Extended Data Table 1c. Effect of antibiotics on adaptation levels.

The Cas1+2 operon was induced in E. coli BL21-AI cells using 0.2% L-arabinose and 0.1mM IPTG overnight, in the presence of either nalidixic acid (50 μg/ml) or rifampicin (100 μg/ml). Following overnight induction, the CRISPR array was amplified and sequenced.

Sample	rep	# of reads spanning the CRISPR array	# of reads supporting unmodified parental array	# of reads supporting acquisition of a new spacer	% expanded arrays
BL21-AI + Nalidixic acid	1	71,941	71,800	141	0.20%
BL21-AI + Nalidixic acid	2	77,774	77,714	60	0.08%
BL21-AI + Rifampicin	1	36,976	34,145	2,831	7.66%
BL21-AI + Rifampicin	2	38,702	28,147	10,555	27.27%