Don't overlook the little guy: An evaluation of the frequency of small plasmids co-conjugating with larger carbapenemase gene containing plasmids.

As the spread of antimicrobial resistance (AMR) genes becomes an increasing global threat, improved understanding of mobile genetic elements which contribute to the spread of antimicrobial resistance genes, becomes more critical. We created transconjugants from the mating of three chromosomally isogenic Klebsiella pneumoniae carbapenemase (bla) positive Citrobacter freundii isolates with a laboratory strain of Escherichia coli and evaluated the movement of small cryptic plasmids (SCPs), p3223 and p1916, when larger bla-plasmids were transferred. In all of the 143 transconjugants, multiple plasmids, both large and small, transferred with each mating. When two blaplasmids were present in the host, frequently (87%; 98/113) both would be transferred during mating. p3223 is found in a wide range of bacterial hosts that harbor AMR genes; p1916 has been identified in only a limited number of publicly available sequences to date. From our evaluation, there is still much to learn about SCPs, and the high rate of co-transfer of multiple plasmids from real-world carbapenemase-producing Enterobacteriales.