  Features of the CRISPR-Cas system, in which bacteria integrate small segments
of phage genome (spacers) into their DNA to neutralize future attacks, suggest
that its effect is not limited to individual bacteria but may control the fate
and structure of whole populations. Emphasizing the population-level impact of
the CRISPR-Cas system, recent experiments show that some bacteria regulate
CRISPR-associated genes via the quorum sensing (QS) pathway. Here we present a
model that shows that from the highly stochastic dynamics of individual spacers
under QS control emerges a rank-abundance distribution of spacers that is
time-invariant, a surprising prediction that we test with dynamic
spacer-tracking data from literature. This distribution depends on the state of
the competing phage-bacteria population, which due to QS-based regulation may
coexist in multiple stable states that vary significantly in their
phage-to-bacterium ratio, a widely used ecological measure to characterize
microbial systems.
