlin: Institute of Veterinary Medicine, 1986; 157-59. 22. Cruickshank JG, Humphrey TJ. The carrier food handler and typhoid salmonellosis. Epidemiol Infect 1987; 98: 223-30.
non-
