Positive Screen
In a positive screen, the goal is to identify those cells that survive post-selection. The selective pressure must be strong enough that most of the cells die, removing their plasmids from the population, and only a small fraction survive. After the surviving cells are collected, their plasmids are PCR-amplified and sequenced using NGS to identify their cDNA or target gene. Positive selection screens are generally very robust, and tens of thousands of genes that were initially present may be narrowed down to a few hundred or fewer ‘winners’.
Negative Screen
Negative screens are a little trickier than positive screens. In a negative screen, the goal is to identify those cells that do not survive the selection mechanism. You’ll infect two sets of cells and subject one set to selection while the other serves as a non-selected control. These two populations are then sequenced using NGS to determine which plasmids have been depleted by selection. In a CRISPR screen, negative screens are often used to identify genes that are essential for growth/survival under certain conditions.
