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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Growth and Importance to Food Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
ADDITIONAL INFORMATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
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: www.EFSonline.uga.edu
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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.
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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.
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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.
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
Bacterial colonies from a dirty apron
active or vegetative bacterial cells. Spores are not easily
after 24 hours.
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.
The third group – called thermophiles – grows well at temperatures between 110oF to 140oF.
These bacteria can cause spoilage in under-processed canned foods.
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As noted earlier, microorganisms are essential in the production of certain foods. Bacteria are
responsible in whole or in part for the following:
Cocoa
Pickles
Vinegar
Coffee
Yogurt
Vanilla
Cheeses
Sauerkraut
Buttermilk
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.
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.
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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.
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.
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, Extension Food Scientist (Retired), and GEORGE A. SCHULER,
Extension Food Scientist (Deceased). This is a revision of Bulletin 704 Bacteria, Yeasts and Molds by Arthur F. Badenhop,
former Extension Food Scientist, James A. Christian and George A. Schuler, Extension Food Scientists.
The Cooperative Extension Service, The University of Georgia College of Agricultural and Environmental Sciences, offers
educational programs, assistance and materials to all people without regard to race, color, national origin, age, sex or
handicap status.
AN EQUAL OPPORTUNITY EMPLOYER
Bulletin 817
Reprinted July 1996
Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, The University of Georgia College
of Agricultural and Environmental Science and the U.S. Department of Agriculture cooperating.
Gale A. Buchanan, Dean and Director