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Transcript
Biology Unit 8a – Viruses & Bacteria Chapter 18 Viruses Infections can be caused in several ways oViruses, bacteria, viroids, and prions can all cause infection. Any diseasecausing 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) capsid nucleic acid lipid envelope polyhedral (foot-and-mouth disease) helical (rabies) Surface proteins capsid nucleic acid surface proteins lipid envelope surface proteins capsid 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 Viruses cause two types of infections. o A lytic infection causes the host cell to burst. host bacterium The bacterophage attaches and 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 forms a circle. The viral DNA directs the host cell to produce new viral parts. The parts assemble into new bacteriophages. o A lysogenic infection does no immediate harm. The prophage may leave the host’s DNA and enter the lytic cycle. The viral DNA is called a prophage when it combines with the host cell’s DNA. Many cell divisions produce a colony of bacteria infected with prophage. Although the prophage is not active, it replicates along with the host cell’s DNA. Viruses cause many infectious diseases o There are many examples of viral infections. o common cold o influenza o SARS o HIV The body has natural defenses against viruses HIV-infected white blood cell 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 anaerobes are poisoned by oxygen • obligate aerobes need oxygen • facultative aerobes can live with or without oxygen Bacteria commonly come in three forms o rod-shaped, called bacilli o spiral, called spirilla or spirochetes o spherical, called cocci Lactobacilli: rod-shaped Enterococci: spherical Archaea have many shapes Spirochaeta: spiral Bacteria and archaea have similar structures o plasmid pili plasma o flagellum flagellum membrance chromosome o pili cell wall plasmid 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 bacteria GRAM 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 conjugation bridge survive by forming endospores (dormant alternate life forms) Clostridium botulinum - Gram-positive, endospore-forming, rod prokaryote. Vegetative and spore stages: note TEM; the magnification flagella. Causes 6000x food poisoning (botulism), wound infections and infant botulism Prokaryotes perform important functions for organisms and ecosystems o Prokaryotes live in digestive systems of animals. o make vitamins o break down food o fill niches o Bacteria help ferment many foods o yogurt, cheese o pickles, sauerkraut o soy sauce, vinegar Lactobacillus acidophilus o Prokaryotes have many functions in Nitrogen fixing bacteria ecosystems o photosynthesize o recycle carbon, nitrogen, hydrogen, sulfur o fix nitrogen o Bioremediation uses prokaryotes to break down pollutants o oil spills o biodegradable materials 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 tissues Streptococcus 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 •Antibiotics do not work on viruses. •Prevention is best method to fight bacterial disease Penicillium chrysogenum Bacteria are gaining resistance to antibiotics oThrough: o overuse o Using them too often and when not ill o underuse o Failure to take the entire course of medicine o misuse o 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