Dissemination of Novel Antimicrobial Resistance Mechanisms through the Insertion Sequence Mediated Spread of Metabolic Genes

Abstract

The widely used biocide triclosan selectively targets FabI, the NADH-dependent trans-2-enoyl-acyl carrier protein (ACP) reductase, which is also an important target for the development of narrow spectrum antibiotics. The analysis of triclosan resistant Staphylococcus aureus isolates had previously shown that in about half of the strains, the mechanism of triclosan resistance consists on the heterologous duplication of the triclosan target gene due to the acquisition of an additional fabI allele derived from Staphylococcus haemolyticus (sh-fabI). In the current work, the genomic sequencing of 10 of these strains allowed the characterization of two novel composite transposons TnSha1 and TnSha2 involved in the spread of sh-fabI. TnSha1 harbors one copy of IS1272, whereas TnSha2 is a 11.7 kb plasmid carrying TnSha1 present either as plasmid or in an integrated form generally flanked by two IS1272 elements. The target and mechanism of integration for IS1272 and TnSha1 are novel and include targeting of DNA secondary structures, generation of blunt-end deletions of the stem-loop and absence of target duplication. Database analyses showed widespread occurrence of these two elements in chromosomes and plasmids, with TnSha1 mainly in S. aureus and with TnSha2 mainly in S. haemolyticus and S. epidermidis. The acquisition of resistance by means of an insertion sequence-based mobilization and consequent duplication of drug-target metabolic genes, as observed here for sh-fabI, is highly reminiscent of the situation with the ileS2 gene conferring mupirocin resistance, and the dfrA and dfrG genes conferring trimethoprim resistance both of which are mobilized by IS257. These three examples, which show similar mechanisms and levels of spread of metabolic genes linked to IS elements, highlight the importance of this genetic strategy for recruitment and rapid distribution of novel resistance mechanisms in staphylococci.

Clinical Relevance of Type II Fatty Acid Synthesis Bypass in Staphylococcus aureus

“…Half of the FA-T r isolates produced endogenous fatty acids despite the presence of triclosan. The well-known FASII routes for triclosan resistance in clinical isolates alter the triclosan target FabI via (i) mutations in fabI or (ii) the presence of sh-fabI (a fabI allele that is horizontally transferred from Staphylococcus haemolyticus) (10,22,28). Among 31 Endo isolates identified in this work, 7 carried fabI polymorphisms previously associated with triclosan resistance (see .…”

Current and Emerging Topical Antibacterials and Antiseptics: Agents, Action, and Resistance Patterns

“…Subsequent screening of available whole-genome sequences revealed the presence of variant sh-fabI alleles that contained single nucleotide polymorphisms (SNPs) predicted to confer triclosan resistance in a range of staphylococcal species (252). It was a concern that sh-fabI was located on a novel mobile genetic element, TnSha1 (253). The acquisition of sh-fabI in S. aureus was almost exclusively associated with a single copy of TnSha1 that was integrated into the chromosome, while in S. haemolyticus the sh-fabI gene was generally found to be part of a larger plasmid-borne IS1272-type element, TnSha2 (253).…”

“…It was a concern that sh-fabI was located on a novel mobile genetic element, TnSha1 (253). The acquisition of sh-fabI in S. aureus was almost exclusively associated with a single copy of TnSha1 that was integrated into the chromosome, while in S. haemolyticus the sh-fabI gene was generally found to be part of a larger plasmid-borne IS1272-type element, TnSha2 (253). S. epidermidis was found to commonly carry both the TnSha1 and TnSha2 elements (253).…”

“…The acquisition of sh-fabI in S. aureus was almost exclusively associated with a single copy of TnSha1 that was integrated into the chromosome, while in S. haemolyticus the sh-fabI gene was generally found to be part of a larger plasmid-borne IS1272-type element, TnSha2 (253). S. epidermidis was found to commonly carry both the TnSha1 and TnSha2 elements (253). IS-mediated transposition of TnSha1 and TnSha2 therefore represents a concerning example of horizontally acquired triclosan resistance determinants.…”

“…To date, there have been very few studies assessing the prevalence of triclosan resistance in clinical isolates of common skin pathogens. Although phenotypic testing for triclosan resistance was not performed, an in silico screen of Ͼ4,000 S. aureus and 300 S. epidermidis genomes revealed that the sh-fabI resistance gene was present in approximately 1.5% of S. aureus and 14% of S. epidermidis isolates (253). The finding of acquired triclosan resistance within staphylococcus isolates is of concern and, given the environmental ubiquity of triclosan, warrants ongoing investigation.…”