Chapter 23
VIRUSES AND BACTERIA
VIRUSES
Viruses are subcellular particles that cannot metabolize on their own and are not considered to be truly
living organisms.
Viruses are infectious agents and are not assigned to any of the six kingdoms.
Viruses are organized associations of macromolecules:
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nucleic acid, DNA or RNA, which carries the blueprint for the replication of progeny virions
contained within a protective shell of protein units called the capsid.
Viruses cannot replicate outside of a living cell.
Once it has invaded a cell it is able to direct the host cell machinery to synthesize new intact infectious
virus particles (virions).
Because viruses are non-motile, they are entirely dependent on external physical factors for chance
movement and spread to infect other susceptible cells.
Viruses are between 20 and 300 nanometers long.
The genome ( DNA or RNA) codes for the few proteins necessary for replication.
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Some proteins are non-structural, e.g. nucleic acid polymerases,
and some are structural, i.e. they become incorporated and form part of the virion.
Phages are viruses that attack bacteria cells.
Classification:
1. Viruses are broadly classified primarily upon the type of genomic nucleic acid, eg. DNA or RNA ,
2. Then further by the number of strands of nucleic acid (eg. double-stranded DNA, double-stranded
RNA or single-stranded RNA, with a positive or negative "sense" of that single strand).
3. Retroviruses are a special category of RNA viruses that require reverse transcription of their
RNA to DNA and then integration of that DNA into the host cell genome before replication can
take place. They carry a reverse transcriptase enzyme as part of the virion.
Viruses infect humans, animals, plants and other organisms.
In humans, they are responsible for chickenpox, mononucleosis, herpes, mumps, warts, influenza,
hepatitis, AIDS, certain types of cancer, etc.
REPRODUCTION
A virus reproductive cycle could be lytic or temperate.
Lytic cycle.
1. Attachment to the host cell.
2. Penetration of the nucleic acid into the cytoplasm of the cell.
3. Replication of viral macromolecules.
4. Assembly of the newly synthesized viral components.
5. Release of the new viruses by destroying the host cell with lytic enzymes.
Temperate or lysogenic cycle.
In the lysogenic cycle the viral genome is replicated along with DNA of the host cell.
In some phages, the viral DNA becomes integrated into the bacterial DNA, now called a prophage.
Bacterial cells carrying prophages are called lysogenic cells.
Lysogenic cells may exhibit new properties.
VIROIDS AND PRIONS.
A viroid consists of a short strand of RNA with no protein coat.
Prions are small polypeptide chains consisting of 208 amino acids.
These particles have been associated to animal and plant diseases.
BACTERIA
Bacteria are prokaryotes.
Two kingdoms: Archaebacteria and Eubacteria.
Some prefer to classify bacteria into two domains: Archaea and Eubacteria.
Bacteria lack membrane bound organelles such as mictochondria and chloroplasts.
CLASSIFICATION
Their unique shapes are useful in classification and identification.
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coccus (sphere) e.g.: Staphylococcus aureus {common skin bacterium}
bacillus (rod) e.g.: Clostridium tetanus {produces tetanus toxin}
spirochete (corkscrew, flexible) e.g.: Treponema pallidum {cause of syphilis}
spirillum (rigid helix) e.g.
vibrio (comma shape) e.g.: Vibrio cholera {cause of cholera}
Most bacteria have a cell wall made of peptidoglycans, sugars linked to short polypeptides.
The cell wall structure varies with the species.
Gram-positive bacteria absorb and retain crystal violet stain.
Gram-negative bacteria do not absorb crystal violet stain.
STRUCTURES
Some species produce a glycocalyx- surrounds cell; secreted by cell.
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Biochemically composed of polysaccharides and / or peptidoglycans.
Sometimes called the capsule or slime layer.
Conveys virulence! {example: Streptococcus pneunoniae, Bacillus anthracis }
Facilitates attachment { example: attachment of encapsulated bacteria to epithelium }.
Some bacteria have hundreds of hair-like appendages known as pili.
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"Common Pili": attachment to surfaces { example: Neisseria gonorrhea use pili for attachment to
mucous membranes of the penis, vagina, and urinary tract }
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"Sex pili" allow for joining of two bacteria during conjugation exchange of DNA.
Most motile bacteria move by means of rotating flagella.
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Bacterium flagella have a structure different from that of eukaryotes.
It consists of a basal body, a hook and the filament.
One circular DNA molecule that contains genes responsible for metabolism, cell growth, and cell
replication.
Plasmid DNA may or may not be present.
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This is a circular DNA containing genes that code for antibiotic resistance enzymes or for
exotoxin production (toxins secreted by the cell, such as tetanus or botulism toxin).
Endospores are formed by (mostly) gram-positive bacteria during adverse conditions.
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Formed inside bacterial membrane, consisting of thick peptidoglycan layers surrounding DNA,
RNA, ribosomes, and essential enzymes.
Highly dehydrated, no metabolism occurring until hydrated again. Special endospore staining
needed to visualize.
REPRODUCTION
1. Bacteria reproduce by binary fission.
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Binary fission replication, that is the splitting of single cell into two cells.
DNA replicates and a transverse wall is formed by the ingrowth of the plasma membrane and
cell wall.
2. In budding, a cell develops a bulge or bud that enlarges and eventually separates from the mother
cell.
3. In fragmentation, walls develop within the cell that eventually separates into several cells.
Genetic material is sometimes exchange between bacterial cells.
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Transformation occurs when a cell takes in fragments of DNA released by another cell.
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Conjugation occurs when two cells of different mating types come together and genetic material
is transferred from one cell to the other by means of a sex pilus.
NUTRITION
Nutrition is varied in bacteria.
Heterotrophic species are saprophitic (saprotrophs) and parasitic.
Autotrophic bacteria are either photosynthetic or chemosynthetic.
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Photoautotrophs obtain energy from light.
Chemoautotrophs obtain their energy by oxidizing inorganic chemicals.
Most bacteria require oxygen to live. They are aerobes.
Some bacteria carry their metabolic functions always in the absence of oxygen. They are obligate
anaerobes.
Facultative anaerobes use oxygen if it is available but can also carry on their metabolic functions
anaerobically.
ARCHAEA
Archaea is an evolutionarily distinct group (domain) of prokayrotes consisting of the methanogens, most
extreme halophiles and extreme thermophiles.
Their cell walls lack peptidoglycan, and their translational mechanism resembles eukaryotes rather than
prokaryotes.
Methanogens produce methane gas from simple carbon compounds.
They live in anaerobic environments like swamp, marine sediments and digestive track of animals.
Extreme halophiles live in saturated salt solutions.
Extreme thermophiles live in temperatures up to 110oC and pH of 1 or 2.
ECOLOGICAL ROLES
Bacteria are essential in the recycling of nutrients in ecosystems.
Bacteria and fungi are the major decomposer organisms.
Some bacteria are symbiotic with other organisms.
Many species are important pathogens of humans, plants and animals.
Pathogenic bacteria produce exotoxins, strong poisons that are secreted by the cells.
Endotoxins are components of the cell wall of most gram-negative bacteria that are release only when the
bacterium dies.