The role of toxin:antitoxin systems and insertion sequences in the loss of virulence in Shigella sonnei

The plasmid, pINV, contains a 30 kb pathogenicity island (PAI) encoding a Type III secretion system (T3SS) which is essential for virulence. During growth in the laboratory, avirulent colonies of (which do not express a T3SS) arise spontaneously. Avirulence in mostly follows loss of the PAI, following recombination between insertion sequences (ISs) on pINV; toxin:antitoxin (TA) systems on pINV promote its retention through post-segregational killing (PSK). We show that avirulence in mainly results from plasmid loss, consistent with previous findings; IS-mediated PAI deletions can occur in , but through different ISs than in . We investigated the molecular basis for frequent loss of the plasmid, pINV. Introduction into pINV of CcdAB and GmvAT, toxin:antitoxin TA systems in pINV from but not , reduced plasmid loss and the emergence of avirulent bacteria. However, plasmid loss remained the leading cause of avirulence. We show that a single amino acid difference in the VapC toxin of the VapBC TA system in pINV also contributes to high frequency plasmid loss in compared to . Our findings demonstrate that the repertoire of ISs, complement of TA systems, and polymorphisms in TA systems influence plasmid dynamics and virulence loss in . Understanding the impact of polymorphisms should be informative about how TA systems contribute to PSK, and could be exploited for generating strains with stable plasmids.