Structure and Functional Tutorial
Bacterial growth is measured as an increase in
numbers rather than size. The conditions
necessary for reproduction of bacteria are highly
variable.
Prokaryotic or binary fission is the mechanism
for cell multiplication. Bacteria reproduce by
prokaryotic fission , resulting in two
genetically identical daughter cells. Bacteria
have a single chromosome, a circular DNA molecule
. The plasma membrane plays a critical role in
separating the DNA replicates.
Review functions of the
Cell wall
Capsule
Pili
Flagella
Flagellin
Genophore
Nucleoid region
Plasmid
Ribososmes
Plasma membrane
Cytoplasm
Endospores
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http://faculty.ircc.edu/faculty/tfischer/images/prokaryote%20overview.jpg
Click here an in depth account of the structure features
and material presented.
All cells regardless of their origins, or level of complexity, share certain characteristics in
common. These include:
1. a plasma membrane which serves to regulate materials that pass in and out of the
cell, as well as, contain the contents of the cell.
2. a cytoplasm, which is the fluid gel contents of the cell. In the case of the prokaryote,
the cytoplasm is a granular heterogeneous mixture whose contents do not contain
membrane bound organelles as are seen in eukaryotes. They do contain, however, the
ever present ribosomes and bacterial chromosome or genophore.
3. ribosome which are universally used as the sites of protein synthesis
4. the chromosome, which in bacteria is a closed circular molecule, that is supercoiled
and condensed into a dense region known as the nucleoid region, to distinguish it from
its eukaryotic counterpart, the membrane bound nucleus.
There are a number of properties that are used to distinguish the prokaryotic and
eukaryotic cells. These include cell size. Generally speaking prokaryotic cells are on the
whole about 10 smaller in diameter than eukaryotic cells. Bear in mind that there is great
variation among cells of each of these groups. As a rule of thumb, eukaryotes vary
between 10 and 100 micrometers while prokaryotes generally range from 1 to 10
micrometers. In addition to size, prokaryotes do not have a membrane bound nucleus, In
eukaryotes the nucleus is bound by a membranous envelope contain the host’s
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chromosomes (thus their name eu – true; karyote refers to kernal or the nucleus). The
name prokaryote is to suggest, before the nucleus.
Finally, prokaryotes, as already mentioned, do not have any membrane bound organelles.
In fact, it is believe and supported by a tremendous amount of evidence that two of the
eukaryotic organelles (the mitochondria and the chloroplasts)were derived from
prokaryotes that came to live in a mutual symbiosis with and were stably acquired by
what is today the eukaryote. The mitochondria were able to take advantage of the ever
increasing amounts of oxygen that was being pumped into the atmosphere by the
photosynthetic cyanobacteria. The chloroplast endowed eukaryotes with the capacity to
capture light photosynthesize their own sugars. This gave rise to various clades of algae
and eventually the more structurally complex plants.
Morphological Diversity of Prokaryotes
Shape
Although some prokaryotes aggregate, most prokaryotes are single celled organisms.
Cells have a diversity of shapes, the most common are the spheres (cocci), rods, (bacilli),
and spiral or helices (spirilla or spirochetes).
Cell surface
Bacterial Cell Wall Most prokaryotes have a cell what is essential to help maintain their shape and protect the
cell from bursting in hypotonic environments. Where plants have cellulose, and fungi
have chitin, prokaryotes have a compound called peptidoglycan as an essential
component of their cell walls. This is a sugar polymer that is crossed linked by short
polypeptide chains. Some antibiotic such as penicillin act directly by disrupting the crosslinking of the bacterial cell walls. Of the two major divisions of the bacteria, the
Archaebacteria do not have peptidoglycan in their cell walls. Only the Eubacteria have
peptidoglycan.
The Gram Stain
Many significant bacteria are classified as either gram positive or gram negative
depending on the organization of the cell wall. A stain can be used to distinguish two
groups of the Eubacteria based on differences in the overall structural composition and
organization of the cell wall. Gram-positive bacteria have simple walls with large
amounts of peptidoglycan. These cells form a complex and retain the dye which is a
bluish purple
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Gram negative bacteria have a more complex cell wall, but less peptidoglycan. As a
result the complex formed by the staining procedure is washed away in gram negative
bacteria. Because of this the staining procedure requires a counterstain to be applied and
this is retained by the unique surface of the gram negative bacteria. This make the gram
negative bacteria appear pink.
Associated with the surfaces of gram negative microbes is a lipopolysaccharide moiety
(LPS). This LPS although it varies from one gram negative species to another, is
responsible for a more general effect produced by the group of gram negative bacteria.
This is an example of an endotoxin and is responsible for fever and achiness associated
with food poisoning. The LPS stimulate a “nonspecific” response from the host immune
system. LPS is also thought to impede the entry of drugs and renders some antibiotic less
effective in treating gram negative infections.
Bacterial Capsule
The capsule is a gelatinous substance that provides cells with additional protection from
host immune responses. By resisting digestion some encapsulated bacteria continue to
divide after being taken up by host’s neutrephils, a first line cellular defense of many
vertebrates. Some strains like Streptococcus pneumonae are completely harmless when
they without a capsule, but are pathogenic when they encode a capsule. The capsule also
contributes to a bacteria’s ability to aggregate and to adhere to the surfaces of tissues and
other substrates.
Pilus
These are surfaces appendages that are used for adherence to surfaces. Some are
specialized for the transfer of DNA in the process of conjugation. The pilus used in
conjugation is specified by plasmids which encode a collection of genes required for
transfer (the tra genes) the plasmid from one bacterial cell to another.
The Bacterial Chromosome
Otherwise known as the genophore, the bacterial chromosome is usually one closed
circular double-stranded molecule. In is not enveloped in a membrane but remains
concentrated in the cytoplasm of the bacterial cell making what is referred to as the
nucleoid region. As with eukaryotes it is associated with proteins, and interestingly the
Archaebacteria but not the eubacteria are found to be associated with histone-like
proteins. The chromosome of Escherchia coli is about 4.6 million base pairs (mbps)
about 1/1000 that of the human genome (3.2 billion base pairs).
Plasmids
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Plasmid are small closed circular supercoiled molecules that exist outside the bacterial
chromosome. They are said to be autonomous, meaning that they are able to replicate
similar to the bacterial chromosome. They are not essential but often confer genetic
properties that extend some kind of benefit to their host, e.g. resistance to antibiotics, or
the ability to metabolize certain substrates perhaps detoxifying environments. A plasmid
that conjugates and confers resistance to antibiotic to its new host is an R plasmid, or an
RTF, a resistance transfer factor. As you can imagine these are of concern in infection,
especially in hospital infections. The use of antibiotic can aggravate the situation and
cause their presence to amplify in treated populations.
Flagella
The bacterial flagella is unique and fifers from the eukaryotic flagella which is consistent
in its structure having the classic “9 + 2” arrangement of microtubules. The bacterial
flagella therefore is an analogous structure and not homologous with the eukaryotic
flagella.
The bacterial flagella rotates rather than whips back and forth like its eukaryotic
counterpart. The flagellum itself in made of many intertwined strands of the protein
flagellin. The anchoring apparatus consist of 35 different proteins arranged in rings.
It is jokingly stated that the bacteria invented the wheel. This elegant machine is driven
by proton pumps.
The flagellum of certain bacteria are also implicated in
disease. For example, the invasion of intestinal epithelial
cells by the Crohn disease-associated adherent-invasive
Escherichia coli (AIEC) strain LF82 depends on surface
appendages, such as type 1 pili and flagella (ref.).
In addition to locomotion by the flagellar movement, there are other modes of
locomotion. Spirochetes have filaments which spiral around the
cell beneath the cell wall. Their movement results in a
“corkscrew” like motion. Two clinically significant
spirochetes are Treponema paliadum, the agent of syphilis,
and Borrelia burgdorferi, the pathogen of Lyme disease.
Some bacteria also demonstrate the phenomenon of gliding.
These bacteria glide through layers of slimy molecules,
which they secrete into their environment. It is thought
that these cells rely on their motors similar to that found
anchoring the flagellum but devoid of the whip-like
flagellar appendage.
When the random movement of bacteria is attracted or
repelled by a specific influence this is referred to as
taxis. Taxis can be positive or negative and can be
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influenced by a variety of stimuli. So, for example, we can
have positive or negative phototaxis, that is attraction
toward or away from light. There is also chemotaxis and
even magnetotaxis.
Endospores
These are resistant cells that are formed by some bacteria and are capable of surviving
harsh adverse condition. Endospores consist of a copy of the bacterial chromosomes
embodied in a thick cell wall. These structures are resistant to extremes of heat and
require autoclaving in order to be destroyed. These endospores will remain dormant until
conditions are again favorable and are responsive to certain signals in their environment.
Inclusion
These are often reserves of nutrients. Inclusion refers to the formation of crystals of
nutrients or minerals that accumulate within the bacterial cell.
Review and Food for Thought
What are four things that all cells share in common?
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What are three distinctions between prokaryotic and eukaryotic cells
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What are some ways that the bacterial chromosome is different from that of a eukaryotic
cell?
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What are the three basic shapes of bacteria?
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By what process do bacteria divide?
Identify two eukaryotic organelles that are believed to have been derived from
intracellular prokaryotes through a endosymbiosis.
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How does the bacterial cell wall differ from that of other cell types that also contain cell
walls?
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What is the function of the cell wall and why do not animals have cell walls?
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What antibiotic interferes with the formation of cell wall biosynthesis?
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Distinguish between the gram positive and gram negative bacteria in terms of their
organization and visual staining properties.
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What is the major component of the bacterial cell wall found in the Eubacteria?
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How do Archaebacteria differ from the Eubacteria in regards to cell wall?
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The bacterial pilus has a variety of functions. How many can you describe?
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What is the role of capsule in disease? Give a specific example.
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What role dose the bacterial pilus and the bacterial capsule share in common?
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The bacterial pilus has a variety of functions. How many can you describe?
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Describe the bacterial chromosome?
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How does the bacterial chromosome compare with that of a human?
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What is another name for the bacterial chromosome?
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Describe plasmids and discuss some of the roles of plasmids in the life of a bacterium
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What is meant by RTFs
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What are three modes of locomotion in bacteria?
1. _____________
2. _____________
3. _____________
What is taxis?
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Describes three modes of taxis.
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A few species of bacteria have the ability to produce highly resistant structures known as
endospores (or simply spores). Identify three spore forming organisms responsible for
producing disease. Give the species name and the particular disease.
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