CBASS limits bacteriophage production while maintaining cell viability in Pseudomonas aeruginosa.

Cyclic-oligonucleotide-based anti-phage signaling system (CBASS) is an immune pathway that recognizes phage infection and generates cyclic nucleotide signals, which activate effectors to limit phage replication. Membrane-acting effectors are proposed to induce cell death; however, this mechanism has not been assessed at endogenous effector expression levels. Here, we examine cell viability outcomes of the CBASS phospholipase effector (CapV) upon activation with 3',3'-cGAMP in Pseudomonas aeruginosa. Surprisingly, exogenous 3',3'-cGAMP or constitutive 3',3'-cGAMP signaling from the synthase (CdnA) enables robust cell growth and viability while abolishing phage production. Constitutive activation of the CapV effector induces no cell fitness cost and selectively blocks many phages. During CBASS activation, phage transcription and initial DNA replication proceed normally, but phages do not reach maximum DNA levels and exhibit impaired DNA packaging. We propose that CapV disrupts capsid assembly at the inner membrane, preventing stable phage DNA packaging, thereby allowing effective CBASS anti-viral activity while preserving host viability.