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Food Safety and toxicology Aims To explain the causes and effects of food poisoning and its relevance to food safety Basically, food is a mixture of chemicals. Usually, food components are distinguished in four categories: 1.nutrients, 2. toxins of natural origin, 3. contaminants, 4. and additives. The nutritients account for more than 99.9% of the food. The main classes of nutrients are : carbohydrates, proteins, fats, vitamins, and minerals, and all of them may pose toxicological risks to the consumer. Food Borne diseases Poisoning Infections Chemical poisoning intoxication poisonous Plant tissue poisonous animal tissue Algae toxins mycotoxins Enterotoxins neurotoxins enterotoxigenic invasive microbial intoxications bacterial toxins toxic metabolites interactions with carbohydrate metabolism What are Food-Borne Diseases? People get sick with a food-borne disease when they consume foods or beverages contaminated with disease-causing microbes, chemicals, insects or other harmful substances. Bacteria, viruses and parasites cause most of these diseases. Toxins, poisons and chemicals can also contaminate food and cause illness. Food Safety • Harmful substances in foods Pathogens • Bacteria, viruses, parasites • Some common pathogens causing foodborne illness – Staphylococcus aureus, Clostridium botulinum, Salmonella, Escherichia coli Chemical contamination • Pesticides • Animal drugs • Pollutants Natural toxins • Methyl mercury • Poisonous plants • Solanine Perceived Risk from Food: % Viewing as a Serious Hazard Major social impact of food borne disease… • Lost productivity time – Days work/school – Financial impact • Risk of mortality • Risk of transmission to family and others The Food Pathogen Problem • Estimates of foodborne disease in U.S./yr: – – – – – 6.5 to 33 MILLION foodborne illnesses Up to 9,000 deaths Hospitalization costs -- > $3 billion Lost productivity -- > $9 billion Under-reported! Adapted from: Council for Agricultural Science and Technology. Foodborne Pathogens: Risks and Consequences, 1994. FDAs Rank of Areas of Concern Most Dangerous 1. Microbial food illness 2. Naturally occurring toxins in foods 3. Residues in foods – Environmental contaminants – Pesticides – Animal drugs, such as hormones, antibiotics 4. Food processing and nutrients in foods Least Dangerous 5. Intentional Food Additives 6. Genetic modification of foods Most food poisoning is caused by? Bacteria Microbes and Food Safety Symptoms of foodborne illness – Diarrhea of more than 3 days’ duration – Fever of longer than 24 hours duration – Headache accompanied by muscle stiffness and fever – Numbness, muscle weakness, tingling sensations in the skin – Rapid heart rate, fainting, dizziness Majority of food-poisoning cases – Result of errors consumers make in handling foods after purchase – Commercially prepared food is “usually” safe Food Safety – Raw meats can contain live, disease-causing organisms – Thorough cooking makes them safe – In the mid-1990s a fast-food restaurant chain in the Northwest served undercooked hamburgers from meat contaminated with bacterium E. coli 0157:H7 • 4 people died • 100s of patrons became seriously ill As a result more Government Inspections and Industry controls were set up through the Hazard Analysis Critical Control Point (HACCP) plan In order to grow and multiply germs need: Time Moisture Food Warmth Flies Waiting Remember it like this Too Many In ideal conditions where there is Moisture, Food and Warmth (37degrees centigrade is ideal), bacteria can double every 10 to 20 minutes. They do this by dividing in to two. This is called Binary Fission These cells are beginning to divide into two After 10 minutes After 20 minutes After 30 minutes After 40 minutes cooking chicken to a core temperature of 75°C should kill most of the bacteria Time : 9.30 Bacteria : 0 Time : 9.40 Bacteria : 12,000 Time : 9.50 Bacteria : 24,000 Time : 10.00 Bacteria : 48,000 Time : 10.10 Bacteria : 96,000 Time : 10.20 Bacteria : 192,000 Time : 10.30 Bacteria : 384,000 Time : 10.40 Bacteria : 768,000 Time : 10.50 Bacteria : 1.5 million From 0 to 1,536,000 in only 80 minutes !!!!!! Knife contaminated by blood Bacterial Food Poisoning • Because harmful microorganisms are present everywhere in the environment, any food can become contaminated if not properly handled before consumption. Many foodborne pathogens are ubiquitous in nature and generally found in soil, water, animals and plants. Pathogens are introduced into a processing plant through the raw materials. Humans and plant equipment may also bring the organisms to a plant. Recontamination of processed food also frequently occurs and contributes to foodborne outbreaks and illnesses. Pathogens can survive for prolonged period on inanimate objects and serve as a source. Bacterial Growth Curve Numbers Of Bacteria rapid multiplication no multiplication Lag Phase Stationary Phase Numbers of bacteria remain constant as the number produced is equal to the number dying Time (hours) Numbers of bacteria decrease When considering a product safety, it is important to know the type of microorganisms or toxins likely to be present, their numbers, and concentrations. In addition, their response to the heat (heat-labile or heat-stable), pH, salts, and other processing conditions is important to consider. The numbers and types of microorganisms present in a finished food product are influenced by : •the original source of the food •microbiological quality the raw or unprocessed •sanitary conditions •the conditions for subsequent packaging, handling, storage, and distribution. Fig. 19-4, p. 669 Bacteria and Temperature 100 HIGH Minimum Growth Temp. Campylobacter Cl botulinum A&B Staph aureus E. coli Salmonella Cl perfringens Listeria Cl botulinum E Bacillus cereus LOW LOW HIGH Bacteria die if heated for a sufficient time. The longer the time, the greater the destruction 63 40 38 Heat resistance • Pathogenic bacteria grow best at human body temperature 37ºC. However the majority will grow between 15-45ºC • Non-sporing cells of bacteria are killed at temperatures above 60ºC. The length of time ranges depending on the organism • Boiling kills living cells, but will not kill all bacterial spores • Fridges should be set below 5ºC. Some bacteria such as Listeria monocytogenes can grow at refrigeration temperatures Bacteria grow quickly Bacteria grow 36 15 7 4 0 Bacteria stop growing, but do not die Bacteria Grow at slower rate Food Safety • Food can provide ideal conditions for bacteria to thrive or produce toxins – Disease-causing bacteria require • Warmth 40°F - 140°F = 4°C - 60°C • Moisture • Nutrients • To control bacteria – – – – Keep Keep Keep Keep hot food hot-above 140F cold food cold-below 40F/4C raw foods separate your hands and the kitchen clean Food Safety • Keep Hot Food Hot – Keep cooked foods at 140°F or higher until served cooking does not destroy all bacterial toxins – If handled improperly can cause illness – Cooked foods should be refrigerated immediately or within two hours at the maximum • Keep Cold Food Cold – Start when you leave the grocery store – At home, put foods into the refrigerator or freezer immediately – When defrosting foods • Thaw meats or poultry in the refrigerator • Marinate meats in the refrigerator Food Safety • Food with an “off” appearance or odor should not be used or tasted • Keep raw foods separate /Prevent crosscontamination – Raw foods, especially meats, eggs and seafood, are likely to contain bacteria – Keep the raw foods and their juices away from ready-to-eat foods – After handling raw foods wash surfaces & your hands, thoroughly with soap • Foods prone to microbial growth – Those high in moisture and nutrients – Those chopped or ground like meats and poultry Foods That Make People Sick • Eggs – Raw, unpasteurized eggs are likely to be contaminated by Salmonella bacteria – Raw pasteurized egg substitutes may contain a few live bacteria • They may not be safe for pregnant women, the elderly, the very young, or those suffering from immune disorders • Raw Produce – Fruits and vegetables are a microbial threat unless they are thoroughly rinsed in running cold water – Ten years ago, meats, eggs, and seafood posed the greatest foodborne illness threat – Today produce equals them Know the Potentially Hazardous Foods • Preparing and serving safe food is important to your customers. • Be careful when you work with and prepare foods that are “potentially hazardous.” – These are foods that have been linked to foodborne illness • Examples of potentially hazardous food are: • • • • • Meat, poultry and fish Milk and egg products Salads and sandwiches made with meat Sliced melons and sprouts Cooked vegetables, cooked rice or beans You are responsible for identifying “potentially hazardous” foods. • Click below to reveal the “potentially hazardous” Potentially foods. Hazardous Contaminated irrigation or flood water Human/animal feces Contaminated seeds Untreated manure Staphylococcus Aureus Found in human nose and throat (also skin) Salmonella Found in animals, raw poultry and birds Bacillus Cereus Found in soil, vegetation, cereals and spices Clostridium Perfingens Found in animals and birds Clostridium Botulinum Found in the soil and associated with vegetables and meats The diseases caused by foodborne pathogens can be classified into three forms: foodborne infection foodborne intoxication foodborne toxicoinfection Toxic bacterial food poisoning some bacteria grow in food and produce a toxin within the food which is then consumed e.g. Bacillus cereus and Staphylococcus aureus. When the food is consumed viable cells of the bacteria do not need to be present. Other microorganisms in food may produce harmful or deadly toxins while growing in the intestinal tract e.g. Clostridium perfringens, enterotoxigenic Escherichia coli and enterohaemorrhagic E. coli Infective bacterial food poisoning infections occur when pathogens are ingested via contaminated food and the bacteria is established in the body, usually growing inside the intestinal tract and irritating intestines e.g. Salmonella spp. and Campylobacter jejuni. The infection may involve subsequent growth in other tissues Intoxication Ingestion of preformed toxins such as staphylococcal enterotoxin, botulinum toxin, Bacillus cereus toxin, and seafood toxins results in food poisoning or Intoxication. Microorganisms present in foods will grow under favorable conditions and produce toxins in the food. Following ingestion, Toxins are absorbed through the gastrointestinal epithelial lining and cause local tissue damage and may induce inflammation resulting in diarrhea or vomiting. In some cases, toxins are translocated to distant organs or tissues such as liver, kidney, peripheral, or central nervous system where they can cause damage. Toxicoinfection Some bacteria cause toxicoinfection, which occurs when ingested bacteria first colonize the mucosal surface and then produce exotoxins in the intestine. Toxins can induce toxic effects on the local cells or tissues and in some cases toxins enter blood stream and induce disease. Examples include cholera toxin produced by V. cholerae; Clostridium produces hydrolytic enzymes such as lecithinase (breaks down lecithin), hyaluronidase and protease (break down extracellular matrix and disrupts tissue structure) Toxins Some bacteria release poisons known as toxins which cause food poisoning. Some toxins, known as exotoxins multiply in food. These toxins are not easily destroyed by cooking and may remain in food once they have developed. Other bacteria produce toxins inside the human body only after the food has been eaten. These are called endotoxins There are many types of exotoxins and they are grouped based on the structure and the mode of action Membrane-disrupting toxin (pore forming): These toxins either insert into the membrane to cause pore formation (example, hemolysin) A–B type toxin: These types of toxins have two subunits; A and B. B binds to cell receptor and A exerts enzymatic activity such as inhibition of protein synthesis or cleavage of target protein. Proteases: Some toxins (i.e., neurotoxins) inactivate metalloproteases (Zinc metalloprotease) action, thereby interfering with nerve impulses. Table Characteristics of bacterial toxins Toxin type Toxins Producing bacteria Membrane Hemolysin E. coli damaging Listeriolysin O L.monocytogenes toxin Perfringolysin O C. perfringens -toxin S. aureus Streptolysin O S. pyogenes Inhibit protein (A–B type Shiga toxin or Shiga-like toxin Diphtheria toxin Edema factor Shigella spp. E. coli C. diphtheriae B. anthracis Mode of action Pore formation Pore formation Pore formation Pore formation Pore formation Target Plasma Cholesterol Cholesterol Plasma membrane Cholesterol N-glycosidase 28S rRNA ADP ribosylation synthesis Adenylate cyclase (i) A-B type toxin receptor Binding B A Toxin Enzymatic action (ii) Pore-forming hemolysin (iii) Membrane active enzyme Phospholipase cleaves phosphatidylcholine(PC) Salmonella • Sources - The intestines of ill people and carriers, animals and animal food, raw meat, raw poultry, raw milk, raw eggs, food pests • Common food vehicles – Undercooked or contaminated cooked meat, raw milk and eggs • Onset period – 6 to 72 hours (usually 12 to 36) Endotoxin in intestine (infective food poisoning) Salmonella • Symptoms - Abdominal pain, diarrhoea, vomiting and fever. Duration is usually one to seven days. • Specific characteristics – Usually requires millions of bacteria to cause illness. Multiplies from 5°C to 47°C under aerobic or anaerobic conditions. Aerobic and Anaerobic • Bacteria which need oxygen to multiply are classed as aerobic • Bacteria which only multiply without oxygen are called anaerobic • Just to confuse you some bacteria including Salmonella, Staphylococcus aureus and Bacillus cereus are classed as facultative anaerobes which means they can multiply with or without oxygen! Salmonella • • • • • • • Specific controls Hygienic farm and abbatoir practices Avoid cross contamination Complete thawing of frozen poultry Thorough cooking to 75°C for one minute High standards of personal hygiene Effective cleaning Safe sewage disposal Clostridium perfingens • Sources - The intestines of humans and animals, faeces and sewage, soil food pests, raw meat and poultry • Common food vehicles – Rolled joints, casseroles, stews, sauces and meat pies when cooking has removed oxygen • Onset period – 8 to 22 hours (usually 12 to 18) Enterotoxin in intestine. (infective food poisoning) Clostridium perfingens • Symptoms - Abdominal pain, diarrhoea, (vomiting is rare) and fever. Duration is usually 12 to 48 hours. • Specific characteristics – Usually requires millions of bacteria to cause illness. Multiplies from 10°C to 52°C under anaerobic conditions. At 46°C it can double every 10 minutes. Produces spores. Illness caused from consuming millions of organisms Staphylococcus Aureus • Sources - Human nose, mouth, skin, hands, spots, boils, septic cuts etc • Common food vehicles – Dairy products. Cold cooked meat and poultry, peeled cooked prawns • Onset period – 1 to 7 hours (usually 12 to 18) Exotoxin produced in food. (Toxic food poisoning) Staphylococcus Aureus • Symptoms - Abdominal pain, diarrhoea, vomiting. Occasionally subnormal temperatures. Duration between 6 and 24 hours. • Specific characteristics – Usually requires millions of bacteria to cause illness. Toxin may survive boiling for up to 30 minutes. Multiplies from 7°C to 48°C under aerobic or anaerobic conditions. Can tolerate relatively high salt content. Endotoxins are part of the cell structure and are cell associated, such as LPS in Gram-negative bacteria or PGN and LTA in Gram-positive bacteria. Some toxins such as cholera toxin and E. coli LT toxin, are designated as enterotoxins because They are responsible for enteric diseases (or gastroenteritis). Cytotoxins are cell- or tissue-specific such as neurotoxins (affect nerve cells), leukotoxins (attack leukocytes), hepatotoxins (attack liver cells), and cardiotoxins (damage cardiac tissue). . Toxin designation is sometimes based on the bacterial species that produced them; Cholera toxin is produced by Vibrio cholerae; Shiga toxin by Shigella species, Diphtheria toxin by Corynebacterium diphtheriae, tetanus toxin by Clostridium tetani, botulinum toxin by Clostridium botulinum. Sometimes the toxin is named on the basis of the action it exerts such as adenylate cyclase produced by Bordetella pertussis, lecithinase by each Clostridium perfringens and Listeria monocytogenes. Toxins may also be designated by a letter such as staphylococcal enterotoxin A (SEA), SEB, SEE, etc., by Staphylococcus aureus. Endotoxin such as LPS, PGN, and LTA are responsible for septicemia. The Lipid A component of LPS is highly toxic. LPS raises the body temperature and activates macrophages to produce IL-1 and TNF-α, which in turn causes the hypothalamus to release prostaglandins. Increased prostaglandin causes elevated body temperature (fever). In essence, this defense strategy is designed to inhibit bacterial growth. Aspirin blocks prostaglandin release and thus reduces body temperature. Endotoxins can trigger a massive inflammatory response leading to septicemia and septic shock. During septic shock, the circulatory system collapses, blood pressure drops, fever increases, heart rate increases, multiple organs fail, and death follows. Bacillus Cereus • Sources - Cereals, especially rice, cornflour, spices, dust and soil • Common food vehicles – Reheated rice, cornflour and spices. • Onset period – 1 to 5 hours. Exotoxin produced in food. (Toxic food poisoning) Bacillus Cereus • Symptoms - Abdominal pain, diarrhoea, vomiting. Occasionally subnormal temperatures. Duration between 12 and 24 hours. • Specific characteristics – Forms spores which produce an exotoxin under aerobic or anaerobic conditions. Both the spores and toxin will survive normal cooking temperatures. Millions required to cause illness. Bacteria multiply between 7°C and 48°C. Clostridium Botulinum • Sources - Fish intestine, soil and vegetables. • Common food vehicles – Low acid processed food contaminated after canning or vacuum packing. Smoked fish, bottled vegetables. • Onset period –2 hours to 5 days (usually 12 to 36 hours). A heat sensitive neurotoxin produced in the food which affects the nervous system. (Toxic food poisoning) Clostridium Botulinum • Symptoms - Difficulties in swallowing, talking and breathing. Double vision and paralysis. Diarrhoea followed by constipation. Fatalities are common and survivors may take several months to recover. • Specific characteristics – Forms spores which produce an exotoxin under anaerobic conditions. Both the spores and toxin will survive normal cooking temperatures. Millions required to cause illness. Bacteria multiply between 3.3°C and 48°C. Pathogenic Bacteria Salmonella Source Raw meat Poultry and eggs Pests and pets Human and animal intestines Dirt and refuse Symptoms Vomiting Nausea Diarrhoea Abdominal pain Average Onset Time 12 - 36 hours after eating Staphylococcus aureus Human nose, throat, ears, skin Septic wounds Animals and raw milk Vomiting Abdominal pain Low temperature 1 – 7 hours after eating Clostridium perfingens Raw meat and poultry Soil, dirt and refuse Raw vegetables Pests and pets Human and animal intestines Diarrhoea Abdominal pain 12 - 18 hours after eating Clostridium botulinum Bacillus cereus Soil Marine sediment Raw fish and meat Animal intestines Dust and soil Cereal, rice and pasta Paralysis Breathing and swallowing difficulty Diarrhoea followed by constipation Nausea Vomiting Abdominal pain Diarrhoea 12 – 36 hours after eating 1 - 5 hours or 8 – 16 hours depending on the form of the food poisoning Campylobacter enteritis Escherichia Coli 0157 Listeriosis That’s the end of lesson 1! Coming next lesson: Foodborne diseases Typhoid and paratyphoid fever Dysentery Some People Face Special Risks A variety of people may face these special risks: • Pregnant women • Very young children • People with chronic illnesses or weakened immune systems • Older adults Why Older Adults? Immune systems weaken with age Stomach acid decreases as you get older Diseases/illnesses may further weaken the body Recognizing Foodborne Illness Can’t see, smell or taste bacteria in food Often takes 1 to 3 days to cause illness, but can take up to 6 weeks 3 Pathogens of Special Importance to Older Adults Escherichia coli O157:H7 Salmonella Listeria monocytogenes E. coli Lives in intestines of healthy cattle and other ruminant animals. Typical food sources: • Undercooked ground meats, • Contaminated produce served raw (lettuce, spinach, sprouts), • Unpasteurized milk Salmonella Salmonella outbreaks have been associated with: • • • • Raw and undercooked eggs Undercooked poultry and meat Raw milk Produce and unpasteurized juice Why eggs? • Salmonella can grow both inside the egg and on the outside of shells Listeria Bacteria widespread in nature, soil, water Survives and grows at refrigerator temperatures! Risky Foods: Refrigerated Ready-to-Eat foods (i.e. deli salads, lunch meats, hot dogs, soft cheeses made with raw milk) Listeria Almost all cases: elderly, pregnant women and immune-compromised persons Mild gastrointestinal symptoms: 8-48 hrs Invasive illness: 2-6 weeks following exposure • Fever, muscle aches, headache, loss of balance, bacteremia, meningitis, encephalitis Moulds and Yeasts • Moulds are a type of fungi that will grow on most foods and at many temperatures. Some are used in food production such as cheese manufacture. Unwanted moulds usually spoil the food but do not cause food poisoning. • Yeasts are another type of fungi that will grow in food. They are used in making food such as bread and beer but also spoil many foods including jam, fruit juice, yoghurts and meats Food Poisoning bacteria •Usually need millions of bacteria to cause illness. •The multiplication of bacteria within the food plays an important part in the disease Spore A resting resistant phase of some bacteria (including Clostridium Perfingens and Botulinum and Bacillus Cereus). The bacterium produces a protective coat which helps it to survive high temperatures (up to 120°C) and lack of water. When favourable conditions return, the spores split open and release the bacteria which are then able to grow and multiply Bacterial cell Spore forming inside cell Spore Formation This is what happens ………….. Cell Spore forms in cell Cell disintegrates Spore is released Spore starts to germinate Spore continues to germinate Now see as, in suitable conditions, the cell begins to divide (binary fission)…………………………. From raw materials to consumer: Chemical, microbiological and technological aspects of food • Most of the food is treated in some way to improve its shelf life, texture, palatability • or appearance. It would be difficult to change this situation. So, it is important to know • what happens to the various food components on the way from raw material to consumer. raw materials Nutrients natural toxins contaminants processing hazardous reaction products contaminants food quality and safety reduction and prevention of health risks additives Figure : Food: from raw material to consumer the four categories of food components are discussed in the following order: 1. natural toxins (including microbial toxins) 2. antinutritives 3. contaminants, 4.food additives and the rationale for their use 5. nutrients. Natural Toxin Endogenous toxins of plant origin 1 Toxic phenolic substances Flavonoids Tannins Coumarin, safrole, and myristicin 2. Cyanogenic glycosides Glucosinolate s 3 Natural contaminants Mixing of edible plants with toxic plants Contamination resulting from intake of toxic substances by animals Contamination of milk with plant toxins Natural toxins in aquatic organisms 4. Microbial toxins Food-borne diseases Bacterial toxins Mycotoxins Aflatoxins Ochratoxin A Antinutritives 3.1 Introduction 3.2 Type A antinutritives (antiproteins) 3.2.1 Protease inhibitors 3.3 Type B antinutritives (antiminerals) 3.3.1 Phytic acid 3.3.2 Oxalic acid 3.3.3 Glucosinolates 3.3.4 Dietary fiber 3.3.5 Gossypol 3.4 Type C antinutritives (antivitamins) 3.4.1 Ascorbic acid oxidase 3.4.2 Antithiamine factors 3.4.3 Antipyridoxine factors Contaminants 4.1 Introduction 4.2 Contamination with heavy metals 4.2.1 Mercury 4.2.2 Lead 4.2.3 Cadmium 4.3 Nitrate 4.4 2,3,7,8-Tetrachlorodibenzo-pdioxin 4.5 Pesticide residues 4.6 Food contaminants from packaging material Food additives 5.1 Introduction 5.2 Use of food additives in relation to their safety 5.2.1 Colorings 5.2.2 Flavoring agents 5.2.3 Preservatives 5.2.3.1 Antioxidants 5.2.3.2 Antimicrobials 5.2.3.3 Antibrowning agents Nutrients 6.2 Macronutrients 6.2.1 Fats 6.2.1.1 Undesirable fat components in raw materials 6.2.1.2 Changes in dietary fats during storage and processing of raw materials, and during manufacturing, preparation and storage of food 6.2.1.2.1 Rancidity 6.2.1.2.2 Oxidation of fats and oils, and adverse health consequences 6.2.1.2.3 Effects of processing techniques on the oxidation of dietary fats and oils 6.2.2 Carbohydrates 6.2.2.1 Changes in dietary carbohydrates during manufacturing and storage of food 6.2.3 Proteins 6.2.3.1 Changes in proteins during processing of raw materials, and during manufacturing, preparation and storage of food 6.2.4 Pyrolysis products occurring in food 6.3 Micronutrients 6.3.1 Hypervitaminoses 6.3.1.1 Vitamin A 6.3.1.2 Vitamin D What is Food Hygiene? • The science of preserving health • It involves all measures necessary to ensure the safety and wholesomeness of food during it’s preparation and storage What does it involve? • Rejecting contaminated food • Decontaminating food • Protecting food from contamination through high standards of personal hygiene, cleaning and disinfection • Preventing any organisms multiplying • Destroying any harmful bacteria by thorough cooking • Discarding unfit or contaminated food Food Poisoning • Normally associated with symptoms such as diarrhoea and vomiting • May also include headache, stomach cramps and fever • Bacteria are responsible for most cases • Other causes include mycotoxins (poisonous chemicals produced by some moulds) Food Poisoning (contd.) • Physical contamination:objects falling in to food – metal, glass, packaging materials etc. • Chemical contamination:- Bleach, cleaning chemicals getting in to food • Natural contamination: Poisonous plants and berries, undercooked red kidney beans Poisonous mushrooms The deathcap – one bite can prove fatal And also…..The deadly Puffer fish! Immunity to Microbes The principal physiologic function of the immune system is to protect the host against pathogenic microbes such as extracellular bacteria, intracellular bacteria, virus or parasites. Important features of immunity to microbes are (a) defense is mediated by both innate and adaptive immunity, (b) different types of microbes stimulate distinct population and subpopulations of lymphocytes, (c) survival of microbes or production of disease depends on their ability to evade immune system, and (d) the tissue injury and disease consequent to infection caused by host response rather than the microbes itself. Extracellular Bacteria These organisms replicate outside the cells, i.e., in circulation, interstitial space, lumen of respiratory tract, and intestinal tract. Examples of extracellular bacteria include Clostridium, Staphylococcus, Bacillus, Streptococcus, Escherichia coli, and Vibrio species. They cause disease either by inducing inflammation or by direct action of toxins or enzymes they produce. Inflammation Bacterial cells provoke inflammation that result in tissue destruction and influx of neutrophils and macrophages at the site of infection. During inflammation, complement activation takes place and cytokines are released from macrophages. Release of enzymes and toxins from neutrophils and macrophages causes cell injury and damage which is also known as suppurative infection (characterized by pus formation). Sequence of events in inflammation include (1) increased blood supply to the site of infection characterized by the development of “redness,” (2) increased capillary permeability and fluid accumulation results in “swelling,” and (3) the recruitment of neutrophils by chemotaxis (C5a) and macrophages result in tissue injury that invoke “pain.” Toxins Exotoxins cause diverse pathological effects resulting in cell injury or cell death (see Chap. 4 for details). For example, Shiga and diphtheria toxins block protein synthesis; botulinum toxin blocks neurotransmitter release; and cholera toxin stimulate cAMP synthesis resulting in Cl− secretion and H2O loss. Endotoxin (LPS and PGN) also called pyrogen, stimulates the release of cytokines IL-1 and TNF-α; activate B-cells; and act as an adjuvant to stimulate macrophage to produce more cytokines. These cytokines, induce fever, decrease smooth muscle contraction, increase membrane permeability, lower blood pressure, and induce shock. Innate Immunity Innate immunity against extracellular bacteria include (1) phagocytosis by neutrophils, monocytes, and tissue macrophages, (2) activation of complement in the alternative pathway or mannose lectin pathway by bacterial peptidoglycan or mannose-binding lectin. The complement activation byproduct C3b enhances opsonization while MAC induces cell lysis. (3) LPS activates macrophages to produce cytokines TNF-α, IL-1, IL-6 which activate neutrophils and macrophages to remove bacteria. These cytokines also could induce fever to retard bacterial growth or may induce “septic shock” or “endotoxin shock” leading to fatal consequences. Antigen-mediated specific T-cell activation results in specific cytokines release which are responsible for isotype switching to produce specific antibodies. 2. Exotoxins are T-dependent antigens and are processed by APC (macrophages, B-cells) and presented via MHC class II molecules to activate CD4+ T-cells and B-cells for cytokine and antibody production, respectively. Certain toxins act as superantigens, which induce T-cells to produce increased cytokines leading to toxic shock syndrome (TSS). For example, staphylococcal enterotoxins (SEA, SEB, SEC, SED, SEF, etc.) bind to MHC class II on macrophages which directly interacts with TCR of T-cells and activate CD4+ T-cells to produce IFNγ, which in turn induce enhanced MHC expression, antigen presentation, and cytokine production in macrophages leading to toxic shock syndrome. 3. Antibodies (IgG, IgM) help in the influenzae, E. coli). Bacteria use surface molecules for adhesion and colonization; however, genetic variation in bacterial surface molecules such as pili, fimbriae, flagella or surface proteins will evade antibodies antibodies developed against previous form of molecules. (2) Some bacteria (Bacillus anthracis, B. cereus, and Pneumococcus spp.) exhibit antiphagocytic mechanism. They possess capsules, which are made of sialic acid and hyaluronic acid (both are also present in host cell membrane) and prevent binding of C3b. As a result these bacterial cells are not recognized by macrophages for destruction. (3) Sialic acid component of capsule also inhibits complement activation. (4) Some intracellular bacteria (Listeria monocytogenes, Salmonella) are capable of scavenging reactive oxygen using superoxide dismutase (SOD) thus avert toxic effects of oxygen radicals. (5) Some bacteria (Staphylococcus aureus, Streptococcus pyogenes) cover Intracellular Bacteria Several foodborne pathogens maintain intracellular life cycle as part of their infection strategy. After binding to specific host cell receptor, they modulate signaling events resulting in cytoskeletal rearrangement and facilitating their own entry. Bacteria trapped inside a phagosome is either escaped by lysing the vacuolar membrane or produce virulence factors that sustain their intracellular life style. Bacteria either spread from cellto-cell, or induce apoptosis, or cause host cell lysis as part of their pathogenic mechanism. The examples of intracellular foodborne bacterial pathogens are Listeria monocytogenes, Yersinia enterocolitica, Shigella spp., and Salmonella enterica. Innate Immunity Natural immunity including gastric acid, bile salts, antimicrobial peptides, mucins, and natural microflora provide some protection; however, many pathogens are resistant to those thus natural immunity is less effective. Furthermore, phagocytes (macrophages and neutrophils) are also less effective since bacteria are resistant to degradation by lysosomal contents.