Dealing with the Evolutionary Downside of CRISPR Immunity: Bacteria and Beneficial Plasmids
Pair-wise competition between transconjugants carrying different types of CRISPR-Cas mutations or deletions and wild-type S. epidermidis RP62a. The change in the relative frequency of plasmid-bearing cells (y-axis) is plotted against the number of transfers (one transfer per day, x-axis). In all cases the growth of wild-type cells was compared against: (A) control wild-type S. epidermidis RP62a (pG0400mut), (B) R5, (C) R60, (D) R14, (E) R7, (F) B15, (G) B39. The black line indicates the average change in relative frequency (the values for each of three independent experiments are shown as a red triangle, blue circle and green rhombus). (H) Predicted changes in frequency for different selection coefficients, s. These are calculated from the equation, dq/dt = –q(1 – q)s, where q is the relative frequency of the plasmid bearing cells and s is the selection coefficient (s>0 indicates that the plasmid-bearing cells are at a disadvantage and s<0 that the plasmid-bearing cells have an advantage). We are assuming 1/100 dilutions or t = 6.64 generations in each transfer.