Infections can be caused in several ways o Viruses, bacteria, viroids, and prions can all cause infection. Any disease- causing agent is called a pathogen.
o A virus is made of DNA or RNA and a protein coat. o non-living pathogen o can infect many organisms o A viroid is made only of single-stranded RNA. causes disease in plants passed through seeds or pollen
o A 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 lipid envelope surface proteins capsid nucleic acid lipid envelope Surface proteins capsid surface proteins 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 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 directs the host cell to produce new viral parts. The parts assemble into new bacteriophages. 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 prophage when it combines with the host cell’s DNA. Although the prophage is not active, it replicates along with the host cell’s DNA. Many cell divisions produce a colony of bacteria infected with prophage. The prophage may leave the host’s DNA and enter the lytic cycle.
Viruses cause many infectious diseases o There are many examples of viral infections. o common cold o influenza o SARS o HIV HIV-infected white blood 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 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 Spirochaeta: spiral Archaea have many shapes
Bacteria and archaea have similar structures o plasmid o flagellum o pili flagellum pili plasmid cell wall chromosome plasma membrance 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 NEGATIVEGRAM 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 ecosystems o Prokaryotes live in digestive systems of animals. o make vitamins o break down food o fill niches Lactobacillus acidophilus o Bacteria help ferment many foods o yogurt, cheese o pickles, sauerkraut o soy sauce, vinegar
o Prokaryotes have many functions in 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 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 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 o Antibiotics 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 antibiotics o Through: 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.