Introduction to Microbiology

The person in charge (PIC) of a food establishment should have basic knowledge of microbiology and how bacteria, viruses, parasites and natural toxins play a role in foodborne illnesses.

The FDA estimates that foodborne diseases cause 76 million illnesses, 325,000 hospitalizations, and up to 5,000 deaths in the United States every year1. These preventable diseases are caused by microorganisms.

A microorganism is any living thing that is too small to see without the aid of a microscope. Most are so small that they must be magnified 1,000 times before they can be seen.

Some microorganisms are pathogens that can cause illness or disease others cause spoilage, which can result in objectionable textures and odors in foods.

Some microorganisms are beneficial; they are used to make products like cheese, bread, pickles, yogurt, beer and wine.

Bacteria, viruses and parasites are microorganisms. Visit the Foodborne Illness List by Food or Foodborne Illness List by Pathogen webpages for details.


Bacteria are living single-celled organisms. There are two general types of bacteria, spore formers and non-spore formers. Think of spores as a plant seed that will grow when conditions are favorable.

These spores are extremely resistant to heat, cold and chemical agents that would kill non-spore forming bacteria. They present a significant health hazard because they can survive pasteurization and sterilization and may reactivate after heating if the temperature of food falls below 135°F.

Spore formers include Clostridium perfringens, Clostridium botulinum, and Bacillus cereus.

Bacteria can cause foodborne illness through infection, intoxication or toxin mediated infection.

Foodborne Infection

Foodborne infection occurs when a microorganism is eaten with food and then multiplies and grows in the body. Since the infection is a result of growth in the body, the time from ingestion until symptoms start to show is relatively long – usually days.

Infection producing bacteria include Yersinia spp, Salmonella, E. coli O157:H7, Vibrio spp, and Listeria monocytogenes.

Foodborne Intoxication

Foodborne intoxication occurs when toxin producing bacteria grow in the food and release toxins into the food, which is consumed. It is the toxin produced by the microorganism that makes the person sick, not the microorganism. Since the illness is a result of absorption of the toxin by the intestinal tract, and not microbial growth in the host’s body, symptoms of intoxication start to show sooner than symptoms of foodborne infections – usually hours.

Toxin producing bacteria include Campylobacter, Clostridium botulinum, Bacillus cereus, and Staphylococcus aureus.

Toxin-Mediated Infection

Toxin-mediated infection combines features of foodborne infection and foodborne intoxication. The bacteria cause illness by producing toxins while growing in the human intestines. The time it takes for symptoms to show is generally, but not always, longer than those for intoxications, but less than those for infections.

Microorganisms that are capable of causing the toxin-mediated infection include all Shigella species, Clostridium perfringens, and E. coli 0157:H7.

Grow Conditions

Bacteria need six conditions to grow. These six conditions can be remembered by using the acronym, FAT TOM2. If you can remove any of these conditions, you will inhibit pathogen growth.

    • Food – Food, such as carbohydrates or proteins, provides the energy source the bacteria need to grow. Obviously, you can’t keep food out of a food establishment, but you can keep all areas clean and free of food debris.
    • Acidity – Pathogens (disease causing microorganisms) cannot grow in an acidic environment. Pathogens will not typically grow in food if the pH is below 4.6.
    • Temperature – Pathogens grow well at temperatures between 41°F and 135°F. This temperature range is known as the “temperature danger zone”. Keep foods below 41°F or above 135°F to inhibit pathogen growth. Check hot holding and cold holding equipment regularly to ensure it is maintaining proper temperatures.
    • Time – Pathogens need time to grow. Allow food to remain in the danger zone for as little time as possible. If you must hold potentially hazardous foods in the danger zone, develop a written process to use time as a control.
    • Oxygen – Some pathogens need oxygen to grow, some only grow in the absence of oxygen.
    • Moisture – Pathogens need moisture to grow. The amount of water available in a food is known as its Water Activity and is measured from 0.0 to 1.0. Water activity of 0.0 is no water, 1.0 is pure water. Pathogens generally need 0.85 water activity or higher to survive.


Unlike bacteria, viruses are not alive. They cannot grow or reproduce outside a host cell. Viruses are even smaller than bacteria and have a relatively simple form. A virus enters an acceptable host cell in order to reproduce and the host cell is either harmed or killed by the invading virus.

Essentially all foodborne viruses are transmitted through a fecal-oral route. That means the virus is in the feces of an infected person. The infected person gets some of the virus filled feces on his or her hands, and then touches food, which transfers the virus to the food. The food is served to another person who ingests the virus and the cycle continues.

Hand washing is the most effective method for stopping the transmission of viruses in food. In addition to hand washing, bare hands should never touch ready-to-eat food. Employees who display symptoms of foodborne illness must be excluded from the facility. Viruses commonly associated with foodborne illness are Hepatitis A and Norovirus.


Parasitic infections from food are much less common in the United States than bacterial or viral infections. However, because the number of parasitic infections from food is rising, they are of growing concern.

The increase is due to globalization of the food supply, changes in eating habits, the popularity of raw or lightly cooked foods, increased international travel, an aging population, more people with deficient immune systems, and an increasing number of immigrants from areas where parasites are common.

Unlike bacteria, parasites are live multi-cellular organisms that do not multiply while in or on food. Like viruses, parasites require a specific host or chain of hosts to grow and reproduce themselves. Most parasites are transmitted from host to host. They may be transmitted from humans to humans and between animals (such as seafood) and humans.

Several parasites have emerged as significant causes of foodborne and waterborne illness and can be prevented by proper cooking or freezing. Like viruses, some parasites are transmitted through a fecal-oral route. Hand washing is critical to prevent the spread of such parasites.

Some common parasites are Giardia duodenalis, Cryptosporidium parvum, Cyclospora cayetanensis, Toxoplasma gondii, Trichinella spiralis, Taenia saginata (beef tapeworm), Taenia solium (pork tapeworm), and Anisakis simplex.

Natural Toxin

Toxins exist naturally in many plants, fungi and animals. Toxic substances may result from natural decomposition processes. Plants and animals that are normally safe can pick up natural toxins, chemicals and pollutants from their environment.

Common natural toxins are Ciguatera poisoning, shellfish toxins, scromboid poisoning, and mushroom toxins.

Prevention Practices

Use the following practices to prevent the spread of foodborne illness:

  • Get food from an approved source.
  • Use proper hygiene, hand washing, and gloves/utensils to avoid contamination of ready-to-eat food.
  • Keep cold food below 41°F to inhibit the growth of bacteria.
  • Keep hot food above 135°F to inhibit the growth of bacteria.
  • Cook food to recommended internal temperature to destroy vegetative bacteria.
  • Maintain a clean environment.
  • Cool food properly to prevent reactivation of spores.


  1. http://www.fda.gov/Food/ResourcesForYou/HealthEducators
  2. ServSafe Coursebook Fifth Edition, Page 2-3.

Contact Information

Telephone: (702) 759-0588


Updated on: January 4, 2022

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