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Background: SME carbapenemases are increasingly reported, especially from North and South America. Here, we describe an SME-4-producing Serratia marcescens (SME-Sm) clinical isolate from Argentina and compare its genome with other SME-Sm and Sm isolates recovered from public databases.Methods: Sm isolates were characterized by WGS using Illumina technology, susceptibility testing and MIC determination. Carbapenemase activity was revealed by biochemical tests based on imipenem hydrolysis. A whole-genome phylogeny was estimated for all the Sm isolates retrieved from public databases with kSNP3 and a whole-genome phylogenetic analysis based on non-recombinant core SNPs was inferred for Sm complete genomes and for those encoding any blaSME variants.Results: Sm163 was resistant to amoxicillin, temocillin, aztreonam and carbapenems, remaining susceptible to extended-spectrum cephalosporins. WGS analysis of Sm163 revealed a genome of 5139329 bp and a chromosomally encoded blaSME-4 carbapenemase gene located on a genomic island closely related to SmarGI1-1 of Sm N11-02820. Comparison of the Sm genomes revealed that the 14 SME-Sm isolates possess this genomic island inserted at the same loci, that 13/14 belong to clade 1 and that 11/14 form a well-defined subcluster of cluster I of Sm clade 1, while Sm163 belongs to clade 2, suggesting that an SME-encoding genomic island may have been transferred between isolates from different clades.Conclusions: To the best of our knowledge this is the first report of an SME-4-encoding Sm from Argentina. The blaSME-4 gene is located on a SmarGI1-1-like genomic island. The genome of Sm163 belongs to clade 2, unlike all the other SME-Sm isolates, which belong to clade 1. [ABSTRACT FROM AUTHOR]

• Eleven bla PER-2 -harboring plasmids from Enterobacterales were fully sequenced. • The bla PER-2 gene was encoded on a variety of plasmid types (IncA, C, FIB, HI1B, N2). • IS Pa12 -composite transposons are suggested to be involved in bla PER-2 origin recruitment and dissemination. • A novel IS Pa12 -composite transposon, Tn 7390 , carrying the bla PER-2 gene was identified. • IS Kox2 -like elements seem to play a role in bla PER-2 gene dissemination. The production of PER-like extended-spectrum β-lactamases has recently been associated with reduced susceptibility to the last resort drugs aztreonam/avibactam and cefiderocol. PER-2 has been mainly confined to Argentina and neighboring countries. Until now, only three plasmids harboring bla PER-2 genes have been characterized but very little is known about the involvement of different plasmid groups in its dissemination. The diversity of genetic platforms associated with bla PER-2 genes from a collection of PER-producing Enterobacterales was analysed by describing the close environment and the plasmid backbones. Full sequences of 11 plasmids were obtained by short read (Illumina) and long read (Oxford Nanopore or PacBio) sequencing technologies. De novo assemblies, annotation and sequence analysis were performed by Unicycler, Prokka and BLAST. Plasmid analysis revealed that the bla PER-2 gene is encoded on plasmids of different incompatibility groups (A, C, FIB, HI1B, N2), indicating that this gene may have been disseminated through a variety of plasmids. Comparison with the few publicly available nucleotide sequences describing the bla PER-2 genetic environment, including those from the environmental species Pararheinheimera spp. (considered as the progenitor of bla PER genes), indicates a role of IS Pa12 in bla PER-2 gene mobilization from the chromosome of Pararheinheimera spp. Also, the bla PER-2 gene was carried by a novel IS Pa12 -composite transposon, Tn 7390. In addition, its association with IS Kox2 -like elements in the close genetic environment in all plasmids analysed suggests a role of these insertion sequence elements in further dissemination of bla PER-2 genes. [ABSTRACT FROM AUTHOR]

• Second description worldwide of LMB-1, a novel subclass B3 MBL in Citrobacter freundii 164 from Argentina. • LMB-1 confers reduced susceptibility to carbapenems and gives inconsistent results with carbapenemase confirmatory tests. • The bla LMB-1 gene was located on a 176-kb IncA/C2 plasmid. • The genetic environment of bla LMB-1 indicates an IS CR -like mobilization from the chromosome of Rheinheimera pacifica. • C. freundii 164 expressed CMY-150, a novel CMY-2 variant that increases ceftazidime and aztreonam MICs when expressed in E. coli. Carbapenemase-producing Enterobacterales expressing OXA-48, KPC, NDM, VIM or IMP enzymes are increasingly reported worldwide. We have characterized LMB-1, a novel metallo-β-lactamase (MBL) of Ambler class B3 from Citrobacter freundii 164 (Cf 164) clinical isolate from Buenos Aires, Argentina. Cf 164 displayed reduced susceptibility to carbapenems but gave inconsistent results with carbapenemase confirmatory tests, indicating the presence of a weak carbapenemase. Analysis of whole-genome sequencing (WGS) of Cf 164 using Resfinder revealed four β-lactamase genes coding for CTX-M-8, PER-2, TEM-1 and CMY-150, a novel chromosomally-encoded CMY variant. Kinetic parameters of purified CMY-150 did not reveal any carbapenemase activity. However, CMY-150 conferred higher minimum inhibitory concentrations (MICs) to E. coli for ceftazidime and aztreonam compared with CMY-2. The in-house-developed β-lactamase search software (ResMiner) in WGS data revealed a novel subclass B3 MBL named LMB-1. LMB-1 conferred resistance to penicillins and expanded-spectrum cephalosporins and reduced susceptibility to carbapenems in E. coli. The bla LMB-1 gene was located on a 176-kb IncA/C2 plasmid. LMB-1 shared 99% amino acid sequence identity with the MBL encoded in the chromosome of Rheinheimera pacifica, it's likely progenitor. Despite repeated attempts, LMB-1 could not be purified, thus only specific activities could indicate hydrolysis of carbapenems. Here we report on CMY-150, a novel CMY-2 variant that confers increased ceftazidime and aztreonam MICs to E. coli and the first description of LMB-1 in Argentina. This work underlines the need for several carbapenemase-producing Enterobacteriaceae (CPE) confirmatory tests, as this novel enzyme might have been missed using only one. Image, graphical abstract [ABSTRACT FROM AUTHOR]