Plasmids are extrachromosomal, self-replicating genetic elements which naturally exist in bacterial cells. They are the platforms on which various transposable elements are assembled and recombined. Additionally, plasmids are considered the key agents in the dissemination of various antimicrobial resistant genes through horizontal gene transfer. Hence, this contributes to the evolution of prokaryotes which enable them to survive in niches which were previously lethal. This was the case with members of the Enterobacteriaceae which were identified to be resistant to different classes of β-lactam antibiotics reported around the Mediterranean countries. Numerous studies have demonstrated that the origin of the worldwide spread of genes giving rise to carbapenems resistance were associated with the IncL and IncM plasmids.
Plasmid pVS20 (91 kb) was isolated from a clinical Serratia marcescens strain from a local Melbourne hospital. The plasmid is conjugative, can only replicate in members of the Enterobacteriaceae and confers multiple antimicrobial resistance. The complete DNA sequence of pVS20 and laboratory derived mutants was determined. Bioinformatic analysis of the sequence revealed that pVS20 is an IncM plasmid which features a backbone that is interrupted by a large (31.3 kb) nested transposable element located in the backbone gene mucB.
The nested structure denoted Tn6607 is built on a Tn1721 related platform and contains remnants transposon of Tn1000-like, Tn5393c, Tn1, IS1, Tn1696 (incorporating the integron In4), and three copies of IS26. The loss of resistance traits observed with the pVS20 mutant appears to be due to the activity of the IS26 elements, which render Tn6607 inheritably unstable. Tn6607 in pVS20 is closely related to a similar element in another IncM plasmid isolated from Serratia marcescens, R1215. We investigate the possibility of dissemination of the complex resistance region to other plasmids and to the chromosomes of strains unable to maintain pVS20.