Most phage display protocols involve a basic step of preparing a stock (proliferation) of phage or phagemid particles from infected bacteria and titrating these stocks to determine the concentration of infectious particles. For phage vectors (and helper phage), for phage vectors (and helper phage), the concentration of these variable particles is determined by counting phage plaques on the host bacterium. In order to ensure that a pure culture of phage that can be propagated is obtained, it is desirable to perform plaque separation according to this step by proliferating the phage vector or the helper phage at any time.
For phagemid vectors, titration of phagemids involves the counting of bacterial infections, as well as clones resistant to the resulting antibiotics.
A convenient alternative to determining the infectivity of phage or phagemid preparation products is to assess the concentration by absorbance. The extinction coefficient depends on the size of the phage particle, which in turn depends on the size of the genome. In general, for one helper phage such as VCSM13, lOD270 = 1.1 x 1013 particles/ml. However, calculations based on absorbance may overestimate the concentration of propagating, infectious particles.
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