Unit 5 – Bacteria
• In this unit, we look at one of the oldest and most
efficient type of organisms.
Bacteria
• Bacteria are very small prokaryotic organisms with a
wide variety of features, metabolisms, and abilities.
• As they are prokaryotes, they lack membranebound organelles but some feature small proteinbound compartments that work in a similar manner.
• Bacteria are invisible to the naked eye individually.
By the time a group of bacteria (known as a
colony) are visible, the group will contain millions of
cells.
Bacteria
• Bacteria are often thought of a disease-causing. In
fact, many species are completely neutral to
humans and still others are beneficial.
• Harmful types of bacteria are called pathogenic (a
pathogen is anything that causes sickness/disease).
• Many bacteria do not actively try and harm
people, but simply release substances (as wastes, or
otherwise) that are toxic to humans.
Cell Structure
• The structure of
bacteria contains
many of the same
elements of eukaryotic
cells, but is
distinguished by having
no organelles.
• All chemical reactions
that would occur in
organelles now occur
in the cytoplasm.
Cell Structure
• The DNA of bacteria is
a single circular piece.
• It is somewhat folded in
on itself to form a
nucleoid region.
• This however, is not the
same as a nucleus,
which is a membranebarrier around the
DNA.
Cell Structure
• Bacterial DNA is also
“bare” compared to
that of eukaryotes.
• Bacterial DNA is not
associated with a
collection of proteins to
form chromatin and so
it is theoretically more
exposed to damaging
factors.
Cell Structure
• DNA is still expressed
through the DNA-RNAProtein pathway.
• Instead of occurring in
the nucleus and rough
ER, this process occurs
in the cytoplasm.
• Ribosomes are freefloating in the cytosol
rather than bound to
the rough ER
membrane.
Cell Structure
• The exterior of the cell is
made of up multiple
layers.
• The innermost layer is
the plasma membrane.
This works much the
same as the ones
found in other types of
cells.
Cell Structure
• The next layer is the cell
wall. This works much
like a plant’s cellulose
cell wall in that it
provides shape and
structure to the cell.
• However, bacteria use
the sugar
peptidoglycan rather
than starch to build the
wall.
Cell Structure
• The cell wall offers
protection and as well
contains some
important enzymes.
• Penicillin works by
disrupting/destroying
the cell wall.
Cell Structure
• Cell walls come in two
major types. The types
were first discovered by
Hans Christian Gram in
the early 20th century.
• He used a special stain
(crystal violet) to dye
bacterial cells.
• Some cells would retain
the dye when washed
off but others would
lose it.
Cell Structure
• The cells that held the
stain were called Gram
Positive.
• These cells have a thick
cell wall surrounding
the cell membrane.
The cell wall takes up
the dye and retains it
even when it is tried to
be washed off.
Cell Structure
• Alternatively, some
cells are Gram
Negative.
• These cells have a
thinner cell wall and a
second cell membrane
outside of the cell wall.
• As the wall is covered
by another membrane,
the dye does not stick
and is readily washed
away.
Cell Structure
• The outermost layer of
a bacteria is the
capsule.
• This serves partially as a
protective coat.
• Additionally, its
stickiness helps the
bacteria adhere to
surfaces.
Cell Structure
• Extending from the cell
membrane are pili
(singular: pilus).
• These are made of
similar materials as the
cytoskeleton.
• Pili are used somewhat
for movement but are
primarily for attaching
to other cells.
Cell Structure
• Another similar
structure is the
flagellum (plural:
flagella).
• These are larger
structures used to
propel the cell.
• A single cell may have
one or many flagella.
Shapes/Arrangements
• Prokaryotes come in
three main shapes:
• Spherical bacteria are
known as cocci.
• Elongated, rod-shaped
bacteria are known as
bacilli.
Shapes/Arrangements
• The remaining group
have a variety of
shapes but are often
grouped under the title
spiral-shaped or
comma-shaped.
Shapes/Arrangements
• Within each group, we
can also classify
prokaryotes based on
how they cluster
together.
• Remember, these are
not multicellular
organisms, there is no
true division of labor,
etc. These are simply
individual cells living
close together.
Shapes/Arrangements
• Clustered cells get the
prefix “staphyl-”. Some
species in this group
can cause very serious
infections in people
(staph infection).
• Circular grouped cells
are staphylococci.
Shapes/Arrangements
• When cells form long
fibers or chains, they
get the “strepto-”
prefix.
• Strep throat is caused
by these type of
bacteria.
• Spherical bacteria of
this type are
streptococci while rodshaped bacteria are
streptobacilli.
Prokaryotic Reproduction
• Prokaryotes reproduce
by a simple and quick
process called binary
fission.
• In this process a single
cell divides into two,
creating two identical
daughter cells from the
original mother cell.
Prokaryotic Reproduction
• During binary fission,
the DNA is replicated.
The single doublestranded circular DNA
is copied to form two
double-stranded
pieces and each is
drawn towards one of
the poles of the cell as
it elongates.
Prokaryotic Reproduction
• Elongation of the cell
results in the cell
membrane/wall/
capsule becoming
longer.
• As this happens, the
two copies of the DNA
begin to separate and
go to each end.
Prokaryotic Reproduction
• Eventually, once all the
contents are evenly
divided between the
cells, the
membrane/wall begins
to narrow at the
center, diving the two
cells in half.
• This continues until the
two cells have
separated completely.
Prokaryotic Reproduction
• Due to the fact that each cell produces two new
cells, the growth of bacteria is exponential (the rate
of growth keeps getting larger and larger.
• However, eventually the rapid growth of bacteria
will outpace the levels of food and wastes in the
area. At this point, reproduction will slow to a level
that simply keeps the population constant, and not
increasing.
Prokaryotic Reproduction
Prokaryotic Reproduction
• Bacterial reproduction is asexual – that means that
the cells produced are identical to the parent. This
means that new mutations and combinations of
traits are not created as they would be in the case
of sexual reproduction.
• Bacteria can reproduce very rapidly, some as fast
as every 20 minutes. This allows bacterial colonies to
increase in size very rapidly and take advantage of
nearby resources.
Prokaryotic Reproduction
• Just because bacteria reproduce asexually does
not mean that they do not have mechanisms to
increase the diversity of the population and to
spread/share genes.
• One mechanism is through basic mutation. In all
organisms, the DNA sequence eventually becomes
altered. Either by faulty copying or by various
chemicals or radiation, the sequence can be
altered, giving the possibility of new genes.
Prokaryotic Reproduction
• Another mechanism is transformation. This is the
process by which prokaryotes can take up “loose”
pieces of DNA from the environment and
incorporate it into its genome.
• This allows prokaryotes to gain entire new traits and
abilities and even traits to be shared from very
different creatures/species.
Prokaryotic Reproduction
• Many viruses work by inserting their own DNA into
the host cell and using that to take over the cell’s
machinery to make more viruses. However,
sometimes the virus will integrate itself into the
bacteria but for one reason or another not be able
to completely hijack the cell.
• This in the way the virus may bring new genes into a
bacteria from the viral DNA but not end up being
destroyed by the virus.
• This process of sharing genes is called transduction.
Prokaryotic Reproduction
• Another way of sharing genes is through
conjugation. One cell will make a copy of a portion
of its genes and then transfer the DNA to another
cell through a specialized pilus. This allows useful
genes to be spread through a colony easily.
• This technique is harnessed in many biotechnology
fields spread new traits to an entire population.
Prokaryotic Reproduction
Bacterial Nutrition
• Bacteria and Archaea show the widest range of
metabolisms in all of life. They are able to take up
various different sources of energy and various
types of carbon sources.
• If a species can take up energy from sunlight, it is
known as an phototroph. (photo = light)
• All other species, which get their energy from the
chemical bonds in some kind of foodstuff are called
chemotrophs. (chemo = chemical)
Bacterial Nutrition
• Organisms need carbon to make all of their organic
compounds.
• If they can make their organic compounds from the
inorganic compound carbon dioxide (CO2) they
are known as autotrophs. (auto = made on its own,
automatic)
• However, many organisms need some form of prebuilt organics. If an organism need any kind of
organic compounds from its diet, it is an
heterotroph. (hetero = something other than itself).
Bacterial Nutrition
• Combining the two sets, we create four types of
bacteria:
Where they get their carbon from
Where they get their
energy from
Carbon dioxide
Organic nutrients
From sunlight
photoautotroph
photoheterotroph
From chemicals
chemoautotroph
chemoheterotroph
Bacterial Nutrition
• Photoautotrophs work much like plants, getting their
energy from the sun and carbon from airborne
carbon dioxide. Cyanobacteria are a possible
precursor to plants/chloroplasts.
• Chemoautotrophs are often found in unusual
environments such as underwater hot vents. They
get energy from breaking down molecules such as
hydrogen sulfide or ammonia. They can build their
own carbon compounds and only need carbon
dioxide as a carbon source.
Bacterial Nutrition
• Photoheterotrophs is a very uncommon situation
where the organism gets its energy from sunlight but
still requires at least some organics that it cannot
make itself.
• Chemoheterotrophs are similar to animals in that
they both obtain energy and carbon from ingested
materials. This is a common metabolism type, with
the bacteria feeding on other microorganisms and
stray organic materials.
Bacterial Nutrition
• Along with these categories, the fact of if a cell
does or does not need oxygen to survive plays a
key role.
• Organisms that use oxygen (those that respire) are
called aerobes.
• Specifically, if an organism can ONLY survive with
oxygen it is called an obligate aerobe.
Bacterial Nutrition
• However, not all life need oxygen to live.
• Life that does not need oxygen are called
anaerobes.
• Again, those that can ONLY live without oxygen are
obligate anaerobes.
• However there are some species that can use
oxygen, but can also survive in an oxygen-free
environment. These are facultative anaerobes. E.
Coli is one such organism.
Bacterial Nutrition
• In an anaerobic environment, instead of respiration,
the main method of energy production is
fermentation.
• Fermentation often produces an alcohol as a byproduct rather than carbon dioxide.
• Humans put suitable yeasts (fungi) in anaerobic
situations to force them to ferment the sugars of
grains and fruits to make beers and wines.
Bacterial Nutrition
• Many bacteria are important in the recycling of
nitrogen through the environment.
• These work to convert atmospheric nitrogen (N2)
into chemicals such as ammonia, nitrites, nitrates
and nitrogen dioxide in a process called nitrogen
fixing/fixation. Unlike the nitrogen gas, the other
nitrogen compounds are readily used by other
organisms and thus help organisms along the food
chain.
• Many of these bacteria are associated with the
roots of plants and have a mutually-beneficial
relationship.