The goal of food safety management systems like HACCP is to reduce hazards to safe levels or to eliminate them where possible. Hazards may be biological, chemical or physical. Of these, biological hazards pose the greatest risk to food safety. Immuno-compromised individuals such as pregnant women, the sick, babies, and the elderly are particularly susceptible to foodborne injury. Biological hazards include bacteria, viruses, molds, yeasts and parasites. It is important to have a basic understanding of these biological hazards in foods so that you will know how to control them.


Bacteria are single-celled microorganisms with no nucleus or membrane-bound organelles. They possess cell walls that may consist of a thin or thick peptidoglycan layer used to class them into two groups i.e. gram positive or gram negative bacteria respectively. Under a light microscope (X100 objective lens) bacteria mostly appear as either round (cocci) or rod-like (bacilli) cells but they may also be spiral (spirilla), cork screw (Spirochetes), or comma-shaped (vibrios).

Structure of bacteria
Gram positive and gram negative bacteria. Notice difference in peptidoglycan layer (blue). Creative Commons License.
Slide showing gram positive (purple) and gram negative (pink) bacteria. Creative Commons License.

Bacteria can live in or on the surface of other living organisms including human skin and the gastrointestinal tract. They are also ubiquitous in all types of physical environments. Many are harmless, such as the ones used to make yogurt and the ones in our gut. However, others are notorious for causing foodborne disease. We call these infectious biological agents pathogens. According to the Centers for Disease Control and Prevention (CDC), the most common foodborne illnesses caused by pathogenic bacteria comes from infection with SalmonellaClostridium perfringensCampylobacter, and Staphylococcus aureus.

The ability of bacteria to cause injury is dependent on the size of their population. Given the right conditions such as nutrients, pH, temperature, oxygen and moisture, bacteria can multiply every 20 minutes by a process called binary fission. The minimum number of bacterial cells that can result in illness is called the minimum infective dose (MID). Some bacteria have a low MID, making them highly infectious. For example, Escherichia coli (E. coli) with an MID of only 15 bacterial cells is much more infectious than Staphylococcus aureus (Staph) which has an MID of 100,000 cells.

Foodborne illnesses caused by ingestion of bacteria is referred to as food infection. In a food infection, the bacteria invades the intestine and other body structures causing illness. An infected person will have gastrointestinal symptoms such as diarrhea, stomach aches, nausea and vomiting. Foodborne illness caused by ingestion of bacterial toxins is called food intoxicationSymptoms of intoxication may also be gastrointestinal if the toxin is an enterotoxin, e.g. Staphylococcal enterotoxin. However, if the toxin is a neurotoxin, such as botulin from Clostridium botulinum, it can cause dizziness, blurred or double vision, the inability to swallow, muscle weakness and paralysis. The time that it takes from infection or intoxication to the manifestation of symptoms is called the onset time. Different bacteria can take different onset times ranging from hours to several days. This often makes it hard to identify which food was responsible for our foodborne illness. Was it our last meal, or was it something we ate last week?

How Bacteria Cause Injury

Top Four Foodborne Bacteria Based on the CDC

BacteriaMain SourceOnset TimeSymptoms
SalmonellaIntestine of humans and animals6hrs – 3 daysDiarrhea, fever, stomach cramps, nausea, vomiting, and head ache
Campylobacter jejuniIntestine of humans and animals2-5 daysFever, stomach cramps, nausea, and vomiting
Clostridium perfringensSoil, intestine of humans and animals, low-oxygen environments8-24 hours Diarrhea and stomach cramps
Staphylococcus aureusSkin and nasal passage of humans and animals1-2 daysNausea, vomiting, stomach cramps, and diarrhea


Viruses are very tiny infective agents much smaller than bacteria. They are so small in comparison, that they can invade and multiply inside of a bacteria cell. Their structures are simple, consisting of just a nucleic acid and a protein coat.

Structure of virus. Creative Commons License.

Unlike bacteria that cannot survive on their own, they need a host to infect in order to multiply. Once they invade a host cell, e.g., bacteria or human cell, they are able to instruct the DNA of the host cell to make more viruses. Eventually they may cause lysis (bursting) of the cell. This is call lytic infection. Some viruses may not multiply in the cell immediately. Instead, they go through a latent phase of no activity for a while before manifesting later. We refer to this infection as latent infection. Viruses are generally excreted from the body via the throat, nasal cavity, and stool. Therefore, proper handwashing and personal hygiene are critical to controlling these pathogens. Viruses of major concern are Norovirus and Hepatitis A. Fortunately, viruses have poor heat resistance, so they can be inactivated by proper cooking at temperatures reaching 85oC or higher. For this reason, viruses are not a major concern with products that are prepared using high temperature cooking. However, they are very problematic with minimally processed and ready-to-eat foods.


Molds are a type of fungus. They are multi-cellular in structure and so can be seen with the naked eye. This is in contrast to bacteria and viruses which require the aid of a microscope or electron microscope respectively to observe. You probably have seen molds growing on bread, strawberries, on walls, and even in your refrigerator. They are very adaptable, being able to grow in a wide variety of harsh environments.

Moldy Bread. Creative Commons License.
Mold structure under microscope. Creative Commons.

However, they thrive best in environments that are damp and have temperatures between 25oC – 30oC. Grains must be stored below 13% moisture to prevent mold growth and production of a type of mycotoxin called aflatoxinAflatoxins are potent carcinogens. Since molds are aerobic, they need oxygen to grow. They have less resistance to heat than bacteria, and so proper cooking, high enough to kill bacteria will eliminate molds.


Yeasts are single-celled microorganisms but are much larger than bacteria. They are generally egg-shaped and grow by budding. This is a process by which a small bud grows on the parent cell which gradually enlarge and then break off to make a new cell. This process continues with each cell producing buds that form new cells.

Saccharomyces cerevisiae (Yeast) Yeast. Notice new buds being formed. Creative Commons License.

Yeasts are not a food safety concern but can cause spoilage due to fermentation, resulting in production of carbon dioxide gas and alcohol. They can be destroyed by heat as little as 77oC. Therefore, if yeast contamination is evident, then it is usually an indication of gross undercooking or post-process contamination.


Parasites are organisms that live in or on host organisms in order to get their food and survive. The three classes of parasites are protozoa, helminths, and ectoparasites. Protozoa microscopic parasites that are generally transferred to humans by a vector such as mosquito or other insect in which the vector lives. Once in the body they can infect the blood, intestine, or other tissue causing serious illness. Helminths are worms such as flatworms, thorny head worms, and round worms. Unlike protozoa they are macroscopic which means they can be seen by the naked eye. They are capable of infecting the gastrointestinal tract, blood and other tissue. For example, tapeworm and roundworm larva called cysticerci (cysts) can live in the meat muscles of beef and pork. If the meat is not cooked properly, the larvae can infect the intestinal cell walls of humans and develop into adults to produce eggs which pass through the feces. If the infected individuals do not follow good hygiene practices such as washing their hands after using the toilet, they can transfer the eggs to food during handling and hence infect other people. Once eaten, egg embryos (oncospheres) penetrate intestinal cell walls and migrate to muscles to where they develop into cysticerci. This condition is known as cysticercosis.

USDA inspectors look for signs of cysts in meat and meat organs (kidney, liver and heart) during post-mortem inspections to ensure that infected meat is removed and not allowed to reach commerce. However, meat that is moderately infected with cysts may pass an inspection. These cysts are subsequently destroyed by freezing and cooking. For example, freezing pork that is less than 6 inches thick at -15oC for 20 days, can destroy cysts and worms. Heat treatment of 71oC can also destroy cysts and worms in pork.

Ectoparasites are macroscopic organisms that live on the skin of other organisms such as humans. They cause damage, irritation, and are vectors of disease-causing microorganisms. They can live on the skin for months. Example of common ectoparasites of concern include ticks, fleas, lice and mites.

Courtney Simons
Courtney Simons is a food science professor. He holds a BS degree in food science and a Ph.D. in cereal science from North Dakota State University. He also holds Masters degrees in both Environmental Science and Instructional Design from Wright State University.
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