Movement of IS 26 -Associated Antibiotic Resistance Genes Occurs via a Translocatable Unit That Includes a Single IS 26 and Preferentially Inserts Adjacent to Another IS 26

Abstract

The insertion sequence IS26 plays a key role in disseminating antibiotic resistance genes in Gram-negative bacteria, forming regions containing more than one antibiotic resistance gene that are flanked by and interspersed with copies of IS26. A model presented for a second mode of IS26 movement that explains the structure of these regions involves a translocatable unit consisting of a unique DNA segment carrying an antibiotic resistance (or other) gene and a single IS copy. Structures resembling class I transposons are generated via RecA-independent incorporation of a translocatable unit next to a second IS26 such that the ISs are in direct orientation. Repeating this process would lead to arrays of resistance genes with directly oriented copies of IS26 at each end and between each unique segment. This model requires that IS26 recognizes another IS26 as a target, and in transposition experiments, the frequency of cointegrate formation was 60-fold higher when the target plasmid contained IS26. This reaction was conservative, with no additional IS26 or target site duplication generated, and orientation specific as the IS26s in the cointegrates were always in the same orientation. Consequently, the cointegrates were identical to those formed via the known mode of IS26 movement when a target IS26 was not present. Intact transposase genes in both IS26s were required for high-frequency cointegrate formation as inactivation of either one reduced the frequency 30-fold. However, the IS26 target specificity was retained. Conversion of each residue in the DDE motif of the Tnp26 transposase also reduced the cointegration frequency.

First description of food-borne Salmonella enterica resistance regions R1 and R3 associated with IS26 elements

“…R3 was present in two Salmonella Rissen isolates from a swine carcass and a meat handler, collected at the same abattoir. Furthermore, a new rearrangement of resistance region R1, harboring the bla TEM-1 gene flanked by IS26 elements, was identified in Salmonella Typhimurium and Salmonella 4, [5],12:i:-, from different samples. This study highlights the zoonotic potential of Salmonella spp.…”

“…Salmonella enterica, like Salmonella Typhimurium DT104, the monophasic variant Salmonella 4, [5],12:i:-and Salmonella Rissen, has been involved in important multidrug-resistant (MDR) human infections associated with several sources [1e3]. The 4, [5],12:i:-variant has been linked to poultry, cattle, pig and pork products, with pigs being the likely reservoir of infection [1].…”

“…The 4, [5],12:i:-variant has been linked to poultry, cattle, pig and pork products, with pigs being the likely reservoir of infection [1]. Indeed, as recently demonstrated, animal-husbandry-associated environments seem to contribute to enhancing Salmonella's pathogenic potential [2].…”

“…In fact, in those isolates, resistance genes are often associated with integrons, insertion sequences (IS) and transposons, clustering within chromosomal antimicrobial resistance regions [4]. IS26 has been particularly implicated in the dissemination of chromosomal regions containing resistance genes by facilitating their mobilization between distinct genetic areas [5].…”

“…In Salmonella strains, namely S. Typhimurium and the monophasic variant S. 4 [5],12:i:-, IS26 elements have been linked to the presence of resistance regions conferring an R-type ASSuT resistance pattern [7,8].…”