University of Georgia Cooperative Extension Bulletin 817
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What are Bacteria, Yeasts & Molds?
Table of Contents
Looking Beyond What Your Eyes Can See.............................................3
The Universal Cell..............................................................................................3
Bacteria................................................................................................................4
Bacterial Eating Habits....................................................................................5
Growth and Importance to Food Industry...................................................5
Sugar-Loving Yeasts......................................................................................6
Growth and Importance to Food Industry...................................................6
Moisture and Air-Loving Molds..................................................................7
Growth and Importance to Food Industry...................................................7
Additional Information...................................................................................8
This publication is part of the UGA EFS “Starting a New Food Business” packet. For more information, please contact the UGA Food Science specialists.
Extension Food Science Outreach Program
240 Food Science Building
The University of Georgia
Athens, GA 30602-7610
(706) 542-2574
fax (706) 542-9066
Website: http://www.EFSonline.uga.edu
Grateful appreciation is expressed to the individuals and
organizations who contributed to this publication.
Prepared by
William C. Hurst, Extension Food Scientist,
James A. Christian, Former Extension Food Scientist,
and George A. Schuler, Former Extension Food Scientist
What are Bacteria, Yeasts & Molds?
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University of Georgia Cooperative Extension Bulletin 817
Looking Beyond What Your Eyes Can See
Just what are microorganisms? They are small living forms of life which we cannot see with the
naked eye. Bacteria, yeasts and molds are three types of microorganisms. Some people often confuse,
and almost always misunderstand, their functions, but they are just as real and alive as you are. They
eat and grow. They reproduce and die.
Have you ever wondered just how small microorganisms really are? Molds can be seen with only
slight magnification -- use of an ordinary magnifying glass. Yeasts must be viewed through a microscope that magnifies several hundred times. Bacteria can best be seen when studied with a more
powerful scope that enlarges 1,000 times.
Bacteria, yeasts and molds can be found everywhere. Scientists have gathered them from clouds
above mountain tops and in the deepest parts of the ocean. They are present on animals, people and
even in the air we breathe.
Microorganisms have a direct impact on our daily lives. Some are helpful. They aid our bodily processes by helping break down complex foods into simpler substances. Some, called germs, are harmful to us by the role they play in causing diseases.
The Universal Cell
All living organisms, large and small, have one thing in common – the cell. This is a tiny living factory capable of converting simple food substances into energy and new cell material and of reproducing itself.
Larger organisms, including people, are composed of
billions of cells with many different roles. They make
up your body’s parts from your brain to your big toe.
Microorganisms, on the other hand, are made up of a
very few or even a single cell capable of carrying on
all of life’s processes.
So a basic understanding of cell structure and function is essential to understanding the actions of bacteria, yeasts and molds.
Since the cell is the basic unit of all living things, you
might think it is a simple structure. Nothing could be
farther from the truth. The cell is complex in its makeup and its function. Many scientists have spent their
lives studying it.
The main parts of the cell are the nucleus, cytoplasm and cell wall. The nucleus is the control center.
It directs cell division or the formation of new cells. They cytoplasm contains the parts which convert
food material into energy and new cell material. The cell wall or membrane holds everything together
and controls the passage of material into and out of the cell.
University of Georgia Cooperative Extension Bulletin 817
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What are Bacteria, Yeasts & Molds?
Bacteria
Bacteria make up the largest group of microorganisms. People often think of them only as germs and
the harm they do. Actually, only a small number of bacteria types are pathogenic (disease causing).
Most are harmless and many are helpful.
There are thousands of different kinds of bacteria. Some differ only slightly and it takes a highly
trained person to identify them. There are also groups which differ greatly in growth habits and
appearance and are quite easily identified. But regardless of minor differences, most bacteria can be
classified according to the five basic cell shapes illustrated.
In addition to their different shapes, their cell arrangement varies. For example, some cocci are always
grouped in pairs (diplococci). Others are arranged in chains (streptococci). Still others are bunched
(staphylococci). Diplococci are the kind which cause pneumonia. Streptococci are often associated
with “strep throat”. Staphylococci are familiar to many because of their role in “staph infections” and
some types of food poisoning.
Bacteria also vary somewhat in size, but average about 1/25,000 inch. In other words, 25,000 bacteria
laid side by side would occupy only one inch of space. One cubic inch is big enough to hold nine trillion average size bacteria -- about 3,000 bacteria for every person on earth.
Microorganisms, including bacteria, can also be grouped according to their requirement for oxygen.
Some grow only in the presence of oxygen (aerobes). Others grow only in the absence of oxygen (anaerobes). Some are able to grow with or without oxygen (facultative anaerobes).
Under natural conditions, anaerobes grow only in places protected from the air, such as deep in the
soil or under water. They can also grow under man-made anaerobic conditions, such as in canned or
vacuum packed foods which have not been processed or handled properly.
What are Bacteria, Yeasts & Molds?
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University of Georgia Cooperative Extension Bulletin 817
Bacterial Eating Habits
Bacteria and other microorganisms need food in order to grow and multiply. They vary in their food
needs, but nearly everything we consider as food can also be used as food by some type of bacteria.
To be used by bacteria, a food substance must pass into the cell where it can be processed into energy
and new cell material. Because most foods are too complex to move into a bacterial cell, they must
be broken down into simpler substances. Enzymes do this by increasing the rate of biochemical reactions.
Produced within the bacterial cell, enzymes move through the cell wall to break down the food on the
outside into a form bacteria can use.
Growth and Importance to Food Industry
Bacteria reproduce by a process called binary fission -- one cell divides and becomes two. Some can
reproduce at a very rapid rate under proper conditions. If food and moisture are adequate and the
temperature is right, certain bacteria can reproduce in as little as 20 minutes. Within 20 minutes, one
cell becomes two; in 40 minutes, there will be four, and so on. In only eight hours, the original cell
will have multiplied to nearly 17 million new bacteria. Of course, conditions don’t remain favorable
for such a rate of reproduction for long. If they did, we could be buried in bacterial cells.
We must use a microscope capable of magnifying 500 times in
order to see a single bacterial cell. However, if that cell is allowed to grow on suitable food or solid media, it will reproduce
rapidly into a colony consisting of millions of cells. The colony
is visible to the naked eye. The photo shows hundreds of bacterial colonies which grew and reproduced within 24 hours after a
dirty apron was touched to a place of nutrient agar.
Plate counting, a technique by which colonies of bacteria are
grown from single cells, makes it possible to determine the number of bacteria in a food sample without the aid of a microscope.
Bacterial colonies from a dirty apron after
24 hours.
An important thing to consider in the growth of bacteria is the
ability of certain types to produce spores. A spore is a dormant
or resting state of a bacterial cell.
There are certain basic differences between spores and active or vegetative bacterial cells. Spores are
not easily killed. In fact, conditions which will quickly kill active bacteria have little or no effect on
spores. A temperature of 180oF will kill bacterial cells within minutes, but bacterial spores can resist
this temperature indefinitely. This is important to us in that all processing times for canned foods are
calculated by using both the time and temperature required to kill bacterial spores.
Another important method of classifying bacteria is by their ability to grow and reproduce at various
temperatures. Some are cold loving and grow well under refrigeration. Called psychrophilic bacteria,
this group grows at temperatures less that 50oF. They are responsible for many types of spoilage in
refrigerated foods, such as slime formation on meats and ropiness in milk.
The second group – mesophilic bacteria – includes those which cause disease and food poisoning.
They grow only at moderate temperatures of 50oF to 110oF. You can see, therefore, why it’s so important to store unprocessed foods under refrigeration.
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What are Bacteria, Yeasts & Molds?
The third group – called thermophiles – grows well at temperatures between 110oF to 140oF. These
bacteria can cause spoilage in under-processed canned foods.
As noted earlier, microorganisms are essential in the production of certain foods. Bacteria are responsible in whole or in part for the following:
Buttermilk
Cheeses
Cocoa
Coffee
Pickles
Sauerkraut
Vanilla
Vinegar
Yogurt
Sugar-Loving Yeasts
Yeasts are small, single-celled plants. They are members of the family fungi (singular, fungus), which
also includes mushrooms. Fungi differ from other plants in that they have no chlorophyl.
Bacteria thrive on many different types of food. But most yeasts can live only on sugars and starches.
From these, they produce carbon dioxide gas and alcohol. Thus, they have been useful to man for
centuries in the production of certain foods and beverages. They are responsible for the rising of
bread dough and the fermentation of wines, whiskey, brandy and beer. They also play the initial role
in the production of vinegar.
Growth and Importance to Food Industry
Some yeasts are psychrophilic and so they can grow at relatively low temperatures. In fact, the fermentation of wines and beer is often carried out at temperatures near 40oF. Because some kinds are
psychrophiles, they can create a spoilage problem in meat coolers and other refrigerated storage
areas.
Unlike bacteria, which multiply by binary fission, yeasts
reproduce by a method called budding. A small knob or
bud forms on the parent cell, grows and finally separates to
become a new yeast dell. Although this is the most common method of reproduction, yeasts also multiply by the
formation of spores.
Because they can grow under conditions of high salt or
sugar content, they can cause the spoilage of certain foods
in which bacteria would not grow. Examples are honey,
jellies, maple syrup and sweetened condensed milk. Foods
produced by the bacterial fermentation process, such as
pickles and sauerkraut, can also be spoiled by yeasts which
interfere with the normal fermentative process.
Certain yeasts are pathogenic. However, yeast infections are much less common than are bacterial
infections.
What are Bacteria, Yeasts & Molds?
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University of Georgia Cooperative Extension Bulletin 817
Moisture and Air-Loving Molds
Probably the best known microorganisms, molds are widely distributed in nature and grow under
a variety of conditions in which air and moisture are present. They are also plants and a part of the
fungi family.
Nearly everyone has seen mold growth on damp clothing and old shoes. So, many may find it hard
to believe that mold is a microorganism. However, the mold we see with the naked eye is actually a
colony of millions of mold cells growing together.
Molds vary in appearance. Some are fluffy and filament-like; others are moist and glossy; still others
are slimy.
Growth and Importance to Food Industry
Unlike bacteria, molds are made up of more than one cell. Vegetative cells sustain the organism by
taking in food substances for energy and the production of new cell material. Reproductive cells produce small “seed” cells called spores. Unlike bacterial spores, mold spores are the source of new mold
organisms. Bacterial spores generally form only when environmental conditions are unfavorable.
Molds produce a stem consisting of several cells. Together, these cells form a “fruiting body”. The
fruiting body produces the spores, which detach and are carried by air currents and deposited to start
new mold colonies whenever conditions are favorable.
Mold spores are quite abundant in the air. So, any food allowed to stand in the open soon becomes
contaminated with mold if adequate moisture is present. Some types of molds are also psychrophiles
and can cause spoilage of refrigerated foods.
Molds are important to the food industry. Among
their many contributions are the flavor and color
they add to cheeses and the making of soy sauce.
They also play a role in the making of such chemicals as citric and lactic acid and many enzymes.
Molds can also cause problems in foods. Certain
kinds can produce poisons called mycotoxins.
Mycotoxins have only recently been discovered
and little is known about what causes molds to
produce them.
Probably the best known use of molds is in the
drug industry, where they help produce such antibiotics as penicillin.
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What are Bacteria, Yeasts & Molds?
Additional Information
This publication is a brief introduction to the field of microbiology -- the study of all invisible forms
of life. If you would like to know more about this field, we recommend one or more of the following
books. Two of them deal directly with the role microbiology plays in the food we eat.
Food Microbiology. Frazier, W. C. McGraw-Hill Publishing Company, 1967.
Microbiology (Second Edition). Pelczar, Michael J., Jr. and Roger D. Reid. McGraw-Hill, 1965.
Practical Food Microbiology and Technology. Weiser, Harry H. AVI Publishing Company, Inc., 1962.
Bulletin 817 / Reviewed March 2010
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What are Bacteria, Yeasts & Molds?
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University of Georgia Cooperative Extension Bulletin 817