1.I used the LCMV plaque assay protocol as mentioned in paper (Raymond M. Welsh and Mina O. Seedhom. Curr Protoc Microbiol. 2008 February ; CHAPTER: Unit–15A.1) for estimatiion of virus titre of LCMV-WE virus preparartion. Although in the protocol vero cells are used, but I used MC57G (mouse fibro sarcoma) cell lines also. Instead of using neutral red as mentioned in the protocol I used crystal violet after fixing with formaldehyde and washing. I saw plaques only in MC57G cells not in vero cells. After that I tried to repeat it but I couldn't, even after repeating it several times.
2. Since LCMV is a non-cytopathic virus then how can we detect plaques as mentioned in the protocol above.
I work with LCMV-Arm, so my situation is slightly different, but I can say couple things:
1. The virus indeed has very low cytopathic potential, but some CPE is actually there. So you can also try to perform TCID50 assay. More complicated than the plaque assay, but in my experience more reliable. CPE will be +/- evident at day 5-7 after inoculation and will result in appearance of some dead cells on the monolayer which will form something looking like a network.
2. You can change the culture medium. I also tried to use EMEM or 199 with or without 2 mM L-Gln, 2% or 5% or 10% FBS, but neither worked. Then I tried DMEM+5% FBS and finally plaques appeared. So, you can also try it.
3. Consider mycoplasma infection of your Vero cells. In my experience plaque assay worked until my cells catched this contaminant. Since that time I fail to get plaques.
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