Carriage of type II toxin-antitoxin systems by the growing group of IncX plasmids

The stable maintenance of certain plasmids in bacterial populations has contributed significantly to the current worldwide antibiotic resistance (AbR) emergency. IncX plasmids, long underestimated in this regard, have achieved recent notoriety for their roles in transmission of resistance to carbapenem and colistin, the last-line antibiotics for Gram-negative infections. Toxin-antitoxin (TA) systems contribute to stable maintenance of many AbR plasmids, and a few TA systems have been previously described in the IncX plasmids. Here we present an updated overview of the IncX plasmid family and an in silico analysis of the type II TA systems carried in 153 completely sequenced IncX plasmids that are readily available in public databases at time of writing. The greatest number is in the IncX1 subgroup, followed by IncX3 and IncX4, with only a few representatives of IncX2, IncX5 and IncX6. Toxins from the RelE/ParE superfamily are abundant within IncX1 and IncX4 subgroups, and are associated with a variety of antitoxins. By contrast, the HicBA system is almost exclusively encoded by IncX4 plasmids. Toxins from the superfamily CcdB/MazF were also identified, as were less common systems such as PIN-like and GNAT toxins, and plasmids encoding more than one TA system are probably not unusual. Our results highlight the importance of the IncX plasmid group and update previous much smaller studies, and we present for the first time a detailed analysis of type II TA systems in these plasmids.