Two new Salmonella genomic islands 1 from Proteus mirabilis and description of blaCTX-M-15 on a variant (SGI1-K7).

Objectives: To characterize the structure of Salmonella genomic islands 1 (SGI1s) from two clinical Proteus mirabilis isolates: one producing an ESBL and the other a penicillinase.

Methods: WGS completed by PCR and Sanger sequencing was performed to determine sequences of SGI1s from Pm2CHAMA and Pm37THOMI strains.

Results: Two new variants of SGI1 named SGI1-Pm2CHAMA (53.6 kb) and SGI1-K7 (55.1 kb) were identified. The backbone of SGI1-Pm2CHAMA shared 99.9% identity with that of SGI1. Its MDR region (26.3 kb) harboured two class 1 integrons (an In2-type integron and an In4-type integron) containing in particular a qacH cassette (encoding a quaternary ammonium compound efflux pump). These two integrons framed a complex region (harbouring among others blaCARB-4) resulting from transposon insertions mediated by IS26 and successive transposition events of ISs (ISAba14 isoform and the new ISPmi2). The second variant (SGI1-K7) had the same backbone as SGI1-K. Its MDR region (29.7 kb) was derived from that of SGI1-K and was generated by three events. The two main events were mediated by IS26: inversion of a large portion of the MDR region of SGI1-K and insertion of a structure previously reported on plasmids carried by prevalent and successful MDR clones of Enterobacteriaceae. This last event led to the insertion of the blaCTX-M-15 gene into SGI1-K7.

Conclusions: This study confirmed the great plasticity of the MDR region of SGI1 and its potential key role for the dissemination of clinically significant antibiotic resistance among Enterobacteriaceae.