Food microbiology studies microorganisms that affect food quality and safety, both beneficially and harmfully. It focuses on microbial growth, identification, and prevention. The microbiological world includes bacteria, yeasts, molds, viruses, parasites, algae, and prions. Bacteria, yeasts, molds, viruses, and parasites can impact foods, while algae are not typically food microorganisms. Food microbiology examines the characteristics and functions of these microbes and their effects on foods and human health.

1. Food Microorganisms
Food microbiology encompasses the study of microorganisms
that have either a
beneficial or deleterious effect on the quality and safety of
foods.
Food microbiology focuses on the microorganisms that are
found in and around foods.
It includes studies of microbial growth characteristics,
identification, and prevention.
Specific areas of interest are: foodborne illness, food spoilage,
food preservation,
sanitation, and food legislation. These interest areas comprise
this course.
The microbiological world consists of bacteria, yeasts, molds,
viruses, parasites,
algae and prions. All except algae are considered food
microorganisms. Let’s look at
each group briefly.
Bacteria
Bacteria are prokaryotes because they do not possess nuclei.
They have a normal
double stranded DNA genome. Sometimes there may be
extracellular DNA fragments
known as plasmids. Plasmids can often encode proteins that
make the cell enhanced
such as antibiotic resistance.

2. Plasmids can be shared with other
cells usually of a similar genus or
species. The DNA clusters in the
center of the cell forming a nucleoid.
(If it had a membrane around it, it
would be a nucleus). Closely
associated with the nucleoid are the
ribosomes. Ribosomes translate
DNA into amino acids which are the
building blocks of proteins.
Discuss plasmids and food
microbiology
The main interior is called the
cytoplasm. This is where water,
enzymes, and nutrients all work to
power the cell. There are three main
functions, energy production,
metabolism and catabolism. Energy
naturally is in the form of ATP
generated from metabolic reactions.
Metabolism also consists of enzymes
that build up the components of the
cell. Catabolism consists of enzymes
that break down nutrients. All three
work congruently.
Figure 1. Diagram of bacterial structures.
Discuss catabolic enzymes and food
microbiology

3. The cytoplasm is surrounded by a
cytoplasmic membrane which
determines what goes in and out of
the organism. Water, dissolved gases
(e.g. CO2 and O2) and lipid-soluble
molecules simply diffuse across the
phospholipid bilayer. Water-soluble
ions generally pass through small
pores in the membrane. All other
molecules require carrier molecules to
transport them through the
membrane.
Discuss membrane function and food
microbiology
Immediately outside of the
cytoplasmic membrane is the cell
wall. This is the component that
stains differently in the Gram stain.
Red is gram negative a thinner peptidoglycan cell wall. Blue or
purple is gram positive
usually a much thicker peptidoglycan cell wall. Its main
function is structural. The cell
wall provides the bacteria shape: cocci, rods, spirals, etc.
External to the cell wall in SOME bacteria is a capsule layer.
This is a polysaccharide/protein layer that can protect the cell
from desiccation. There are other benefits. Similar to a
capsule, some cells produce a slime layer. The major
difference in the two is the rigidity of the polysaccharide.
Pili are the next structures external to the cell wall and capsule.
These are hair-like structures that assist in attaching to other

4. cells and surfaces. Some specialized pili are used for passing
plasmids between bacterial cells.
Discuss functions of slime layers and pili in food microbiology
Flagella are long hair-like structures that are used mostly for
motility. Not all bacteria
possess flagella and most that do are motile.
The last structure we’ll cover is the spore. Spores are produced
by some species
making them resistant to hostile conditions such as heat and
drying. They serve as
survival mechanisms when environmental conditions are not
suitable for growth and
replication.
Figure 2. Diagram of cell wall and cell membrane. The cell wa
ll is
above and the cell membrane below in the diagram. Above the
cell
wall is the external environment and below the membrane the
cells’ cytoplasm.
Figure 3. Slime layer (above)
and capsule (below).
Yeasts
Yeasts are eukaryotic microorganisms classified in the kingdom
Fungi (along with

5. molds) There are over 1500 known species. Most reproduce
asexually by budding.
There are no known food pathogenic (illness causing) yeasts.
There are a few
medically pathogenic yeasts such as Candida. Most
yeast are fermentative, meaning they ferment
carbohydrates. As they ferment these carbohydrates
they produce byproducts. In some cases the byproducts
are desired, e.g. wine and beer. In other cases the
byproducts are considered spoilage. The genus
Saccharomyces (sugar fungi) is one of the most common
yeasts responsible for wine, beer, and bread. Yeast are
common in the environment and can be found in higher
numbers where there are high levels of sugars.
Molds
Molds are also eukaryotic fungi. They
differ from yeasts in that they are
multicellular. There are thousands of species of molds. They
grow
structures called hyphae and reproduce by producing spores.
Molds
secrete catabolic enzymes from their hyphae to “dissolve”
organic
matter and make the byproducts available for their growth.
Molds
can grow in extreme conditions, including dry and acidic.
Different
molds are able to degrade starch, cellulose and lignin into
sugars.
Molds also secrete other molecules that can have functions
related to
food microbiology. On the beneficial side some secrete
antibiotics.
On the negative side a few secrete mycotoxins. Mycotoxins are

6. chemicals that make humans ill.
Parasites
Parasites are organisms that derive nourishment and
protection from other living organisms known as hosts.
Like yeasts and molds, there are thousands of species.
Parasites that use humans as hosts are considered
pathogenic. Unlike bacteria, yeasts or molds, they
have no cell wall, but instead have a cytoskeleton. In
that respect they function more like animal cells.
Parasites that are present in foods that cannot infect a
Figure 4. Yeast cells in scanning electron
microscopy. Note the cell buds (new yeast
cells).
Figure 5. Mold. Hyphae
at the bottom and spores
at top.
Figure 7. Giardia
(protozoan) trophozoite.
Figure 6. Giardia
cyst.
human are considered adulteration. There are no food
“spoilage” parasites. The harm
they do may be small to quite significant.
Parasites are of different types and range in size from tiny,

7. single-celled, microscopic
organisms (protozoa) to larger, multi-cellular worms
(helminths) that may be seen
without a microscope. The protozoa typically have a life cycle
where they go from an
active unicellular microbe (trophozoite) to a spore-like cyst.
The cyst form is dormant
and can withstand harsher environmental conditions. Most
often a host is required to
transform from cyst to active form creating a life cycle.
The helminthes are probably the scariest of the food microbes
and the definition of
“micro” may no longer apply. Some of these worms grow to 1-
2 meters in humans and
one species reaches 12 meters in cows. The worms can be flat
(a tape worm) or round.
Viruses
Viruses are “parasitic” organisms that can only reproduce inside
a host. They are
extremely small and cannot be seen in normal microscopy.
There are no food
“spoilage” viruses. Viruses unlike all of the rest of the food
microbes above can have
either DNA or RNA. It’s genome is surrounded by a protein
coat. Some steal lipid
layers from their host to have a third layer. Basically, the virus
must invade a host cell,
take over its reproduction system, produce new viruses, then
finally lyse the host cell to
start the process over. Fortunately, viruses do not survive well
outside of hosts. The
main pathology of viruses is due to this cell lysis. However,
there are some newer

8. theories looking at the virus genetic mutations and cancers.
Human papillomavirus is
one such virus. Viruses can use bacteria as hosts. When the
bacteria are desired
cultures, such as in cheese-making, this can become a major
problem.
Prions
Prions are unique microorganisms, if they can even be called
microorganisms. They
are made up of only protein, no nucleic acids. Prions are
believed to induce misfolding
of native (targeted) proteins that normal cells make. These
abnormal proteins become
prions themselves. Prions are responsible for several
spongiform encephalopathy’s.
As the number of misfolded proteins increases a localized area
or gap forms in brain
tissue. The progression of the disease is dependent on the
creation rate of the native
host protein that is targeted by the prion. Some prion diseases
take many years to
decades to show pathologies.
SUMMARY
Food microbiology encompasses the study of microorganisms
that have either a
beneficial or deleterious effect on the quality and safety of
foods. This food
microbiological world consists of bacteria, yeasts, molds,
viruses, parasites, and
prions. A basic understanding of each microbial group is
required to later examine
their good or bad effects on food and human health.