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