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5.5: Bacteria and Humans

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    Where does cheese come from?

    Bacteria are often used to make cheese from milk. But making foods is not the only beneficial role of bacteria. For example, they also play an essential role in your gut!

    Helpful Bacteria

    Can we survive without bacteria? Could bacteria survive without us? No and yes. No, we could not survive without bacteria. And yes, bacteria could survive without us.


    Bacteria can be used to make cheese from milk. The bacteria turn the milk sugars into lactic acid. The acid is what causes the milk to curdle to form cheese. Bacteria are also involved in producing other foods. Yogurt is made by using bacteria to ferment milk (Figure below). Fermenting cabbage with bacteria produces sauerkraut.

    Yogurt is made from milk fermented with bacteria
    Figure \(\PageIndex{1}\): Yogurt is made from milk fermented with bacteria. The bacteria ingest natural milk sugars and release lactic acid as a waste product, which causes proteins in the milk to form into a solid mass, which becomes the yogurt.


    In the laboratory, bacteria can be changed to provide us with a variety of useful materials. Bacteria can be used as tiny factories to produce desired chemicals and medicines. For example, insulin, which is necessary to treat people with diabetes, can be produced using bacteria.

    Through the process of transformation, the human gene for insulin is placed into bacteria. The bacteria then use that gene to make a protein. The protein can be separated from the bacteria and then used to treat patients. The mass production of insulin by bacteria made this medicine much more affordable. During transformation, bacteria can take up any DNA from the environment. Therefore, transformation allows scientists to insert any DNA into a bacteria, potentially producing many different proteins. This makes the bacteria greatly useful to people.


    Bacteria also help you digest your food. Several species of bacteria, such as E. coli, are found in your digestive tract. In fact, in your gut, bacteria cells greatly outnumber your own cells!


    Bacteria are important in practically all ecosystems because many bacteria are decomposers. They break down dead materials and waste products and recycle nutrients back into the environment. The recycling of nutrients, such as nitrogen, by bacteria, is essential for living organisms. Organisms cannot produce nutrients, so they must come from other sources.

    We get nutrients from the food we eat; plants get them from the soil. How do these nutrients get into the soil? One way is from the actions of decomposers. Without decomposers, we would eventually run out of the materials we need to survive. We also depend on bacteria to decompose our wastes in sewage treatment plants.

    Harmful Bacteria

    With so many species of bacteria, some are bound to be harmful. Harmful bacteria can make you sick. They can also ruin food and be used to hurt people.


    There are also ways that bacteria can be harmful to humans and other animals. Bacteria are responsible for many types of human illness, including:

    • Strep throat
    • Tuberculosis
    • Pneumonia
    • Leprosy
    • Lyme disease

    Luckily most of these can be treated with antibiotics, which kill the bacteria. It is important that when a medical doctor prescribes antibiotics for you, you take the medicine exactly as the doctor tells you. You need to make sure the bacteria is killed.

    Food Contamination

    Bacterial contamination of foods can lead to digestive problems, an illness known as food poisoning. Raw eggs and undercooked meats commonly carry the bacteria that can cause food poisoning. Food poisoning can be prevented by cooking meat thoroughly, which kills most microbes, and washing surfaces that have been in contact with raw meat. Washing your hands before and after handling food also helps prevent contamination.


    Some bacteria also have the potential to be used as biological weapons by terrorists. An example is anthrax, a disease caused by the bacterium Bacillus anthracis. Inhaling the spores of this bacterium can lead to a deadly infection, and, therefore, it is a dangerous weapon. In 2001, an act of terrorism in the United States involved B. anthracis spores sent in letters through the mail.


    • Bacteria can be used to make foods and medicines.
    • Bacteria play an important role in animal digestion.
    • Bacteria recycle nutrients in the environment.
    • Bacteria are responsible for many types of diseases in humans.
    • Some bacteria can contaminate food and cause food poisoning.
    • Some bacteria have been used as biological weapons by terrorists.

    Explore More

    Use the resources below to answer the questions that follow.

    Explore More I

    1. How does the gut bacteria differ between the small intestine and the large intestine?
    2. What can happen if something causes a reduction of "good" bacteria in your gut?
    3. How do the number of bacterial cells in your intestines compare to the total number of cells in your body?

    Explore More II

    1. What is one of the uses of serotonin in the body?
    2. What have scientists discovered about the relationship between gut bacteria and serotonin?
    3. What do scientists hope to do with this information?


    1. How are bacteria helpful in ecosystems?
    2. How are bacteria beneficial to your health?
    3. List two foods produced with the help of bacteria.
    4. What are three examples of diseases caused by bacteria?
    5. How can you prevent food poisoning?
    6. How are bacterial diseases treated?

    5.5: Bacteria and Humans is shared under a CK-12 license and was authored, remixed, and/or curated by CK-12 Foundation via source content that was edited to conform to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

    CK-12 Foundation
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