We also found positive correlation between the existence of
a spacer 100% identical to a phage sequence and the existence of
that phage in the same gut sample. In 21.2% of cases where
a sample had one or more spacers fully matching to a phage contig,
the targeted phage was present in the sample (although usually at
low levels), as compared with only 13% of cases where spacers with
a lower match were found (P < 1010, Chi-square for independence).
Spacers matching the phage precisely suggest a more recent acquisition
since not enough time has elapsed for accumulation of mismatches,
particularly taking into account the typically fast pace of
viral evolution. A possible interpretation of this result is therefore
that these spacers were recently acquired in response to phage predation,
and active phages still coexist with resistant bacteria in the
same sample.