Food Microbiology:
Delicious or…
…Dangerous?
Introduction to Food Microbiology
Objectives:
•Provide a general introduction to microbiology
•Introduce the concept that microbes are found
in the food that we eat every day
•The microbes can be helpful in producing
some types of delicious food
•The microbes can also be harmful, as in the
example of the E. coli spinach contamination
•Explain methods of food sterilization to prevent
contamination in the future.
General Introduction to
Bacteria
Fun Facts about Bacteria
Necessary to make many everyday foods including
yogurt, cheese, sauerkraut, soy sauce, vinegar
Used to clean up various forms of pollution
including oil spills, pesticides, sewage
Produce antibiotics that we can use to the
fight bacteria that make us sick
Produce Botox so we can all have wrinkle-free
foreheads
Fun Fact: Bacteria comprise about 1/20th of your total body weight!
Classification of Bacteria
•Bacteria are the most abundant organisms on earth.
•Of the three major domains of classification, bacteria take up two.
•The domain Bacteria is often referred to as eubacteria
•The most common type of bacteria found on earth
•Examples: Streptococcus, Escherichia coli, Clostridium botulinum,
Salmonella
•The domain Archaea is often referred to as archaebacteria
•Archaea are thought to resemble the very first cells on earth
•They are more rare and live in extreme conditions
Identification of Bacteria
Bacterial Shapes
Sphere shaped:
coccus
Rod shaped:
bacillus
Spiral shaped:
spirochete
•Bacteria have three basic shapes.
•These images are magnified by more than
10,000 times!
•They are all surrounded by some type of cell wall
Staining Bacteria
Gram-negative:
Double membrane doesn’t hold
purple dye
Gram-positive
Thick peptidoglycan layer
holds purple dye
Cell Wall Structure
Gram-Negative
Gram-Positive
Holds Purple Dye
Will Not Hold Purple Dye
DNA
Outer
Membrane
Peptidoglycan
Plasma Membrane
Peptidoglycan
Plasma Membrane
The Genetics of Bacteria
Bacteria Are Prokaryotic
•The main DNA is a long circular loop, one single
chromosome, not contained within a nucleus
•Many bacteria also contain plasmids, a smaller loop of
DNA
•It is the plasmid that allows these asexual organisms to
have variation, and can also make the bacteria pathogenic!
Bacterial Conjugation
Often it is the plasmid that contains genes that are pathogenic to humans!
Bacterial Growth and Division
Bacterial Growth and Division
• Most bacteria divide every 20 minutes
• They will form bacterial colonies
•Bacteria divide by a process called prokaryotic fission
•The product from one bacterium dividing would be two
genetically identical daughter cells
Good vs. Bad Bacteria
• There are many different kinds of bacteria, most of which will not
make you sick and may actually help you:
– Staphylococcus epidermidis lives harmlessly on your skin
– Escherichia play a nutritional role in the intestinal tract by synthesizing
vitamins, especially vitamin K.
– Neisseria subflava lives harmlessly in most people’s mouths
• Fun Fact: The human body contains 20 times more microbes than it
does cells!
• But, there are also some that can make you sick…
Bacteria Can Cause Many Different Illnesses
•Strep Throat - Streptococcus pyogenes
•Ear Infections - Streptococcus pneumonia,
Haemophilus influenzae, Moraxella catarrhalis
•Meningitis – Neisseria meningitidis, Haemophilus
influenzae
•Tuberculosis – Mycobacterium avium, M. tuberculosis
•Gonorrhea – Neisseria gonorrhoeae
•Syphilis – Treponema pallidum
Food Microbiology
Delicious or Dangerous?
Food Microbiology
• Microorganisms are a part of the food we eat every day,
specifically bacteria
• Most of these bacteria are
harmless, non-pathogenic
• BUT some of these bacteria are
harmful to us, pathogenic
• Is the food “delicious or dangerous????”
Delicious Food Microbiology
• Yogurt has bacteria added to it that give it a slightly
sour taste
– “active yogurt cultures”
– Lactobacillus bulgaricus and Bifidobacteria
• Cheeses can be fermented with various bacteria
– Swiss cheese -Propionibacterium
– Blue cheese -Penicillium roqueforti and Lactobacillus
• Finally, many pickled vegetables get their flavor from
bacteria
– Sauerkraut – Leuconostoc and Lactobacillus
– Pickles – Lactic acid bacteria
Dangerous Food Microbiology
Contaminated Food Can Make You Sick
Escherichia coli O157:H7
*Source: spinach, ground beef, frozen pizza
*Symptoms: vomiting, diarrhea (often bloody)
Clostridium botulinum -botulism
*Source: soil, water, non-acid
canned foods
*Symptoms: flaccid paralysis and
possible death (or a smooth face)
Salmonella typhimurium – salmonella
*Source: eggs, chicken meat, dairy products
*Symptoms: headache, chills, vomiting, diarrhea
Facts about Foodborne Illness
• “The CDC estimates that 76 million
people get sick, more than 300,000 are
hospitalized, and 5,000 Americans die
each year from foodborne illness.”
• Uncooked foods are the largest source
of foodborne illness in the US (raw
meat, uncooked eggs, raw vegetables,
etc.)
Case Study
Outbreak of E. coli in Spinach,
2006
What happened?
• There are rare cases of
E. coli 0157:H7
contamination each year
• In late August of 2006,
the CDC noticed a
dramatic spike in cases
• The hypothesis was that
this was a foodborne
illness, but the source
was unknown
Who was affected?
• E. coli O157:H7
outbreak resulted in
205 confirmed
illnesses and three
deaths
• The contaminated
spinach was found in
26 states.
Source traced to spinach
• The CDC narrowed the source down to fresh
spinach coming from an isolated farm in
southern California
– Normally fresh vegetables do not harbor
pathogenic bacteria
• The CDC, in September 2006, reported an
apparent outbreak of E. coli O157:H7 linked
to the consumption of bagged spinach
How did the spinach get
contaminated?
• Environmental risk factors for E. coli
O157:H7 contamination at or near the spinach
field:
– the presence of wild pigs in the proximity of
irrigation wells used to grow produce for ready-toeat packaging
– surface waterways exposed to feces from cattle
and wildlife
• The strain of E. coli is normally found in the
intestinal tract of cows, pigs and deer.
How did the CDC identify
E. coli 0157:H7?
•The CDC cultured the bacteria from the spinach and placed it on a
food source E. coli likes to eat, such as EMB (eosin methylene blue) agar
•E. coli grows pink colonies on EMB agar
•No other bacteria will grow pink on this
media.
•The CDC was able to distinguish the E. coli 0157:H7 strain from other
E. coli strains by doing gel electrophoresis of the DNA.
•The genetic fingerprint will be different- do you know why?
How could our class identify
E. coli?
•
3 methods of potential identification
methods:
1. Look at shape under the microscope
2. Gram staining
3. EMB agar
1.
2.
3.
Symptoms of E. coli 0157:H7
infection
• Type of enterohemorrhagic Escherichia
coli (EHEC)
– Hemorrhagic (bloody) diarrhea
– Abdominal pain
– Kidney failure
How does E. coli 0157:H7 make
you sick?
• E. coli 0157:H7 produces a toxin,
verotoxin, similar to a shiga
toxin
• Verotoxin invades the cells of
the intestine and inhibits protein
synthesis
– Inhibition of protein synthesis =
cell death
How can we avoid getting sick?
• Cook the food
– E. coli 0157:H7 (on spinach)
can be killed by cooking at
160° F for 15 seconds
– Cook meat until grey/brown
in color and juices run clear
to kill bacteria
How can food contamination be
prevented pre-consumer?
• Irradiation is one possible
solution
– Process of irradiating foods
with ionizing radiation (gamma
rays or high energy electrons)
to destroy microorganisms
contaminating the food
•
– According to CDC, irradiation
does NOT change nutritional
value, composition of the food,
or make the food dangerous
Cool fact: astronauts eat irradiated food in
space!
Summary
• Bacteria are the most abundant organisms on earth
• Some bacteria never cause us harm and can be helpful
to us (non-pathogenic)
• Some bacteria are harmful to us (pathogenic)
– Toxins produced by bacteria can damage our cells
– Outbreak of E. coli 0157:H7 in spinach
• Contamination and infection can be prevented by a
variety of methods
– Cooking – you can do at home
– Irradiation – can be done by producers