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.

Promysalin was previously described as a narrow spectrum molecule with a unique species-specific activity against Pseudomonas aeruginosa. Here we demonstrate that promysalin is active against Gram-positive and Gram-negative bacteria using a microdilution assay. Promysalin acts on Gram-positive bacteria with a mechanism of action involving cell membrane damage with leakage of intracellular components. The evaluation of MICs and MBCs on 11 promysalin analogs, synthesized utilizing diverted total synthesis, allowed the identification of the structural moieties potentially involved in cell membrane interaction and damage. The mechanism of action of promysalin against Gram-negative bacteria is still not clarified, even if a synergistic effect with the bisguanidine chlorhexidine on cell membrane disruption has been observed.

A series of lipopeptidomimetics derived from teixobactin have been prepared that probe the role of residues (1-6) as a membrane anchor and the function of enduracididine. The most active compounds, with a farnesyl tail and End10 to Lys10 or Orn10 substitution have potent activity (MIC 8 μg mL) against S. aureus. These results pave the way for the synthesis of simple, cost-effective yet potent lipopeptidomimetic antimicrobials.