It is important to know the titre of the amplified virus to determine the optimum amount of virus to use for future amplifications and protein production. To little virus will result in low virus titres or poor protein yields and to much virus will waste virus stocks and may result in the generation of mutant virus genotypes. To determine virus titre we either use plaque assay or an in-house quantitative PCR based method (baculoQUANT). Plaque assay titration is carried out using Sf cells seeded in dishes and infected with serial dilutions of virus inoculum. The cells are then covered with a semi-solid agarose overlay and incubated for 3-5 days. Once they reach confluence they are stained with neutral red and virus plaques (clear areas) can be visualised over a red background of live cells (only live cells take up the dye). This method requires some degree of technical expertise but is a direct measure of virus infectivity.
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