Evolution of IncHI2 plasmids via acquisition of transposons carrying antibiotic resistance determinants

Objectives: To investigate the relationships between IncHI2 plasmids conferring resistance to antibiotics isolated in Australia and those from other countries.

Methods: PCR, restriction digestion, cloning and DNA sequencing were used to characterize transposons and determine their location in IncHI2 plasmids recovered from Salmonella enterica isolates from Australian animals.

Results: Tn10, carrying the tet(B) tetracycline resistance determinant, was found in IncHI2 plasmids pSRC26 and pSRC125 recovered from S. enterica serovar Typhimurium from Australian cattle and in IncHI2 plasmids from serovar Infantis isolates from chickens. Its location was the same as seen in the IncHI2 reference plasmid R478. The location of Tn1696-related mercury and multiple antibiotic resistance transposons was also the same in all of the Australian plasmids, and the mer end was in the same position as the mer module in R478. However, R478 has lost the tnp end (including most of the integron) and some adjacent sequence. The sequence adjacent to the tnp end of the Tn1696-related transposons in the Australian plasmids is in the bla(CMY-8)-carrying plasmid pK29, but only 22 bp from the transposon inverted repeat remains. These plasmids all belong to the same evolutionary lineage. Neither transposon was found in TP116, which represents a separate lineage.

Conclusions: Transposon locations are useful markers for lineages of closely related plasmids. The configuration surrounding the Tn1696-like transposons in the Australian IncHI2 plasmids is ancestral to those found in R478 and pK29, each of which has part of the transposon and adjacent sequence replaced by other resistance genes.