MICROBIOLOGY OF FOODBORNE ILLNESS REVIEWThe vast majority of foodborne illnesses are the direct result of microbiological hazards. These hazards are caused by pathogenic (disease causing) microorganisms. Pathogenic organisms can include: bacteria, viruses, parasites, and mold. Contamination by bacteria is the number one cause of foodborne illness.
Microbiological Causes of Foodborne Illness
- Bacteria
Bacteria are single cell organisms which multiply and increase in number through cell division given the right environmental conditions. These ideal conditions include: temperature warmth (around 90°F), moisture (water activity, Aw) food source, desirable pH (7, neutral). Aerobic bacteria require oxygen, while anaerobic bacteria require no oxygen for cell division (reproduction). Many pathogenic bacteria are facultative and grow in either aerobic or anaerobic conditions.- Molds
Molds are a multi-cellular form of fungi, which can grow on almost any substance used for food if the conditions are right. Mold spore cases are present in the environment. These spore cases break and release thousands of microscopic mold spores, each capable of growing mold under the right conditions. These conditions include a damp, dark, environment which provide the right environment for the growth of spores which swell and burst their walls and send out short threads. These threads "root" into the food to provide nourishment for the mold spores. The tip end of the threads are spread out in the air above the surface of the food forming little goblues which breaks up to form new spores. As the spores on the surface of the food ripen the food develops an unpleasant musty odor which destroys the fresh flavor of food.
Fuzzy brightly colored mold growth can blow into the air and onto other foods. Molds cause a musty odor and destroy the fresh flavor of food. Certain molds may produce poisonous toxins called mycotoxins. Aflatoxin is a mycotoxin that grows on nuts, corn, wheat and other grains. It may be found in products made from these foods including breads and peanut butter. Ingestion of aflatoxin usually causes low grade fever in humans, but can produce cancer in trout rats, and ducks. Other illnesses thought to be made worse by the presence of aflatoxins include Reyes syndrome, cirrhosis and kwashiorkor (Jones, 1992, Jay, 1986).
However, some molds are beneficial and are used in the production of antibiotics such as penicillin and in soy sauce production. Molds are specifically grown to create blue cheeses such as Roquefort and stilton and that which grows on the rind of camembert providing distinct flavors.
- Viruses
Viruses are the smallest known living organism. They do not have a cell wall, a membrane or nucleus. When it reproduces, it takes over the life processes of the host cell. The host cell continues to live and reproduces other viral cells. The cells in food products are dead and the virus requires a living cell to reproduce, therefore, food products are only carriers. Water, air, soil, people and surfaces may also act as carriers for viruses.
Several viruses can be spread by people who handle food and do not have good personal hygiene habits. A person may excrete the virus organism in their feces, urine or through sneezing. If they do not wash their hands after using the restroom or sneezing before handling food, they will contaminate the food. Foods that are not often heated after handling such as bakery products uncooked oysters or clams, sandwiches, salads and desserts could transmit a viral illness.
- Parasites
Parasites are organisms that live or feed off another organism. In general, they may be found in raw animal products or seafood. They are destroyed by thorough cooking. Examples include: Trichinella Spiralis worm (a round worm) found in wild game or pork, and the Anisakis Spiralis, commonly referred to as "cod fish worm" or "seal worm" and found in fish.
- Yeasts
Yeasts are single-celled organisms which as they grow convert its food through the process of fermentation into alcohol and carbon dioxide. To multiply and grow, yeast needs moisture, food in the form of sugar or starch and a warm temperature 79°-80°F is best). Wild (naturally occurring) yeast spores are constantly floating in the air and can land on uncovered liquids and foods resulting in yeast contamination. In general, yeast contamination in food creates a slime on the food surface, bubbles and an alcoholic smell or taste. In cottage cheese, it may cause a pink discoloration. They can be destroyed by heating to 136°F for 15 minutes. In the food processing industry, carefully cultured yeasts are used in the production of beer, wine and bread.
Yeasts are responsible for few illnesses in humans and there is no evidence that they are transmitted by food or that the wild (naturally occurring) yeast in foods are harmful to humans.
Environmental Factors Affecting Bacterial Growth
Bacteria reproduce by cell division. Generally under the "right" conditions a single cell bacteria will reproduce every 20 minutes. A single cell bacteria can become 72 billion in 12 hours. The right optimal conditions vary with microorganisms.
Bacteria can exist in a vegetative state or dormant state. In the vegetative state, they reproduce and produce waste products. If the bacteria are spore formers they develop a thickened cell wall which allows them to survive environmental conditions such as cooking, freezing, high salt conditions, drying and high acid conditions. Spore forming bacteria are generally found in foods like vegetables that are grown in the soil and animal products. When the environmental conditions become acceptable they may return to the vegetative state and resume reproduction.
"RIGHT" Conditions For Growth Include:
- Food Source - In general, this would include high protein, low acid, moist protein rich foods. The Food and Drug Administration defines these foods as potentially hazardous. Potentially hazardous food means a food that is natural or synthetic and is in a form capable of supporting:
- The rapid and progressive growth of infectious or toxigenic microorganisms.
- The growth and toxin production of Clostridium botulinum or
- In shell eggs, the growth of Salmonella enteritidis.
Potentially hazardous food includes an animal FOOD (A FOOD of animal origin) that is raw or heat-treated; a FOOD of plant origin that is heat-treated or consists or raw seed sprouts; cut melons; and garlic and oil mixtures.
Potentially hazardous food does not include:
- An air-cooled hard-boiled egg with shell intact;
- A food with a water activity (Aw) value of 0.85 or less;
- A food with a hydrogen ion concentration (pH) level of 4.6 or below when measured at 24°C (75°F);
- A food, in an unopened hermetically sealed container, that is commercially processed to achieve and maintain commercial sterility under conditions of non-refrigerated storage and distribution; and
- A food for which a variance granted by the regulatory authority is based upon laboratory evidence demonstrating that rapid and progressive growth of infectious and toxigenic microorganisms or the slower growth of C. botulinum cannot occur.
Foodborne illness outbreaks attributed to fresh fruits and juices and vegetables are another risk. In response to this concern, FDA has issued recommendations that all fresh juice products be pasteurized.
- Temperature - The majority of bacteria are Mesophilic and grow best in the temperature range of grow 60-110°F which includes human body temperature (98.6°F). Potentially hazardous foods held in the Temperature Danger Zone-40°F-140°F, are at risk for increased bacterial contamination if they remain in this temperature range more
than 2 hours.
However, all bacteria do not have the same temperature requirements for growth. Psychrophilic bacteria grow in cold temperatures - 32-60°F and Thermophilic bacteria grow best in hot temperatures -110 -171°F.
According to the Food and Drug Administration (FDA) category's of pH Foods based on pH values are
pH Scale
berries
sauerkraut
tomatoes
peaches
apples
vinegarseafood
potatoes
beans (string, wax, green)
meat
corn
cow's milk
carrots
- Time - In general bacteria multiply rapidly, given warmth, moisture, and time. Under ideal conditions a single cell can produce over one million cells in five hours. Food should not be in the Temperature Danger Zone for more than two hours. The two-hour time frame is culminative and includes all steps in processing, preparation and serving of potentially hazardous foods.
- Air/Oxygen - Bacteria reproduce with or without the presence of oxygen. Aerobic bacteria require oxygen, while anaerobic bacteria require no oxygen. Facultative bacteria can survive in environments that may or may not have oxygen present. Most bacteria that are responsible for foodborne illnesses are facultative.
- Moisture - All bacteria need moisture in a useable form to grow and reproduce. The amount of water that is not part of the food and is available for use by bacteria for their growth is called water activity. The symbol for water activity is Aw. The water activity of water is 1.0. In general, the water activity of potentially hazardous foods ranges from 0.97-99. Freezing, drying, salting or adding sugar to a food reduces the amount of water available and slows or prevents bacterial growth.
Approximate Aw Values of Selected Foods
Water
Fresh meat, Poultry,Fish
Most cheese
Cured meat
Parmesan cheese
Dried whole milk
0.99 -0.99
0.100 - 0.99
0.101 - 0.95
0.102 - 0.76
0.02
Jams, jellies
Uncooked rice
Dried fruit
Cereal
Sugar
0.75 - 0.94
0.80 - 0.87
0.55 - 0.80
0.10 - 0.20
0.19Bacteria Growth Patterns
Bacterial growth, in the "right" conditions has a four stage pattern. The four stages are:
- Stage I-Lag Phase-Little or no growth occurs at this stage. They are becoming acclimated to their environment.
- Stage II-Log Phase-Rapid growth occurs as the bacteria have adapted to their environment and all the environmental conditions are "right".
- Stage III-Stationary Phase-The rate of bacterial growth is equal to the death rate. Numbers of bacteria do not increase at this stage.
- Stage IV-Decline (death) Phase-The rate of death exceeds the rate of growth. Many of the essentials needed for survival are no longer present or build up of by products from growth kills them off.
Creating Barriers to Microbial Growth
Each of the factors described contributes to the growth of pathogenic bacteria in food. However, it is the interaction between them which allows for extensive bacterial growth. A series of barriers which address the factors affecting bacterial growth need to be developed and put in place to reduce the risk of foodborne illness.Types of Foodborne Illness
A foodborne illness is a disease which is transmitted to people from the food they eat. An individual may get a foodborne illness from coming into contact with an individual who is ill with a foodborne illness. The transmission of the illness can be one, or all of the following:
- Ill human contaminates food eaten, makes someone sick
Person - Food - Person- Food contains harmful foodborne illness causing bacteria, person eats food, gets sick
Food - Person- Person to person contact
Person - Person
Definition of Foodborne Illness Outbreak
The Center for Disease Control and Prevention (1999) defines a foodborne disease outbreak "as an incident...in which 2 or more persons experience a similar illness after ingestion of a common food and epidemiological analysis implicates the food as the source of the illness." State and local regulatory agencies have established reporting requirements for suspected foodborne illness outbreaks. Check with your local health authority for this criteria.
Three types of foodborne illness
- Foodborne Infection - Direct result of consuming food containing harmful living microorganisms.
- Foodborne Intoxication - Direct result from consuming food containing toxins that have been produced by harmful bacteria. They can cause illness even after the bacteria is dead. These toxins may be naturally occurring in foods such as mushrooms, certain plants and animals (i.e., puffer fish).
- Foodborne Toxin-Mediated Infection - Results from consuming food containing harmful living microorganisms. Once in the human intestine they produce harmful toxins.
Revised 6/00
University of Rhode Island
Cooperative Extension Food Safety Education