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Food Safety



Microbiological Hazards


Microbiological hazards include disease-causing bacteria, viruses, and parasites. Many of these microorganisms occur naturally in the environment and can be foodborne, waterborne, or transmitted from a person or an animal. Cooking kills or inactivates most pathogens, while proper cooling and storage can control them before or after cooking.


Bacteria are single-celled organisms so small they can only be seen with a micro-scope. Bacteria are everywhere and most are not pathogenic (disease-causing). The human gastrointestinal tract is home to more than 300 species of bacteria. Fortunately, only a few of these cause illness. Some bacteria are beneficial and are used in making foods such as yogurt, cheese, and beer. Others cause food to spoil, but do not cause human sickness. This difference between spoilage bacteria and pathogenic bacteria is important in the prevention of foodborne illness. Since pathogenic bacteria generally cannot be detected by looks, smell, or taste, we rely on spoilage bacteria to indicate that a food should not be eaten. Not many people will eat food that has become slimy or that smells bad. Pathogenic bacteria cause foodborne illness in three different ways:

Infection. Some bacteria damage the intestines directly. This type of illness occurs from eating food contaminated with live pathogenic bacteria. Cells that are alive and producing are vegetative cells. Many bacteria are killed in the acidic environment of the stomach, but some survive, pass through to the small intestine, and begin to grow in number. When the bacteria have multiplied to a high enough number (this depends on the strain of bacteria, its virulence or strength, and the health and susceptibility of the individual), the person becomes ill.

Salmonella is a classic example of this kind of bacteria. Salmonella exists in the intestinal tracts of animals, including food-producing animals as well as turtles, cats, dogs, birds, rodents, and wild animals. Raw milk and eggs are also sources of Salmonella. While heat easily destroys Salmonella, inadequate cooking allows some of the organisms to survive. Often Salmonella is spread through cross-contamination. This could happen when a cook prepares a piece of raw poultry on a cutting board and then uses the same cutting board without cleaning it to prepare another food that will not be cooked, such as a salad. The second food will not receive any heat treatment to kill the bacteria.
Salmonella can reproduce very quickly; they double their number almost every 20 minutes. When the number of Salmonella are very high, there is a better chance that some of them will survive the harsh environment of the stomach and make it to the small intestine. Once there, they will continue to multiply, eventually causing damage to the intestine and symptoms of foodborne illness. With some bacteria the infectious dose, the number of bacteria necessary to cause illness, is very high, in the millions; while in others it can be as low as 10 organisms.

Intoxication. Some bacteria produce harmful toxins or other chemicals that are then present in the food. It is not the bug itself that causes illness, but rather the toxin the bacteria produce. This can happen even if the pathogen itself has been killed, as long as it had sufficient time to produce enough toxin before dying. Staphylococcus aureus is an example of this type of bacteria.
Staphylococci exist in air, dust, sewage, water, milk, animals, humans, and in food or on food equipment. They are present in the nasal passages and throats and on the hair and skin of 50 percent or more of healthy individuals. Illness is caused by ingesting toxins produced in food by some strains of S. aureus, usually because the food has not been kept hot enough or cold enough. Although cooking easily destroys the bacteria, the toxin produced by the bacteria is very resistant to heating, refrigeration, and freezing. It is not possible to detect the presence of the toxin in food by smell, appearance, or taste. The likelihood of illness increases with the amount of time the bacteria are left at an improper temperature.

Toxico-infection. Some bacteria enter the intestines live, survive the acidic environment of the stomach, and then produce a harmful toxin inside the human digestive system.
Toxico-infection is a combination of the previous two examples in that live cells must be consumed, but the toxin is produced in the intestine and it is the toxin that really causes the illness. An example of an organism that causes foodborne illness in this manner is Clostridium perfringens. Like many bacteria, this organism is widely distributed in the environment in soil, water, dust, and in the intestines of domestic and wild animals. Large numbers of the bacteria, usually in the millions, need to be ingested to cause illness. These bacteria produce a spore, which is a dormant form of the organism. Vegetative cells may form a spore when the conditions for survival are not optimal for the cell, such as high heat or lack of water and food. Heat often does not destroy spores. Once the conditions are conducive to growth again, the spore will again become a vegetative cell. A typical case of foodborne illness occurs when a piece of meat is cooked, but the spores survive. Then, if the meat is not cooled properly the spores revert back to their non-dormant vegetative form and reproduce to high numbers. When food contaminated with C. perfringens is eaten, the organism grows in the small intestine and produces a toxin that causes illness.

Some bacteria use a combination of the above methods to cause illness. Just how many bacteria or how much toxin needs to be consumed before a person becomes ill depends on a number of factors that will be covered later in the section on factors that contribute to foodborne illness. Onset time, the time between when a person eats the food and when the symptoms start, is also highly variable. Most animals, including humans, are carriers of all kinds of bacteria, some of which do cause foodborne illness. Additionally, since many bacteria occur everywhere in nature—in the soil, air, water, etc.—it is not possible to totally eliminate harmful bacteria from the food supply. Bacteria need certain elements to survive and grow:

Water. Bacteria need water in order to carry out the biologic processes of life, much as humans do. Drying foods and adding salt or sugar to a food, which bind the available water, inhibit microbial growth. This is why foods such as dry pasta do not spoil as easily as a piece of meat. The amount of available water in a food is called water activity.

Food. Bacteria prefer foods that are high in protein such as meats, dairy products, and seafood.
High-fat foods can protect bacteria from acidic juices in the stomach, and can help the bacteria to survive and pass into the small intestine. Foods with a high water and protein content, which are more likely to foster the growth of pathogens are called potentially hazardous foods.

Proper temperature. While many pathogenic bacteria grow best at temperatures near human body temperature, they can survive and grow over a temperature range from 40 degrees F to 140 degrees F (or from4 degrees Cto 60 degrees C). This range is called the danger zone, and keeping foods out of the danger zone is one of the main ways of controlling bacterial growth. Bacteria do not grow as well at the extremes of this range, but they can still survive and grow at those extremes. Some bacteria can even grow at temperatures below or above the danger zone, but their growth is very slow at those temperatures. Refrigerator temperature is 40 degrees F (5 degrees C) to inhibit bacterial growth.

Time. Some bacteria grow very quickly; it only takes 20 to 30 minutes for their numbers to double.
Following table shows the number of bacteria that would be present after 10 hours for an organism that reproduces every 20 minutes.

Multiplication Rate for a Bacterium that Reproduces Every 20 Minutes

Time Number of Bacteria
  20 minutes   2
  40 minutes   4
  1 hour   8
  2 hours   64
  3 hours   512
  4 hours   4,096
  5 hours   32,768
  6 hours   262,144
  7 hours   2,097,152
  8 hours   16,777,216
  9 hours   134,220,000
  10 hours   1,073,700,000
Given the right conditions, a single cell can multiply to more than one billion cells in that amount of time, about the same amount of time it would take for the center of a big deep pot of chili to cool below 40 degrees F. For most bacteria, large numbers of them must be consumed before illness occurs. There are exceptions to this, such as E. coli 0157:H7 in which consuming as few as 10 cells can cause foodborne illness.

Oxygen. Some microorganisms need oxygen to live, while others do not. Bacteria that need oxygen are called aerobic, those that do not are anaerobic. For example, Clostridium botuli-num, the pathogen that causes botulism, only grows in the absence of oxygen. Many patho-genic bacteria, called facultative anaerobes, can grow with or without oxygen but have a preference.

Proper pH or acidity. Microorganisms prefer foods that are neutral (pH 7) or slightly acidic. Many foods fall into this category—eggs, meat, poultry, fish, sauces, soups, gravies.


Several viruses also cause foodborne illness. Viruses differ from bacteria in that they are smaller, require a living animal or human host to grow and reproduce, do not multiply in foods, and are not complete cells. Ingestion of only a few viral particles is enough to produce an infection. Humans are host to a number of viruses that reproduce in the intestines and then are excreted in the feces. Thus, transmission of viruses comes from contact with sewage or water contaminated by fecal matter or direct contact with human fecal material. Raw or undercooked molluscan shellfish (oysters, clams, mussels, and scallops) are the food most often associated with foodborne viral diseases. Human pathogenic viruses are often discharged into marine waters through treated and untreated sewage. As shellfish filter contaminants from these polluted waters, they store them within their edible tissues. Shellfish grown and harvested from polluted waters have been implicated in outbreaks of viral diseases. Therefore, proper handwashing and using a clean water supply are vital to controlling the spread of foodborne viruses.

Scientists do not know as much about viruses as they do about bacterial pathogens. One problem has been a lack of good laboratory methods to detect viruses. Without rapid, easy, inexpensive testing methods it has not been possible to study how viruses are transmitted, the number of people who become ill from foodborne viral infections, or the best methods to control viruses. Because of these problems, health care providers often do not order tests to detect viral infections, which causes foodborne illness from viruses to be even more underreported than for bacteria.
Epidemiologists estimate that 67 percent of foodborne illnesses can be attributed to viruses, although only a small percentage of total foodborne illness deaths are due to viruses.
Hepatitis A is a virus commonly associated with foodborne infections. The incubation period for hepatitis A, before a person develops any symptoms, is anywhere from 10 to 50 days. It is during this period before symptoms appear that a carrier is most infectious and most likely to spread the disease. Hepatitis A, and many other viral and bacterial pathogens, is most often transmitted via a fecal-oral route. The fact that a person is infectious even before they know they have the disease makes it difficult to control.

Protozoa and Parasites

Some parasites also cause foodborne illness. Parasites must live on or inside a living host to survive. The most common foodborne parasites are Anisakis simplex, Cryptosporidium parvum, Toxoplasma gondii, Giardia lamblia, and Cyclospora cayetanensis. Giardia, Cryptosporidium, and Toxoplasma are all protozoa, or single- celled organisms. Giardia has been identified more than any other pathogen in waterborne disease outbreaks, but there also have been foodborne Giardia outbreaks. Cryptosporidium is also primarily a waterborne pathogen. An estimated 21 percent of waterborne outbreaks from drinking water are due to parasitic agents, mainly Giardia and Cryptosporidium. These two parasites are the most common cause of human parasitic infections.
Toxoplasma gondii is common in warm-blooded animals, including cats, rats, pigs, cows, sheep, deer, chickens, and birds. It can be found in feces and raw meat from these animals. While not a problem for healthy adults, it can cause a very severe infection in unborn babies and in people with immune system disorders. Anisakis simplex and related worms are found in raw or undercooked seafood.
As with viruses, parasitic infections are underreported due to poor testing and diagnostic methods. Trichinosis, caused by the parasitic worm Trichinella spiralis and associated with eating undercooked pork.

Bovine Spongiform Encephalopathy (BSE)

Bovine spongiform encephalopathy, or mad cow disease, is a fatal brain disease of cattle. The brain of affected animals appears sponge-like under a microscope.
The human form is Creutzfeldt-Jakob disease (CJD). It is a rare disease that occurs in approximately one person per million. A new type of CJD, new variant or nvCJD, has been linked to BSE exposure. This new variant of CJD differs from the traditional type mainly in that it affects younger people. Although scientists know little about the origin, transmission, and nature of spongiform encephalopathies, the most accepted theory is that the causative agent is a prion, a type of pathogenic protein.
Infective prions are thought to be found only in the brain tissue, spinal cord, and retina of infected animals, not in meat or milk products. Gelatin and beef tallow both undergo a manufacturing process thought to produce a product free of the BSE causative agent.

Major foodborne illness pathogens

Organism name Source and food involved Symptoms
  Bacillus cereus   Source: soil, dust.
Foods involved: rice dishes, pasta sauces, puddings, soups, casseroles, pastries, meat milk.
  Onset: 6-15 hours after eating.
Duration: 24 hours.
Symptoms: watery diarrhea, abdominal cramps, nausea, vomiting.
  Campylobacter jejuni   Source: soil, water, intestinal tracts of animals.
Foods involved: raw or undercooked poultry and meat, unpasteurised milk and dairy products.
  Onset: 2-5 days after eating.
Duration 2-10 days.
Symptoms : diarrhea (sometimes bloody), abdominal pain, fever, headache, possible complications of meningitis and arthritis.
  Clostridium botulinum   Sources: spores of these bacteria are wide spread in the soil and intestinal tracts of animals.
Produces a very potent toxin in an anaerobic(oxygen less) environment.
Foods involved : improperly processed canned foods, especially low acid foods and meats; garlic in oil products; smoked fish.
  Onset : generally 12-36 hours after eating, but range is 4 hours to 8 days.
Duration: several days to a year.
Symptoms: muscle weakness, dizziness, double vision, difficulty speaking and swallowing, progressive paralysis of the respiratory system that could lead to death.
  Clostridium perfringens   Source: sewage, soil, dust, water, intestinal tracts of animals and humans.
Foods involved: improperly held, cooled or reheated foods. Meat, poultry, stews, casseroles.
  Onset : 8-24 hours after eating.
Duration : 24 hours.
Symptoms : intense abdominal pain, diarrhea.
  Escherichia coli O157:H7   Source : E coli I a normal inhabitant of the intestines of all animals, including humans. E coli O157:H7 is a rare variety of E coli produces large quantities of toxins that cause severe damage to the lining of the intestine.
Foods involved : undercooked or raw ground beef, raw milk and cheeses made from raw milk, unpasteurised juices.
  Onset : 3-8 days after eating.
Duration : the illness lasts for an average of 8 days.
Symptoms : severe cramping, abdominal pain, and diarrhea which is initially watery but become bloody. Occasionally vomiting occurs. Low fever may be present. Some victims, particularly the very young and very old, develop hemolytic uremic syndrome (HUS), characterized by renal failure and hemolytic anemia. Can cause death.
  Listeria monocytogenes   Source : soil, water, intestinal tracts of animals. Can grow at low temperatures and can withstand heat, cold, and drying better than most pathogens.
Foods involved : unpasteurised milk, soft cheeses, undercooked meat and poultry, chilled ready-to-eat foods, raw vegetables if fertilized with manure containing the organism.
  Onset : 1 day to 3 weeks after eating.
Duration : unknown.
Symptoms : fever,headache,nausea,vomiting. Primarily affects immuno-compromised persons and pregnant women. Can cause fetal death, as well as meningitis, encephalitis, and septicemia.
  Salmonella   Sources : intestinal tract of animals and humans.
Foods involved : undercooked eggs, raw meats and poultry, unpasteurised milk and dairy products, shrimp, custards, sauces, cream desserts.
  Onset : generally 6-48 hours after eating.
Duration : 1-2 days or may be prolonged, again depending on host factors and ingested dose.
Symptoms : nausea, vomiting, abdominal cramps, diarrhea, fever, headache. Arthritis symptoms may follow 3-4 weeks after onset of above symptoms.
  Shigella   Sources : principally a disease of humans and other primates. The organism is frequently found in water polluted with human feces.
Foods involved: salads(potato, tuna, shrimp, macaroni, and chicken), raw vegetables, milk and dairy products, poultry. Contamination of these foods is usually through fecally contaminated water and unsanitary handling by food handlers.
  Onset : 12-48 hours after eating.
Duration : 1-2 days, may have symptoms for months.
Symptoms : abdominal cramps, diarrhea, fever, sometimes, vomiting. Stools may contain blood, pus, or mucus. Arthritis is a possible complication following illness.
  Staphylococcus aureus   Source : humans and animals are the primary carriers. Staphylococci are present in the nasal passages and throats and on the hair and skin of 50 percent or more of healthy individuals.
Foods involved: meat and poultry products, eggs, salads (potato, tuna, shrimp, macaroni and chicken), cream-filled desserts, sandwich fillings, milk and dairy products. Foods that require considerable handling during preparation and that are kept at slightly elevated temperatures after preparation are frequently involved in staphylococcal food poisoning.
  Onset : generally ½ - 8 hours after eating.
Duration : 1-2 days.
Symptoms : diarrhea, vomiting, nausea, abdominal pain, cramps, and prostration. Rarely fatal.
  Vibrio vulnificus   Source : seawater contaminated with human feces.
Foods involved : raw or undercooked oysters, clams, shrimp, crabs.
  Onset : 2-76 hours after eating.
Duration : 1-8 days.
Symptoms : chill, fever, and/or prostration. People with liver conditions, low stomach acid, and weakened immune systems are especially susceptible. Death in those individuals is about 50 percent.
  Vibrio parahaemolyticus   Source: marine coastal environment.
Foods involved : raw or improperly cooked fish and shellfish. Infection is more likely in the warmer months of the year
  Onset : 4-96 hours after eating.
Duration : average is 2.5 days.
Symptoms : diarrhea, abdominal cramps, nausea, vomiting, headache, fever, and chills.
  Vibrio cholerae   Source : shellfish.
Foods involved : raw or improperly cooked shellfish.
  Onset : 48 hours after eating.
Duration : 6-7 days.
Symptoms : diarrhea, abdominal cramps, and fever.
  Yersinia enterocolitica   Source : soil, water, intestinal tract of animals, especially pigs.
Foods involved : meats, oysters, fish, and raw milk.
  Onset: 24-48 hours after eating.
Duration : 2- 3 weeks.
Symptoms : fever, abdominal pain, diarrhea and/or vomiting, may mimic appendicitis. Sometimes misdiagnosed as crohn’s disease. The major complication is the performance of unnecessary appendectomies, since one of the main symptoms of infections is abdominal pain of the lower right quadrant.
Protozoa and parasites
  Anisakis ap. and related worms   Source : parasites are in the flesh of fish.
Foods involved : fish- especially cod, haddock, fluke, pacific salmon, herring, flounder, and monkfish.
  Onset : 1 hour to 2 weeks after eating.
Duration : varies, worm must be coughed up or removed surgically.
Symptoms : acute abdominal pain, much like acute appendicitis accompanied by a nauseous feeling.
  Cryptosporidium parvum   Source : this single-celled protozoa infects humans and other animals.
Foods involved : contaminated water or any food contaminated by fecal matter.
  Onset : 1-12 days.
Duration : 2-4 days, up to 4 weeks.
Symptoms : severe watery diarrhea.
  Cyclospora cayetanensis   Very little is known about this organism.
Foods involved : contaminated water, as well as various types of fresh produce treated with contaminated water.
  Onset : 1 week.
Duration : days to weeks, may reoccur.
Symptoms : watery diarrhea, loss of appetite, weight loss, bloating, increased flatus, stomach cramps, nausea, vomiting, muscle aches, low-grade fever, and fatigue.
  Giardia lamblia   Source : single-celled protozoa that lives in the intestines of people and animals.
Foods involved : most frequently associated with consumption of contaminated water.
  Onset: 1-2 weeks after infection.
Duration : 4-6 weeks, but sometimes months to years.
Symptoms : diarrhea, abdominal cramps, nausea. Especially infects hikers, children, travelers, and institutionalized patients.
  Trichinella spiralis   Source : worm larvae in animal meat.
Foods involved : undercooked pork and wild game meats.
  Onset : 8-15 days.
Duration : days to weeks.
Symptoms : muscle pain, vomiting, nausea.
  Hepatitis A virus   Source: Hepatitis A virus is excreted in feces of infected people.
Foods involved : water, shellfish, and salads are the most frequent sources. Contamination of foods by infected workers in food processing plants and restaurants is common.
  Onset : 10-50 days after exposure.
Duration : 1-2 weeks, or several months if symptoms are severe.
Symptoms : fever, malaise, nausea, anorexia, and abdominal discomfort. After 3-10 days patient develops jaundice with darkened urine. Severe cases can cause liver damage and death.
  Norwalk virus   Source : human fecal contamination.
Foods involved : water is the most common source of outbreaks and may include water from municipal supplies, wells, recreational lakes, swimming pools, and water stored aboard cruise ships. Salads and raw or undercooked shellfish are the foods most often implicated in Norwalk outbreaks.
  Onset : 24-48 hours.
Duration : 24-60 hours.
Symptoms : nausea, vomiting, diarrhea, and abdominal pain. Headache and low-grade fever may occur.
Natural toxins
  Ciguatera   Source : fish accumulate this toxin from eating algae in tropical waters.
Foods involved : marine finfish most commonly implicated in ciguatera fish poisoning include the groupers, snappers, jack , mackerel, and triggerfish.
  Onset : within 6 hours after eating.
Duration : several days to months.
Symptoms : numbness and tingling, nausea, vomiting, diarrhea, headache, acute sensitivity to temperature extremes, vertigo, muscular weakness, arrhythmia, bradycardia or lachycardia, and reduced blood pressure.
  Scombrotoxin   Source : from consumption of foods that contain high levels of histamine.
Foods involved : fishery products that have been implicated in scombroid poisoning include the tunas (e.g., skipjack and yellowfin), bluefish, sardines, mackerel, amberjack, and abalone.
  Onset : 30 minutes after eating.
Duration : 3 hours to several days.
Symptoms : tingling or burning sensation in the mouth, rash on the upper body, drop in blood pressure. Frequently headaches and itching of the skin are encountered. The symptoms may progress to nausea, vomiting, and diarrhea and may require hospitilisation.
  Shellfish toxins   Source : many different toxins cause shellfish poisoning. Shellfish accumulate the toxins from feeding on “red tide” algae.
Foods involved : mussels, oysters, and scallops.
  Onset : few minutes to few hours, depending on the toxin.
Duration : varies with toxin.
Symptoms : tingling, burning, numbness, drowsiness, incoherent, speech, difficulty breathing, lack of muscle coordination.
  Tetrodotoxin   Source : this toxin is one of he most potent toxins in nature.
Foods involved : pufferfish, also known as fugu.
  Onset : 20 minutes to 3 hours after eating.
Duration : varies.
Symptoms : headache, epigastric pain, nausea, diarrhea, and/or vomiting may occur. The second stage of intoxification is charecterised by increasing paralysis. Many victims are unable to move, even sitting may be difficult. There is increasing respiratory distress. Paralysis increases and convulsions, mental impairment, and cardiac arrhythmia may occur. Death usually oocurs within 4-6 hours, with a known range of about 20 minutes to 8 hours.
  Aflatoxin   Source : produced by certain strains of the fungi Aspergillus flavus and A. parasiticus that grow in fields or during storage.
Foods involved : corn products, peanuts and peanut products, cottonseed, milk (from cows eating moldy feed), and tree nuts such as brazil nuts, pecans, pistachio nuts, and walnuts.
  Onset : varies with dose.
Duration : varies.
Symptoms : acute short term- fever, jaundice, swelling, vomiting, pain, enlarged liver. Chronic long term- cirrhosis and cancer of the liver.
  Ergot   Source : toxin produced by the fungus claviceps purpura.
Foods involved : rye, wheat, barley, oats.
  Onset : varies with dose.
Duration : varies with dose.
Symptoms : gangrene, convulsions, dementia.


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