a, Growth arrest (measured as culture OD 600 ) induced by target transcription in wild-type L. seeligeri (but not the ∆CRISPR mutant) harbouring an aTc-inducible protospacer RNA. Arrowhead indicates addition of 100 ng ml–1 aTc. Each data point represents the mean ± s.e.m. of three biological replicates. b, Wild-type and ∆CRISPR L. seeligeri cultures carrying an aTc-inducible target transcript were exposed to 100 ng ml–1 aTc for 3 h as in a, then diluted (at time 0 h) to OD 600 = 0.05 in fresh medium in the presence or absence of aTc, and growth was monitored over 24 h. Each curve represents the mean ± s.e.m. of three biological replicates. c, Immediate reduction in CFU upon phage infection of L. ivanovii RR3 or ΩCRISPRVI strains. The indicated strains were infected with ϕRR4 at an MOI of 2, and CFU titres in the infected cultures were monitored over time. Pre-infection (P) and mock-infection titres were also measured. Each bar represents the mean ± s.e.m. of three biological replicates. d, Cell vitality within ΩCRISPRVI cultures during phage infection. Cell vitality was measured in samples of cultures from c by monitoring conversion of nonfluorescent resazurin to fluorescent resorufin at each time point. The resorufin signal from heat-killed cells was subtracted from all samples as background, and each signal was normalized to the pre-infection value. Live cell standards (10% and 50%, mixed with heat-killed cells) are shown to demonstrate the quantitative capability of the vitality assay. Each bar represents the mean ± s.e.m. of three biological replicates. e, Phage-susceptible L. ivanovii ΩCRISPRVI(spcP) cells harbouring a spacer against an aTc-inducible plasmid target RNA (or empty vector control) were treated with aTc for 1 h to pre-activate Cas13a, then infected with ϕRR4 at an MOI of 1. Viable CFUs were counted before infection (PRE), 7 h after infection (T7) or after mock infection (UN). Two-sided Student’s t-test, ***P = 0.0005. Each bar represents the mean ± s.e.m. of three biological replicates.