On his new Zika Diaries blog, Dr. Vincent Racaniello writes: Counting Zika virus. Excerpt (but I know you'll click through to read his whole post):
Last time on Zika Diaries, I recounted how we had obtained different strains of Zika virus. The next step was to make more of them. To do that we needed cells that can be infected with the virus, and a way to measure the amount of virus made.
Here’s another way to look at this problem: we have four small vials of different Zika viruses, each containing less than 1 milliliter of fluid. That won’t last very long. We need to make more Zika virus.
In general, viruses are fussy about what cells they reproduce in (you do know that viruses cannot reproduce without going inside a cell, right?). For example, we have been using HeLa cells for many years to produce stocks of poliovirus. But Zika virus does not multiply in HeLa cells! It’s necessary to find the right cell match for each virus.
A quick scan of the modest Zika virus literature revealed that Vero cells are the right cells for amplifying stocks of the virus. Vero is an immortal cell line produced years ago from kidneys of African Green monkeys. They are used for growing a number of different viruses.
We took a small amount of each of our stocks of Zika virus, added it to Vero cells growing in a plastic dish, and in a few days the cells had all died. Virologists call this ‘cytopathic effect’. It’s a way of knowing the virus is replicating in cells.
Our stocks of Zika virus had just gone from 1 ml to 10 ml each. But how do we know how many virus particles we had? Put another way, what is the virus titer?