Experimental evolution reveals genetic routes for adaptive loss of the antibacterial type VI secretion system

The type VI secretion system (T6SS) is a contractile nanomachine used by Gram-negative bacteria to deliver effector proteins into target cells, contributing to both interbacterial competition and pathogenesis. Although T6SS gene clusters are present in recently isolated commensal and pathogenic Escherichia coli strains, they are absent from classical laboratory strains that have been propagated for decades in pure cultures, suggesting that T6SS can be lost in the absence of competition. Here, we combined experimental evolution with whole-genome sequencing to track the fate of the enteroaggregative Escherichia coli (EAEC) Sci1 T6SS during competition with either T6SS-susceptible or T6SS-immune bacteria. After ∼640 generations, T6SS activity was largely maintained during competition with T6SS-susceptible bacteria, whereas ∼90% of clones evolved with T6SS-immune bacteria lost or attenuated T6SS activity through diverse mutations within the sci1 promoter, essential T6SS structural genes, or the rfaH transcriptional antiterminator. We identified two RfaH-binding ops elements within the sci1 cluster, revealing antitermination as a regulatory element of EAEC T6SS transcription, which is conserved among Enterobacteriaceae. Our findings highlight how experimental evolution can reveal the selective forces shaping T6SS maintenance and identify new regulatory components controlling its activity.