lab_19_bacteriophage_titer

lab_19_bacteriophage_titer - Miramar College Biology 205...

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Lab Exercise 19: Isolation & Titration of Bacteriophage Page 1 of 3 Miramar College Biology 205 Microbiology Lab Exercise 19: Isolation & Titration of Bacteriophage Background Viruses are obligate intracellular parasites whose existence blurs the line between inert chemical and organism. At their most simplistic, they are nothing more than a protein capsid surrounding genetic material. These parasites, which cannot rightly be called alive, are responsible for an inordinate number of human and plant diseases. As viruses are many times smaller than even the smallest bacteria, they are invisible to even light microscopy, and their existence was not known until the advent of the electron microscope. Prior to their identification, viruses were only known as filterable infectious agents . It was known that certain environmental samples, though filtered to eliminate bacteria, would still cause disease when inoculated into a culture of susceptible host cells. In addition to infecting eukaryotes, viruses also infect bacteria although typically with incredible host cell specificity. In fact, the ability to be infected with a known phage type is used to identify some strains of bacteria. The action of these bacteria-eating viruses was known long before the virus itself was ever discovered. It was known only that a cell-free filtrate when added to a bacterial culture, resulted in a seeming disappearance of the bacterial cells, evidenced by a lessening of the turbidity of the broth culture. The unknown entity was coined a bacteriophage (phage = eat) by Félix d’Hérelle in 1917. We know today that the bacteriophage was not eating the bacteria, but was in fact undergoing what we call lytic infection . During the lytic cycle, the virus infects the cell and then subverts the cellular machinery to
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