Foods that allow microorganisms to grow are called parasites. True or false?

Foods that allow microorganisms to grow are called parasites. True or false? It is false. Microorganisms require food substances in order to grow. These foods are often known as substrates.

In the laboratory, microorganisms are grown on growth media which are designed to simulate the substrates they would encounter in nature. Microorganisms can be heterotrophic or autotrophic.

Serratia marcescens bacteria colonies on an agar plate
Serratia marcescens bacteria colonies on an agar plate

Heterotrophic bacteria depend on a food substrate while autotrophic bacteria do not entirely depend on a substrate for food. This is because heterotrophs have to take in food particles while autotrophs use sunlight to make food.

A microorganism needs to live on a substrate that provides all the nutrients and minerals that are needed for it to be able to survive, grow and reproduce. Substrate type will vary depending on the type of species.

Substrates also occur inside organisms, for instance, we know that animals have bacteria living in the intestine that help to break down food and form minerals. Other food substrates can be surprising, for instance, there are bacteria that feed on oil, and help in the cleanup after an oil spill.

Microscopic fungi, protists, and algae are also considered to be microorganisms along with bacteria. Fungi do feed on substrates as well, for instance, Penicillium mold which feeds on fruit.

Algae do not need a substrate for food as they use sunlight to form food by photosynthesis. Foods that microorganisms feed on are not parasites, but microorganisms can themselves be parasites.

Cyanobacteria are a type of bacteria that can also produce sugar from photosynthesis. Autotrophs still do require other minerals taken up from the environment to help make sugars in photosynthesis.

Most bacteria and all fungi do rely heavily on taking in nutrients from some sort of substrate. To grow bacteria in the lab, scientists need to use a growth medium that simulates the natural substrate of a particular species of microorganism.


A substrate needs to supply all the nutrients a microorganism needs and it needs to be at the correct pH and temperature. This is because pH and temperature will affect the enzyme activity of the microorganism.

Bacteria such as Lactobacillus can grow at a low pH (below pH 7), while others, such as Vibrio cholera grows at a pH of 9.0.

The temperature may also influence the growth rate of the microorganism. Each organism has a  temperature range at which it grows best.

It is important that the temperature is not too high as most bacteria will die at a high temperature. However, there are a few species that can survive and even thrive at high temperatures.

Some microorganisms may need oxygen for cellular respiration, while others do not need oxygen. This is also important in ensuring the survival of the microorganism.

Microorganisms that feed on waste can often survive in oxygen-poor environments while other species require oxygen. Species that require oxygen to grow and survive, include Pseudomonas bacteria, while species that don’t and can’t use oxygen include Clostridium bacteria.

Other species of bacteria can live for a short while without oxygen. These are facultative anaerobes such as Staphylococcus and Escherichia coli.

Many microorganisms live in water and use nutrients in the water to live and grow. Some protists, for instance, may feed on bacteria that live in the water.

Bacterial species abundance and diversity are also different in different aquatic environments because of differences in the nutrients in the water.

Researchers have found that pH, oxygen levels, and nutrients all influence the bacterial community in water. The same is true of bacteria in other habitats, such as in the soil.

Human impact

Some microorganisms are parasites that harm their hosts. For instance, the malaria parasite Plasmodium, which is a protist that lives inside red blood cells.

However, many microorganisms are useful to us. For example, our gut bacteria help to digest our food and keep harmful bacteria under control. We have also used bacteria to produce food and alcoholic drinks commercially.

Commercial application

We have taken advantage of the break down of substrates by yeasts and bacteria to produce foods and alcoholic beverages. Yeasts produce alcohol as a waste product of their fermentation of a food substrate such as malt, hops, and barley.

Beer is produced when hops, barley, and malt are used as substrates for the yeast. In comparison, wine is produced when grapes are used as the food for the yeast.

Growth media

A growth medium is an artificial substrate that scientists have developed to use to grow bacteria or other microorganisms.

The growth medium has to be sterile and has to supply all the nutrients that the organism needs to live and grow. In addition, the medium has to be incubated at the correct temperature.

This is because the microorganisms such as bacteria have specific temperature requirements for growth.

In the case of bacteria, growth media are mixed and sterilized by placing in an autoclave. An autoclave is a machine that sterilizes by using a combination of high moist heat (steam), and pressure. The medium has to be sterilized to make sure that only the microorganism of interest will grow.

Bacteria growth media take the form of agar plates or test tubes. Agar is a jelly-like substance which provides a good semisolid surface for bacterial growth.

Various nutrients are added to the agar depending on what species of bacteria is to be grown. Nutrients can be added using exact amounts of various inorganic salts and organic material.


Animal tissue and plant tissue may be added depending on species of bacteria. Some bacteria are cultured in a liquid broth rather than on agar plates.

Agar plates are made by pouring agar into a Petri dish and allowing it to set.  Agar can also be poured into test tubes to produce what is called a slant.

The bacterial species that the person wants to grow is then added to the growth medium and incubated. In other words, you leave the medium and bacteria to grow under good temperature conditions.

After a certain time period, you will be able to see a colony of bacteria with your eyes. This will contain thousands of individual bacterial cells.

A growth medium can be created for organisms other than bacteria. For instance, one can grow Paramecium, a protist, by adding hay to distilled water.

Amoeba can be grown by adding rice grains to a medium consisting of certain amounts of salts such as potassium chloride and magnesium sulfate.


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  2. Editors of Encyclopedia Britannica (2018). Growth medium. Retrieved from
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