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Spacer frequency distribution of alleles characteristic of STEC isolates for CRISPR1 (A) and CRISPR2a (B). Shiga toxin-producing E. coli (STEC) spacer arrays are drawn on the x axis, and frequency is labeled on the y axis. The darker the color of the bar, the more frequently the spacer occurs within the 9 alleles, with black representing 9 times.
