biology unit 8a – viruses & bacteria chapter 18. viruses
TRANSCRIPT
BiologyUnit 8a – Viruses & Bacteria
Chapter 18
Viruses
Infections can be caused in several ways
oViruses, bacteria, viroids, and prions can all cause infection.
Any disease-causing agent is called a pathogen.
oA virus is made of DNA or RNA and a protein coat.onon-living pathogen ocan infect many organisms
o A viroid is made only of single-stranded RNA.•causes disease in plants
•passed through seeds or pollen
oA prion is made only of proteins.•causes misfolding of other proteins •results in diseases of the brain
Viruses differ in shape and in ways of entering host cells.
o Viruses have a simple structure. o genetic material o capsid, a protein shell o maybe a lipid envelope, a protective
outer coat
enveloped(influenza)
helical(rabies)
polyhedral(foot-and-mouth
disease)
capsid nucleic acid
lipidenvelope
surfaceproteins
capsid
nucleic acid
lipid envelope
Surface proteins capsidsurfaceproteins
nucleic acid
Bacteriophages infect bacteria
capsid
DNA
tail sheath
tail fiber
Viruses enter cells in various ways
o bacteriophages pierce host cells.o Viruses of eukaryotes enter by
endocytosis or by fusing with the membrane
colored SEM; magnifications:large photo 25,000; inset 38,000x
o A lytic infection causes the host cell to burst.
host bacterium
The bacterophage attachesand injects it DNA into a host bacterium.
The host bacterium breaks apart, or lyses. Bacteriophages are able to infect new host cells.
The viral DNA directs the hostcell to produce new viral parts.The parts assemble into newbacteriophages.
The viral DNA forms a circle.
Viruses cause two types of infections.
o A lysogenic infection does no immediate harm.
The viral DNA is called a prophagewhen it combines withthe host cell’s DNA.
Although the prophage is notactive, it replicates along withthe host cell’s DNA.
Many cell divisions produce acolony of bacteria infectedwith prophage.
The prophage may leave thehost’s DNA and enter thelytic cycle.
Viruses cause many infectious diseases
o There are many examples of viral infections.o common cold o influenzao SARSo HIV
HIV-infected whiteblood cell
The body has natural defenses against viruses
Vaccines are made from weakened pathogens
o A vaccine stimulates the body’s own immune response.
o Vaccines prepare the immune system for a future attack.
o Vaccines are the only way to control the spread of viral disease.
Bacteria & Archaea
Bacteria and archaea are both single-celled prokaryotes.
o Prokaryotes can be grouped by their need for oxygen. • obligate anaerobesare poisoned byoxygen • obligate aerobes need oxygen• facultative aerobes can live with or without oxygen
Bacteria commonly come in three forms
o rod-shaped, called bacillio spiral, called spirilla or spirocheteso spherical, called cocci
Lactobacilli: rod-shapedEnterococci: spherical Spirochaeta: spiral
Archaea have many shapes
Bacteria and archaea have similar structures
o plasmid o flagellum o pili
flagellum
pili
plasmid
cell wall
chromosome
plasmamembrance
This diagram shows the typical structure of a prokaryote. Archaea and bacteria look very similar, although they have important molecular differences.
o The amount of peptidoglycan within the cell wall can differ between bacteriaGRAM NEGATIVE GRAM POSITIVE
Archaea have different lipids entirely
Gram staining identifies bacteria
o gram-positive stains purple, more peptidoglycan
o gram-negative stains pink, less peptidoglycan
Gram-negative bacteria have a thin layer of peptidoglycan and stain red.
Gram-positive bacteria have a thicker peptidoglycan layer and stain purple.
Bacteria have various strategies for survival
o Prokaryotes exchange genes during conjugation
o Bacteria may survive by forming endospores(dormant alternate
life forms)
conjugation bridge
TEM; magnification 6000x
Clostridium botulinum - Gram-positive, endospore-forming, rod prokaryote. Vegetative and spore stages: note the flagella. Causes food poisoning (botulism), wound
infections and infant botulism
Prokaryotes perform important functions for organisms and
ecosystemso Prokaryotes live in digestive systems of animals. o make vitaminso break down food o fill niches
Lactobacillus acidophilus
o Bacteria help ferment many foods
o yogurt, cheeseo pickles, sauerkraut o soy sauce, vinegar
o Prokaryotes have many functions in ecosystemso photosynthesize o recycle carbon, nitrogen,
hydrogen, sulfur o fix nitrogen
o Bioremediation uses prokaryotes to break down pollutants
o oil spillso biodegradable materials
Nitrogen fixing bacteria
Bioremediation in soil
Some bacteria cause disease
o Bacteria cause disease by invading tissues or making toxins.
o A toxin is a poison released by an organism.
Clostridium botulinum – food poisioning
Normally harmless bacteria can become destructive
o may colonize new tissuesStreptococcus bacteria on skin
They are fairly harmless unless they come in contact with other tissue such as muscle or fat through an open wound.
Antibiotics are used to fight bacterial disease
oAntibiotics may stop bacterial cell wall formation
Penicillium chrysogenum
•Antibiotics do not work on viruses. •Prevention is best method to fight bacterial disease
Bacteria are gaining resistance to antibioticsoThrough:
o overuse o Using them too
often and when not ill
o underuse o Failure to take the
entire course of medicine
o misuseo Used in agriculture
to increase rate of growth but bacteria already in the food becomes resistant.
A bacterium carries genes for antibiotic resistance on a plasmid.
A copy of the plasmid is transferred through conjugation.
Resistance is quickly spread through many bacteria.
That’s all until next week