How low? Which selection round(s) are you talking about? Actually I dont strictly follow any rule about titers for deciding what to do during phage selection, usually I dont even titrate,The only relevant criteria during my experiments are those emerging from phage pool ELISA. But it would be necesary toknow if your titers are really so low to indicate a technical problem in phage production/selection/rescue.
As Gertrudis mentioned, the answers depend very much on the details of your experiments - the quality and diversity of your initial library, the stringency of your selection, the round of selection. You expect low titers after the first selection round, as you get rid of most of the non-binding phages. The titers should gradually recover with successive rounds as the population of phages is increasingly dominated by phages enriched for the right (specific binding) or wrong (unspecific binding) reasons.
If the titers do not improve with successive rounds, your panning strategy might be to harsh. If the best binders in the original library are of poor affinity, you may have to lower stringency in the first rounds to avoid losing your binders right from the start (shorter washing steps), and may have to use additional diversification/selection steps for affinity maturation.
Is it possible for you to check if the target protein actually IS on the immobile phase (ELISA wells, immuno tube, beads)? Of course it won't be possible to enrich target-specific phages if the target is not present.
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