Phage
Bacteriophage Viruses Classification of Viruses Capsomere RNA Membranous of capsid Head Viruses are small packages of genes Capsomere DNA envelope Tail DNA Capsid sheath Consist of protein coat around nucleic acids RNA Tail (DNA or RNA) fiber Glycoprotein Glycoprotein 18 ¥ 250 nm 70–90 nm 80–200 nm 80 ¥ 225 nm
20 nm 50 nm 50 nm 50 nm (a) Tobacco mosaic (b) Adenoviruses: (c) Influenza (d) Bacteriophage: virus: helical polyhedral viruses: complex-polyhedral RNA-virus DNA-virus enveloped helical DNA-virus RNA-virus Figure 18.4. Colorized TEMs
The Lytic Cycle (virulent phage)
Viruses • Bacteriophage 1 Attachment. The T4 phage uses 2 Entry of phage DNA its tail fibers to bind to specific and degradation of host DNA. T4 receptor sites on the outer The sheath of the tail contracts, Obligate intracellular parasites surface of an E. coli cell. injecting the phage DNA into the cell and leaving an empty • Cannot grow or reproduce by itself capsid outside. The cell’s 5 Release. The phage directs production DNA is hydrolyzed. • Have no independent metabolic pathways for energy of an enzyme that damages the bacterial cell wall, allowing fluid to enter. The cell synthesis swells and finally bursts, releasing 100 • Reproduce (replicate) only by using host cell to 200 phage particles. machinery Phage assembly • Non-cellular • (Is it living?)
Synthesis of viral genomes 4 Assembly. Three separate sets of 3 and proteins. The phage DNA proteins self-assemble to form phage Figure 18.1: directs production of phage heads, tails, and tail fibers. The T4 Bacteriophages proteins and copies of the phage phage genome is packaged inside genome by host enzymes, using infecting an E. coli Head Tails Tail fibers the capsid as the head forms. bacterium cell 0.5 mm components within the cell. Figure 18.6
Culture of virulent (lytic) phages Koch’s postulates • Bacteriophage In order to prove definitively that a particular pathogen causes a disease, researchers must: • Agar plate with 1. Find the pathogen in each individual that has the “bacterial lawn” disease. (solid white field of bacteria) 2. Isolate the pathogen from a diseased individual, and grow the pathogen in a pure culture. • Plaques: clear, ÿ But cannot grow viruses independent of host cells. bacteria-free region growing 3. Induce the disease in healthy individuals by around one infecting them with the pure pathogen. original viral- 4. Isolate the pathogen from the newly infected infected bacterium = pfu: plaque-forming unit individuals.
Heyer 1