Bacteria (Lesson 22)

Karen Overstreet, Roy, Heli J.


Did you ever have a 24-hour bug? Flu-like symptoms, such as nausea, vomiting, diarrhea, and stomach cramps, that last only a day or less are often from a food-borne illness. Harmful bacteria (germs) can be in food. Proper food handling can keep food safe. The first step in understanding food-borne illness is to learn how and why disease-causing bacteria spread. Then you are better able to prevent food-borne illness.

What You Will Learn

Upon completing the lesson, you should be able to:

  • Define food-borne illness.
  • Identify some foods that are most commonly involved in food-borne illness.
  • Identify the system that reduces the risks of microbial contamination.
  • Demonstrate an understanding of bacteria's role in food-borne illness by
    • identifying five factors that bacteria depend upon for growth.
    • listing the temperature range (danger zone) in which bacteria quickly multiply.
    • stating the pH range that inhibits bacterial growth.

The characteristics of food-borne disease have changed over the past 50 years:

  • There are new pathogens.
  • There are new modes of transmission.
  • We have a more global food supply.
  • We have changing consumer preferences and food choices.
  • There are new issues related to antibiotic resistance.

New technology, new safety programs, and new research-based regulations can help ensure the safety of high-risk foods. But what can we do to make our food as safe as possible? The future of food safety depends on education. The responsibility for keeping food safe lies with the person preparing food. That means each and every one of us, whether we are a farmer, processor, food service worker, grocery store manager, or consumer.

Food-borne Illness

A food-borne illness is a disease that is carried or transmitted to human beings by food. The potential to cause food-borne illness can occur through errors in growing, transporting, selling, purchasing, receiving, storing, preparing, and/or serving food.  Since 1938, the Communicable Disease Center (CDC ) has published annual summaries on outbreaks of food-borne illness. An outbreak is defined as an incidence of food-borne illness that involves two or more people who eat a common food, which is confirmed through laboratory analysis as the source of illness. Young children, the elderly and those with a weak immune system are most susceptible to food-borne illness.

Any type of food can be the source for food-borne illness. Generally, it is high-protein foods that are responsible for most food-borne illnesses. High-protein foods are classified as potentially hazardous by the U.S. Public Health Service. They include any food that consists in whole or in part of dairy products or protein foods including eggs, meats, poultry, fish, shellfish, edible crustacea, tofu, and other soy-protein foods. Baked or boiled potatoes, plant foods that have been heat-treated, raw seed sprouts, or synthetic ingredients are also considered potentially hazardous. Potentially hazardous foods are capable of supporting the growth of pathogenic microorganisms. Pathogenic microorganisms are simply disease-causing germs that, if consumed, can cause food-borne illness or make us sick. Potentially hazardous foods do not include foods that have a pH (acidity) of 4.6 or below or an Aw (water activity) level of 0.85 or less under standard conditions.

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Factors Involved in Food Safety

Let's look at the three factors involved in food safety. Can you guess what they are?

Food: It should be purchased from a reliable grocer, and safe food handling should occur throughout storage, preparation, and service. (Remember, not all food is safe when you purchase it, such as fresh poultry.)

People: Those involved in every step of the food chain. (People pose the major risk to safe food, so education is the key.)

Preparation Area: The sanitary condition of the kitchen and the equipment used. (Well-designed equipment and proper procedures for cleaning and sanitizing of equipment, utensils, and food-preparation areas are imperative.)

The Hazard Analysis and Critical Control Points (HACCP ) Food Safety System:  Many processing plants are being required to test meat and poultry for bacteria using a HACCP system. HACCP is a means of controlling processes so the risks of microbial contamination are reduced. (Even with the best efforts of inspectors and industry, we all must realize that food-associated risks can never be zero. Why? Hint: Look at the three factors involved in food safety!) HACCP focuses on problem prevention. We can do the same in the kitchen.

Hazards to Safe Foods

Unsafe food usually results from contamination, which is the presence of harmful substances not originally present in the food.  Some food safety hazards are introduced by humans or by the environment, and some occur naturally.

First, we need to know there are three hazards to safe foods: biological, chemical and physical. Biological hazards cause the greatest percentage of food-borne illness outbreaks. The biological hazard consists of pathogenic microorganisms, viruses, yeasts and molds. Bacteria are the most common cause of food-borne illness. Knowing a little about bacteria and in what environment they flourish is the first step in controlling them.

Chemical hazards include pesticides, food additives, preservatives, cleaning supplies, and toxic metals that leach from cookware and equipment.

Physical hazards consist of foreign objects that accidentally get into the food, such as hair, dirt, metal staples, and broken glass.


A bacterium is a living organism that is made up of a single cell that needs nutrients to live. Bacteria in food causes illness in two ways. Some bacteria are infectious (those that cause illness in humans) and use the nutrients in potentially hazardous foods to multiply. When the contaminated food is eaten, illness may occur. Other bacteria are not infectious themselves. As these bacteria grow and die in the food, they discharge toxins. The toxins, not the bacteria, cause the food-borne illness after they are ingested.

A vegetative microorganism is capable of growth if all conditions are favorable. But most are killed by high heat. Some are resistant to low temperature and may survive freezing. This is why we say refrigerator or freezer storage does not ensure that bacteria will stop growing. However, they are slowed down tremendously at low temperatures.

A few bacteria have the ability to produce spores. Spores are difficult to destroy because they are thick-walled and lie within a vegetative bacterial cell. Spores themselves do not reproduce but lay dormant until conditions are favorable for growth. Then it becomes a vegetative organism, capable of multiplying. Spores are why we should keep harmful bacteria out of food in the first place and why there are recommended canning procedures for food preservation.

Bacteria Reproduction
Bacteria reproduce by enlarging and then dividing in two. Each divides into two more and this continues. Overnight, billions of bacteria can grow if conditions are right. To grow, bacteria must have food, the right acidity (pH), enough time at the right temperature, the right level of oxygen, and moisture.

Under ideal conditions, bacteria grow in a predictable manner.  Once they make it into the food (from a sneeze, unclean hands, unclean cutting board, etc.), they need to adapt to their new environment. This is the lag phase. With time, the bacteria multiply rapidly, known as the log phase. At some point, the bacteria will be at such large numbers, they compete for space and nutrients. Growth slows down and some begin to die in what is known as the stationary phase. As nutrients are used up and their waste products build up, the decline phase occurs as bacterial cells begin to die more quickly. Remember, the waste products or toxins can themselves cause food-borne illness, even if all the disease-causing bacteria are killed. How quickly or slowly the phases move along depends on the temperature, the food supply, the species of bacteria, the amount of initial contamination, and the age of the bacteria. You can see that temperature control and time are the key factors in preventing bacterial growth. If we can prevent bacteria from growing past the lag phase, we have more control in preventing food-borne illness.

Water, wind, food, insects, rodents or animals, including humans, allow bacteria to move from place to place. They are found everywhere, from the soil to the mucous membranes in our mouths and noses. Frequently they end up on our hands, which is how they end up in our food. Most bacteria thrive in a warm, moist, protein-rich environment that is neutral or low in acid. But some species can survive extremes of heat or cold, a low-moisture environment or a high-acid or salt content.

Food and Moisture
Bacteria need both food and water to grow. But since they have no teeth, they must ingest their nutrients in a water solution. Water activity (Aw) is the amount of moisture available for bacterial growth. Bacterial pathogens must have a water activity level above 0.85. Water activity can be lowered by freezing, dehydrating, or mixing with a dissolved substance like sugar or salt. The high sugar content of jams and jellies prevents pathogenic bacteria from growing because the sugar is dissolved in the water, making it unavailable for use by the bacteria. Dry noodles, flours, candy, jams, jellies, and crisp, cooked bacon all have a water activity level below 0.85. Raw bacon, soft cheese, meat, and poultry are examples of high-protein foods that are potentially hazardous because of the high water activity level.

Time and Temperature
Most bacteria reproduce rapidly between temperatures of 60 and 110 degrees F, which coincides with room and human body temperatures. Other species grow better between 110 and 130 degrees F. Still others prefer 32 to 45 degrees F but can grow at temperatures as low as 19 degrees F. Placing food in the refrigerator does not guarantee that bacteria present will not grow. Between 40 and 140 degrees F is known as the Danger Zone, where bacteria can survive and grow. Food should not be allowed in the danger zone for more than two hours once you have purchased it.

Bacteria pH Needs
The pH of a food is a measure of its acidity or alkalinity. Distilled water has a pH of 7.0, which is considered neutral. Any food with a pH below 7.0 is acidic. Pathogenic bacteria prefer a pH of 4.6 to 7.0, so foods within this range are considered potentially hazardous foods. Can you guess what they might be? Meat and poultry should be the first to come to mind. Commercial mayonnaise has an acidic pH of 3.0 and limes 2.0. Bacteria themselves cannot survive in plain mayonnaise. But adding chicken to make chicken salad raises the pH level. If the chicken is contaminated, the resulting pH level would be in the preferred range for pathogenic bacteria to grow.

Internet Activity: Visit Look for the tab Keep Food Safe and find Charts: Food Safety at a Glance.  You can find practical information about safe temperatures for various meats and other charts.

Oxygen Requirements

Bacteria vary in their needs for oxygen. Aerobic bacteria can grow only in the presence of plenty of oxygen. Anaerobic bacteria can grow only in the absence of oxygen. Anaerobes are the bacteria of concern in vacuum-packaged foods, cans or a large pot of food. If you read the ingredient label of these types of foods, you will find that an acidifier, salt or sugar has been added to help increase the safety and shelf life of vacuum-packaged foods. Cans or jars of food are subjected to intense heat under pressure to reduce the risk of bacterial contamination.

Microorganisms can cause both contamination and spoilage. Food that is not spoiled may be contaminated and unsafe to eat. Contamination usually occurs because of odorless and tasteless bacteria. Spoilage bacteria ruin the taste, aroma or appearance so you know the food is not safe to eat. If in doubt, always throw the food out without tasting. See a list of the major Food-borne Pathogens at the end of this section. Every day we find new strains of bacteria. We now have better detection methods, making an unknown bacterium of yesterday a well-known bacterium of today.

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Viruses, Parasites and Fungi

Viruses differ from bacteria in that they are not complete cells, only genetic material in a protein wrapper. Viruses do not increase in number while they are in food. They merely are transported into a susceptible human through the food. Inside the body, they gain entrance to a cell and force it to assist in producing more viruses. Most outbreaks of food-borne or water-borne viral disease are linked to poor personal hygiene or a contaminated water supply.

To survive, a parasite must have a host to live on or inside. A roundworm that prefers mammals is Trichinella spiralis. It is often found in hogs, bear, and moose. Except for arctic strains, freezing the meat at 5 degrees F for 30 days will kill the parasite. Cooking domestic pork to 145 degrees F and letting it rest for three minutes, while its temperature rises a few more degrees will also kill the parasite.

The fungi of most concern to us are yeasts and molds. Molds grow quickly and usually are seen as a fuzzy growth. Mostly, molds affect the quality of foods through spoilage with discoloration, a musty odor or off flavor. Molds can grow under almost any conditions, including those with a high-salt and -sugar content and low pH. Yeasts prefer the high sugar and moisture content typically found in jelly.

Major Food-borne Pathogens


Duration of Illness


Foods Implicated


Campylobacter jejuni

3-5 days/
1-4 days

Diarrhea, fever, nausea, abdominal pain, headache

Raw vegetables, unpasteurized milk and dairy products, poultry, pork, beef and lamb

Avoid cross contamination;
cook foods thoroughly


6-72 hours/
4-7 days

Abdominal pain, headache, nausea, vomiting, fever, diarrhea

Poultry and poultry salads, meat and meat products, milk, eggs, egg custards and sauces, and other protein foods

Avoid cross contamination; refrigerate food; cook meats and meat products properly; avoid fecal contamination by practicing good personal hygiene

Staphylococcus aureus

1-6 hours/
24-48 hours

Nausea, vomiting, diarrhea, dehydration

Warmed-over foods, ham and other meats, dairy products, custards, potato salad, cream- filled pastries and other protein foods

Practice good personal hygiene; proper heating and refrigeration of food

Escherichia coli

12-72 hours/
1-3 days

Bloody diarrhea, severe abdominal pain, nausea, vomiting, diarrhea, occasional fever

Raw and undercooked beef and other red meats, imported cheeses, unpasteurized milk, raw finfish, cream pies, mashed potatoes, other prepared foods

Cook beef and red meats thoroughly; avoid cross contamination; use safe food and water supplies; avoid fecal contamination by practicing good personal hygiene

Listeria monocytogenes

1 day to 3 weeks/ Indefinite

Nausea, vomiting, headache, fever, chills, backache, meningitis

Unpasteurized milk and cheese, raw vegetables, poultry and meats, seafood, prepared and chilled ready-to-eat foods

Use only pasteurized milk and dairy products; cook foods to proper temperatures; avoid cross contamination

Clostridium perfringens

8-22 hours/
24 hours

Abdominal pain, diarrhea

Meat that has been boiled, steamed, braised, stewed or roasted at low temperature for a long period of time or cooled slowly before serving

Use careful time-and-temperature control in cooling and reheating cooked meat dishes and products


1-7 days/

Diarrhea, fever, chills, lassitude, dehydration

Potato, tuna, shrimp, turkey and macaroni salads; lettuce; moist mixed foods

Avoid cross contamination; avoid fecal contamination by practicing good hygiene; use sanitary food and water sources; control flies

Bacillus cereus

8-16 hours/
12 hours

Nausea and vomiting, diarrhea, abdominal cramps

Rice and rice dishes, custards, seasonings, dry food mixes, spices, puddings, cereal products, sauces, vegetable dishes, meat loaf

Use careful time-and-temperature control and quick chilling methods to cool foods; hold hot foods above 140 degrees F; reheat leftovers to 165 degrees F

Clostridium botulinum

12-36 hours/Several days to a year

Vertigo, visual disturbances, inability to swallow, respiratory paralysis

Improperly processed canned foods of low acid foods; garlic in oil products; grilled onions; stews; meat & poultry loaves

Follow recommended guidelines for home-canned products; use careful time-and-temperature control for vacuum packaged foods; refrigerate vacuum packaged foods; purchase garlic in oil in small quantities for immediate use

2/4/2010 12:46:19 AM
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