Bacteria
BSEN-625
Advances in Food Engineering
Bacterial Evolution &
Classification
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Most numerous organisms on
earth
Earliest life forms
Microscopic prokaryotes (no
nucleus nor membrane-bound
organelles)
Contain ribosomes
Infoldings of the cell membrane
carry on photosynthesis &
respiration
Surrounded by protective cell
wall containing peptidoglycan
(protein-carbohydrate)
Bacterial Evolution &
Classification
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Many are surrounded by a
sticky, protective coating of
sugars called the capsule or
glycocalyx (can attach to other
bacteria or host)
Have only one circular
chromosome
Have small rings of DNA called
plasmids
May have short, hairlike
projections called pili on cell wall
to attach to host or another
bacteria when transferring
genetic material
Most are unicellular
Found in most habitats
Bacterial Evolution &
Classification
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Most bacteria grow best at a pH of 6.5 to 7.0
Main decomposers of dead organisms so recycle nutrients
Some bacteria breakdown chemical & oil spills
Some cause disease
Move by flagella, gliding over slime they secrete ( e.g.
Myxobacteria)
Some can form protective endospores around the DNA
when conditions become unfavorable; may stay inactive
several years & then re-activate when conditions favorable
Classified by their structure, motility (ability to move),
molecular composition, & reaction to stains (Gram
stain)
Grouped into 2 kingdoms --- Eubacteria (true bacteria) &
Archaebacteria (ancient bacteria)
Once grouped together in the kingdom Monera
Kingdom Eubacteria (true
bacteria)
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Most bacteria in this
kingdom come in 3 basic
shapes --- cocci (spheres),
bacilli (rod shaped),
spirilla (corkscrew shape)
Bacteria can occur in pairs (
diplo- bacilli or cocci)
Bacteria occurring in chains
are called strepto- bacilli or
cocci
Bacteria in grapelike
clusters are called
staphylococci
Most are heterotrophic
(can’t make their own food)
Shape of bacteria
Kingdom Eubacteria (true
bacteria)
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Can be aerobic (require
oxygen) or anaerobic
(don’t need oxygen)
Subdivided into 4 phyla --Cyanobacteria (bluegreen bacteria),
Spirochetes, Grampositive, &
Proteobacteria
Can be identified by Gram
staining (gram positive or
gram negative)
Examples of bacteria
Gram Staining
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Developed in 1884 by Danish microbiologist, Hans Gram
Bacteria are stained purple with Crystal Violet & iodine; rinsed with
alcohol to decolorize; then restained with Safranin (red dye)
Gram Staining
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Bacterial cell walls either stain purple or
reddish-pink
Gram-positive bacteria
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Thick layer of
peptidoglycan (proteinsugar) complex in cell walls
& single layer of lipids
Stain purple
Lactobacilli are used to
make yogurt, buttermilk ….
Actinomycetes make
antibiotics like tetracycline
& streptomycin
Disease-causing gram +
bacteria produce poisons
called toxins
Gram-positive bacteria
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Clostridium causes tetanus
or lockjaw
Streptococcus cause
infections such as “strep”
throat
Staphylococci cause
“staph” infections
Also cause toxic shock,
bacterial pneumonia,
botulism (food poisoning), &
scarlet fever
Can be treated with
penicillin (antibiotics) &
sulfa drugs
Streptococcus
Gram-positive bacteria
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Cell walls have a thin layer of
peptidoglycan & an extra layer of
lipids on the outside
Stain pink or reddish
Lipid layer prevents the purple stain
& antibiotics from entering
(antibiotic resistant)
Some are photosynthetic but make
sulfur, not oxygen
Rhizobacteria grow in root
nodules of legumes (soybeans,
peanuts…) & fix nitrogen form the
air for plants
Rickettsiae are parasitic bacteria
carried by ticks that cause Rocky
Mountain spotted fever
Spirochetes can cause syphilis &
Lyme disease
Food Related Micros
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Gram-positive bacterium, motile by
means of flagella.
Some studies suggest that 1-10% of
humans may be intestinal carriers of L.
monocytogenes.
It has been found in at least 37
mammalian species, both domestic and
feral, as well as at least 17 species of
birds and possibly some species of fish
and shellfish.
It can be isolated from soil, silage, and
other environmental sources. L.
monocytogenes is quite hardy and resists
the deleterious effects of freezing,
drying, and heat remarkably well for a
bacterium that does not form spores.
Most L. monocytogenes are pathogenic
to some degree.
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Listeria monocytogenes
Listeria monocytogenes
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Has been associated with such foods as:
„ raw milk,
„ pasteurized fluid milk,
„ cheeses (particularly soft-ripened varieties),
„ ice cream,
„ raw vegetables,
„ fermented raw-meat sausages,
„ raw and cooked poultry,
„ raw meats (all types), and
„ raw and smoked fish.
Its ability to grow at temperatures as low as 3°C permits multiplication
in refrigerated foods.
The 1987 incidence data prospectively collected by CDC suggests that
there are at least 1600 cases of listeriosis with 415 deaths per year in
the U.S. T
The vast majority of cases are sporadic, making epidemiological links to
food very difficult
Food Related Micros
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Salmonella typhimurium
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Is a rod-shaped, motile
bacterium -- nonmotile
exceptions S. gallinarum and S.
pullorum--, nonsporeforming
and Gram-negative.
There is a widespread
occurrence in animals,
especially in poultry and swine.
Environmental sources of the
organism include water, soil,
insects, factory surfaces,
kitchen surfaces, animal feces,
raw meats, raw poultry, and
raw seafoods, to name only a
few.
Salmonella ssp
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Has been associated with such foods as:
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Raw meats, poultry, eggs, milk and dairy products, fish, shrimp,
frog legs, yeast, coconut, sauces and salad dressing, cake mixes,
cream-filled desserts and toppings, dried gelatin, peanut butter,
cocoa, and chocolate.
Various Salmonella species have long been isolated from the
outside of egg shells.
The present situation with S. enteritidis is complicated by the
presence of the organism inside the egg, in the yolk.
This and other information strongly suggest vertical transmission,
i.e., deposition of the organism in the yolk by an infected layer hen
prior to shell deposition.
Foods other than eggs have also caused outbreaks of S. enteritidis
disease.
It is estimated that from 2 to 4 million cases of salmonellosis
occur in the U.S. annually.
Clostridium botulinum
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Clostridium botulinum is an anaerobic, Gram-positive,
spore-forming rod that produces a potent neurotoxin.
The spores are heat-resistant and can survive in foods
that are incorrectly or minimally processed.
Seven types (A, B, C, D, E, F and G) of botulism are
recognized, based on the antigenic specificity of the toxin
produced by each strain.
Types A, B, E and F cause human botulism.
Types C and D cause most cases of botulism in animals.
Animals most commonly affected are wild fowl and
poultry, cattle, horses and some species of fish.
Although type G has been isolated from soil in Argentina,
no outbreaks involving it have been recognized.
Clostridium botulinum
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The toxin is heat labile and can be destroyed if heated at 80°C for 10
minutes or longer.
The incidence of the disease is low, but the disease is of considerable
concern because of its high mortality rate if not treated immediately
and properly.
Most of the 10 to 30 outbreaks that are reported annually in the United
States are associated with inadequately processed, home-canned
foods, but occasionally commercially produced foods have been
involved in outbreaks.
Sausages, meat products, canned vegetables and seafood products
have been the most frequent vehicles for human botulism.
The organism and its spores are widely distributed in nature.
They occur in both cultivated and forest soils, bottom sediments of
streams, lakes, and coastal waters, and in the intestinal tracts of fish
and mammals, and in the gills and viscera of crabs and other shellfish.