Microorganisms are living organisms that cannot be seen with the naked eye. They are so small that you need a microscope to see them. Some are good and useful, for example the bacteria in our intestinal tract that aids in digestion. Some are used in food processing such as bread, wine, cheese and yogurt production. Others are needed in helping to breakdown and recycle organic materials that are used as fertilizers. Disease-causing microorganisms (called pathogens) are the ones that causes us to get sick and therefore must be carefully controlled. Sicknesses caused by undesirable microorganisms are called foodborne illnesses. For example Hepatitis A virus may cause liver infection. Some microorganisms produce harmful toxins. When these toxins are ingested, they result in illnesses by intoxication. These toxins may attack the gastrointestinal system (enterotoxins) or the nervous system (neurotoxin). Toxins are not broken down by heat, so proper cooking will not improve the safety of the product if the toxin is already in the food.

Some bacteria may not produce a toxin but can cause an infection when we eat foods containing microorganisms above the minimum infective dose (MID). The MID for some organisms may be large (as many as 100,000 cells) but could also be very small (as little as 10 cells). People with weak immune systems are more prone to infection. Pregnant women, infants, the elderly and the sick are referred to as immuno-compromised and are more susceptible to infections than others.

Microorganisms of significant public health interest include bacteria (the main one), yeast, mold, viruses and parasites. Based on the Center of Disease Control (CDC) the most common foodborne illnesses are those caused by the bacteria Campylobacter, Salmonella, Listeria monocytogenes, Escherichia Coli O157:H7 and the norovirus.


Bacteria vary in size between 1/25000 to 1/1000 of an inch. They come in different shapes, but are primarily round (called cocci) and rod shaped (called bacilli). Some produce spores. These are single-celled reproductive bodies that are capable of reproducing whenever conditions are ideal. Otherwise, they stay dormant (inactive). They are very resistant to harsh conditions such as exposure to chemicals and high temperatures. They are like seeds that are not growing but remain protected until conditions such as food, temperature and water are right for growth. At that time, they convert into vegetative cells which is the growing form of bacteria. Vegetative cells are much more susceptible to kill and control since they are less protected. The job of the food processor is to ensure that spores are never exposed to conditions that will promote their growth and result in spoilage and injury. For example, foods can be heated to a lethal temperature to kill vegetative spores and then cooled quickly so that bacteria stays for as little time in the danger zone as possible (i.e. 4oC – 60oC or 40oF-140oF).

Bacteria replicate by a process called binary fission where the bacterial cell grows and splits itself into two cells. Likewise, those two divide to make 4 then 8, 16, 32, 64 and so on. Given the right conditions, bacteria will double every 20-30 minutes. The rate at which they grow is called the bacterial growth rate. Bacterial grow rate is represented by four phases of growth:

  1. Lag phase: very little to no growth as the bacteria is getting adjusted to its environment
  2. Log phase: rapid and exponential growth as the bacteria is exposed to its ideal conditions for growth
  3. Stationary phase: the rate of growth is the same as the rate of death as the bacteria begins to compete for food and tries to survive in a hostile environment containing toxins from its own waste
  4. Death phase: bacteria are dying at a faster rate than they are growing. At this stage, some may develop spores to secure the next generation

Bacteria are different in their transmission, source, immediate symptoms, long-term consequences, onset time, MID and length of disease. Several of these are covered in the supplementary materials provided. Please review.


Molds are multicellular and therefore are much larger than bacteria. They reproduce by fruiting bodies called spores. They resemble tiny plants with a mesh of tiny interconnected tubular roots. Molds are what you see on moldy bread. They are much more tolerant to cool temperatures than heat and therefore may even be in your refrigerator. They enjoy moist environments and need oxygen to grow. They produce mycotoxin which can cause illness by intoxication. Generally though, molds do not pose much of a public health problem since they are easily controlled by heat. Mold are sometimes used in food processing such as in the ripening of cheese and sausages.


These are single-celled organisms. They are smaller than molds but bigger than bacteria. They are egg-shaped are replicate by budding. Budding occurs when a small protrusion (bud) bud forms on the parent cell and becomes enlarged until it breaks off to form a new cell. Like molds, they are tolerant to cold and offer little resistance to heat. Yeasts are used in the fermentation of breads and wines. Yeast growth in foods may cause spoilage, but is not a public health concern.


These are the smallest of the microorganisms. They are so small that you need a special type of microscope called an electron microscope to see them. They are made up of a single stand of genetic material protected in a casing of protein and fat. Viruses are capable of inserting its genetic code into a host and using that host as a copying machine to run many copies of itself. They are unable to persist outside of the host and are susceptible to heat. As long as foods are prepared under sanitary conditions and properly heat treated, virus infections from food should not be a major problem.


Parasites may be single-celled organisms (protozoa) or they may be large enough to see with the naked eye (worms). They do not replicate independently, but require a living host. Sometimes they use one host and in some cases more than one; as in cattle followed by humans when we eat meat. Examples of parasites of public health interest include beef and pork tape worm (Taenia aginata and Taenia solium) and round worm (Trichinella spiralis) in pork. Infections can be caused from drinking infected water or eating meat that is undercooked.

Sources of Food Contamination

Raw materials are the primary source of contamination. Common ways in which microorganisms enter foods products include:

  1. Soil, water, and in plant environment
  2. Animal feeds
  3. Animal hides
  4. Gastrointestinal tract of animals
  5. Employees
  6. Food utensils and equipment
  7. Food ingredients
  8. Air and dust
  9. Condensation
  10. Vegetable and vegetable products

Conditions Affecting Microbial growth

Understanding the conditions necessary for microbial growth will help us to control them. An easy way to remember these conditions is using the acronym FATTOM which stands for food, acidity, time, temperature, oxygen, and moisture.

Food: provides suitable energy source for growth of microbes. By following good manufacturing practices (GMPs) such as proper cleaning and sanitizing, we can help to reduce exposure of food surfaces to bacteria

Acidity: the pH (an index of acidity) of foods will determine how fast bacteria will grow. All bacteria have an optimum pH level at which they grow. Acidic environments inhibit bacterial growth and makes bacteria more susceptible to thermal kill. Foods can be acidified (lowering of pH) by adding various food-safe acids such as ascorbic, citric and acetic acid (vinegar). Foods with pH above 4.6 are classified as “low acid foods”. A pH of 4.6 or below is important to control bacterial growth including spores. Yeasts and molds favor acidic environment more than bacteria, but have a wide pH range (2-11).     

Time: the longer microorganism stay in the danger zone, or are exposed to ideal conditions, the more they will grow. Therefore exposure time in ideal conditions should be minimized e.g. removing waste by cleaning in a timely manner, and cooling quickly. Controlling time and temperature are important parts of many HACCP plans.

Temperature: different bacteria have different optimum temperature for growth. Based on optimum ranges, they can be classified as:

  1. Psycrophiles and Pychrotrophs: cold-loving microorganisms with optimum temperature of 59oF – 68 oF (15oC – 20oC) and 77oF and 104oF (25oC-40oC)
  2. Mesophiles: warm-loving microorganisms with optimum temperatures of 86oF to 104oF (30oC to 40oC)
  3. Thermophiles: heat-loving microorganisms with optimum temperatures exceeding 40oC (104oF).

The majority of bacteria causing food safety problems are mesophiles

Oxygen: bacteria can be classified based on their need for oxygen

  1. Aerobes: require oxygen to grow
  2. Anaerobes: cannot grow if oxygen is present
  3. Obligate anaerobes: must avoid all oxygen
  4. Facultative anaerobes: will use oxygen if present, but can survive without it

Moisture: dissolved nutrients enter bacteria through their cell walls. Therefore, for bacteria to survive, they need to be surrounded by available water. This available water is referred to as water activity (aW). It indicates the amount of free (unbound) water present. Water activity ranges from values between 0.00 and 1.00 where 1.00 is the maximum amount of free water. Therefore, the water activity of flours would be closer to 0.00 while the water activity of a juice would be closer to 1.00. Water activity in meats can be controlled by adding salts, sugars and nitrate ingredients that are able to bind some of the water. Water activity of 0.7. 0.75 and 0.8 can activate growth of mold, yeast and bacteria respectively.

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.
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