When an F+ bacteria conjugates with an F-, it makes the other bactaria F+ too. So on the long run, all bactaria should be F+.
Is there any mechanism that converts F- to F+? Could it be degeneration of F plasmid?
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So an easy way to convert a cell from F+ to F- is to divide it without correctly replicating the plasmid and transferring it to a daughter cell, leaving you with one F+ and one F-. The cells that are F- now have a selective pressure advantage over the F+ cells, as they now don't require the energy to produce a plasmid when they divide. If that advantage is enough to overcome the frequency of conjugation, you'll end up having a population of F- cells.