Viruses and Bacteria

Viruses and Bacteria Unit 6 Chapter 18

Virus characteristics • nucleic acids enclosed in a protein coat • smaller than the smallest bacterium • nonliving particles • shape determines the cell the virus attacks This picture shows the three major virus shapes.

Naming a virus • named after diseases that they cause or organs that they attack • Ex: poliovirus, small pox virus A virus that infects a bacterium is called a bacteriophage or phage for short.

Viral metabolism • Viruses cannot grow, reproduce, or develop without a host cell. • Viruses share parasitic relationships with their hosts.

Viral structure Capsid • Nucleic acid (DNA or RNA) • Envelope: layer that surrounds the capsid, found mostly in larger viruses (such as those that affect humans)

Viral replication (lytic) • Attach to a host cell with matching receptors. • The viral genetic material enters the cell either through injection or through endocytosis that forms a vesicle. • The host cell copies the viral material. • The viral DNA is transcribed, translated, and reassembled into new virus. • The host cell bursts open, releasing new viruses.

Lytic cycle: rapid replication and assembly, followed with lysis (bursting)

Lytic cycle video Click on image to play video.

Viral replication (lysogenic) • Some viruses go through a lysogenic cycle, a replication cycle in which the virus’s nucleic acid is integrated into the host cell’s chromosome, followed by a lytic cycle. • Attachment • Infection • Genetic material inserted into host DNA (now called provirus) • Replication

Lysogenic cycle video Click on image to play video.

Lysogenic cycle B. Provirus Formation A. Attachment and Entry Provirus Although the provirus is inactive, it replicates along with the host cell’s chromosome. C. Cell Division Bacterial host chromosome A lysogenic virus injects its nucleic acid into a bacterium. The viral nucleic acid is called a provirus when it becomes part of the host’s chromosome. LYSOGENIC CYCLE LYTIC CYCLE The provirus leaves the chromosome. The cell breaks open releasing viruses. Viral nucleic acid and proteins are made.

Lysogenic viral diseases • herpes simplex I • herpes simplex II that causes genital herpes • hepatitis B virus that causes hepatitis B • Chickenpox virus but may become lytic and cause shingles Herpes virus

HIV • Human immunodeficiency virus (HIV) causes a condition called AIDS. • HIV destroys the T cells that are part of the human immune system. • When the T cell counts drop to a particular level, a person is said to have AIDS.

HIV

HIV is a retrovirus. • Retroviruses are viruses that have RNA instead of DNA as the genetic material. • HIV injects its RNA into the host cell. • An enzyme (reverse transcriptase) makes a DNA complement of the RNA. • The viral DNA is embedded into the host cell, which the virus then goes through a lysogenic and lytic cycle.

HIV life cycle RNA Retrovirus DNA is made from the viral RNA. RNA DNA Reverse transcriptase Entering cell Provirus in host chromosome mRNA Retrovirus Cycle New virus parts Exiting cell New virus forming

Cancer and viruses • Some viruses cause cancers. • These viruses cause the cells to divide abnormally, creating tumors.

Prions and viroids • Prions are composed of proteins but have no nucleic acid to carry genetic information • They cause other proteins to fold incorrectly, resulting in improper functioning proteins. • Prions cause mad cow disease. • Viroids are composed of a single circular strand of RNA with no protein coat. • Viroids have been shown to cause infectious diseases in several plants.

Plant virus • The first virus to be identified was a plant virus, called tobacco mosaic virus, that causes disease in tobacco plants. • Plant viruses enter the plant through wounds or insect bites. Tobacco mosaic virus causes yellow spots on tobacco leaves, making them unmarketable.

Neutral plant viruses • Some mosaic viruses cause striking patterns of color in the flowers of plants. Rembrandt tulips

Where do viruses come from? • Some scientists suggest that viruses are nucleic acids that break free from their host cells while maintaining an ability to replicate parasitically within the host cells.

Archaebacteria and Eubacteria

Bacteria characteristics • Unicellular • Prokaryotic: lacks nucleus and organelles • Cell wall • Diverse metabolism • Archaebacteria and Eubacteria may have shared a common ancestor billions of years ago.

Three main types of Archaebacteria • Methanogens: live on anaerobic environment, produces methane gas • Habitat: marshes, lake sediments, the digestive tracts of some mammals, such as cows, sewage disposal

Three main types of Archaebacteria • Halophiles: lives only in water with high concentrations of salt

Three main types of Archaebacteria • Thermophiles: live in the hot, acidic waters of sulfur springs and near cracks deep in the ocean floor, where it is the autotrophic producer for a unique animal community’s food chain.

Eubacteria characteristics • Small • Unicellular • Ribosomes for protein synthesis • Single circular DNA chromosome instead of linear chromosome • May have plasmids (small circular DNA outside of the main chromosome)

Eubacteria anatomy Ribosome Cytoplasm Chromosome Flagellum CellMembrane Gelatin-like capsule Cell Wall

E. coli – What are the parts of this cell?

Eubacteria characteristics • Lives in more hospitable environments than archaebacteria • Diverse nutritional needs • Heterotroph: consume organic matter • Saprobe: digests dead or decaying matter • Autotroph: produces own organic matter through photosynthesis

Photosynthetic bacteria • Most cyanobacteria are blue-green and some are red or yellow in color. • Made up of chains of independent cells. • Habitat: rivers and streams

Cell wall function • A bacterial cell remains intact as long as its cell wall is intact. • If the cell wall is damaged, water will enter the cell by osmosis, causing the cell to burst. • Penicillin can block cell wall production, which can cause bacteria to be destroyed.

Alexander Fleming, discoverer of penicillin in 1928

Gram stain • a technique that determines the differences in the composition of bacterial cell walls (thick or thin) Gram-positive bacteria Gram-negative bacteria

Bacterial shapes • Bacterial cell walls also give bacteria different shapes • Coccus: spheres • Bacillus: rods • Spirillum: spirals

How bacteria grow • Diplo: a paired arrangement of cell growth • Staphylo: an arrangement of cells that resemble grapes • Strepto: an arrangement of chains of cells • Bacteria reproduce asexually by a process known as binary fission.

Binary fission • A bacterium first copies its chromosome. • The original chromosome and the copy become attached to the cell’s plasma membrane for a while. • The cell grows larger, and eventually the two chromosomes separate and move to opposite ends of the cell.

Binary fission • Then, a partition forms between the chromosomes. This partition separates the cell into two similar cells.

Binary fission • Because each new cell has either the original or the copy of the chromosome, the resulting cells are genetically identical.

Conjugation • one bacterium transfers all or part of its chromosome to another cell through or on a structure called a pilus that connects the two cells • Conjugation results in a bacterium with a new genetic composition. • This is the closest form of bacterial sexual recombination.

Metabolic diversity • Requires oxygen (obligate aerobe) • Requires oxygen-free environment (obligate anaerobe) • Can live with or without oxygen, able to perform cellular respiration or fermentation

Endospores • Structure that contains a bacterium’s DNA and a small amount of its cytoplasm • Encased by a tough outer covering • Resistant to drought and extreme conditions • May survive thousands of years • Bacillus anthracis, the bacteria that causes anthrax is able to form spores in harsh conditions and become active under favorable conditions. Spores can spread through inhalation, digestion, or contact.

How to kill endospores • Sterilization: heating under high pressure in either a pressure cooker or an autoclave • Canned foods must be sterilized and acidified to prevent spoiling.

Botulism • C. botulinum can thrive in canned foods where they produce toxins. • When eaten, a person develops severe food poisoning. • Sterilization can kill off C. botulinum.

Helpful bacteria • More bacteria are helpful than cause diseases. • Ex: nitrogen fixing bacteria

Nitrogen fixation • Several species of bacteria have enzymes that convert N2 into ammonia (NH3) in a process known as nitrogen fixation. • Other bacteria then convert the ammonia into nitrite (NO2–) and nitrate (NO3–),which plants can use. • Bacteria are the only organisms that can perform these chemical changes.

Nitrogen fixation • Some nitrogen- fixing bacteria live symbiotically within the roots of some trees and legumes. • Farmers grow legume crops after the harvesting of crops such as corn, which depletes the soil of nitrogen.

Food and medicine • Some foods that you eat—mellow Swiss cheese, crispy pickles, tangy yogurt— would not exist without bacteria.

Harmful bacteria • Disease-causing bacteria can enter bodies through openings, such as the mouth. • How bacteria harm host bodies • Bacterial growth can interfere with the normal function of body tissue. • Bacteria can release a toxin that directly attacks the host.

Diseases caused by bacteria Diseases Caused by Bacteria Treatment Transmission Symptoms Disease Fever, sore throat, swollen neck glands Antibiotic Inhale or ingest through mouth Strep throat (Streptococcus) Antibiotic Inhale Fatigue, fever, night sweats, cough, weight loss, chest pain Tuberculosis Puncture wound Stiff jaw, muscle spasms, paralysis Tetanus Open and clean wound, antibiotic; give antitoxin Rash at site of bite, chills, body aches, joint swelling Antibiotic Lyme disease Bite of infected tick Remove and fill the destroyed area of tooth Bacteria in mouth Destruction of tooth enamel, toothache Dental cavities (caries) Vaccination to prevent, antibiotics Sore throat, fever, heart or breathing failure Inhale or close contact Diptheria

Viruses and Bacteria