Gales, A. C. | bioRxiv Science

bioRxiv2026

Genomic features and instability of linezolid resistance in vancomycin-resistant Enterococcus faecalis ST525

This study investigated the mechanisms of oxazolidinone resistance and genomic background in clinical ST525 Enterococcus faecalis isolates, a poorly characterized lineage reported in Brazilian hospitals as either vancomycin-resistant (VRE) or linezolid- and vancomycin-resistant (LVRE). Five LVREfs from intensive care unit patients (linezolid MICs, 8-32 mg/L; vancomycin MICs, >256 mg/L) carried only G2576T and G2505A (in three of four rrl copies) as known linezolid resistance determinants, despite variable linezolid MICs. During serial in vitro passages without antibiotic pressure, isolates with initial MICs of 8-16 mg/L exhibited progressive MIC reductions accompanied by reversion of mutant to wild-type rrl copies, whereas the isolate with an initial MIC of 32 mg/L maintained high-level resistance without reversion. Additional domain V substitutions emerged during MIC decline, while growth kinetics remained broadly similar. Whole-genome sequencing of a representative isolate (LVREfs L7) revealed a ~2.8-Mb chromosome enriched in mobile genetic elements (~8.6%), lacking a functional CRISPR-Cas system. A plasmid-borne prototype Tn1546 carrying vanA, devoid of internal insertion sequences, was found inserted into a Tn3-family transposase gene, a platform not typically associated with vanA mobilization. Together, these findings indicate that ST525 LVREfs, whose genomic features are consistent with long-term persistence in hospitals, may pose a challenge to routine susceptibility testing. A single mutational mechanism can yield variable linezolid MICs, and the ratio of mutated vs wild-type rrl copies can become heterogeneous in the absence of antibiotic pressure, without correlating directly with these values, contrary to the well-documented copy number-MIC correlation.

molecular biology↗

bioRxiv2019

Virulence potential of a multidrug-resistant Escherichia coli belonging to the emerging clonal group ST101-B1 isolated from bloodstream infection

Escherichia coli EC121 is a multidrug-resistant (MDR) strain isolated from bloodstream infection of an inpatient with persistent gastroenteritis and Zone T lymphoma, that died due to septic shock. Despite causing an extraintestinal infection, it harbors few known virulence factors and was assigned into phylogenetic group B1. To evaluate if the EC121 was pathogenic or opportunistic, its genome was sequenced, and an in vitro characterization of some pathogenicity-associated properties was performed. The data retrieved from genome analysis showed that E. coli strain EC121 belongs to the O154:H25 serotype, and to ST101-B1, which was epidemiologically linked to extraintestinal infections and antimicrobial resistance spread as well. Moreover, it was closely related to Shiga-toxin producing E. coli (STEC). Besides, strain EC121 is an MDR strain harboring 14 antimicrobial resistance genes, including blaCTX-M-2, and more than 50 complete virulence genetic clusters, which are reported to be associated either with DEC or ExPEC. The strain also displays the capacity to adhere to a variety of cell lineages, and invade T24 bladder cells, as well as the ability to form biofilms on abiotic surfaces, and survive the bactericidal serum complement activity. Additionally, it is virulent in the Galleria mellonella model. Altogether, E. coli EC121 unveiled to be a pathogen powered by its multi-drug resistance characteristic. Carry out studies providing accurate information about the virulence potential of all kinds of MDR strains are essential because these studies will help in the development of alternative therapies of infection management and spread control of MDR strains. Authors summaryThe phylogenetic origin of extraintestinal pathogenic Escherichia coli is mostly associated with phylogroup B2, and the majority of the studies regarding extraintestinal infection focus on the most virulent strains, which might also present multidrug-resistant (MDR) phenotype. Strains belonging to phylogroup B1 and isolated from extraintestinal infections are considered as opportunist pathogens and have their virulence neglected. We focus our study in one MDR strain isolated from bloodstream infection that belongs to phylogenetic group B1 to enlarge the knowledge about the virulence of this kind of strain. We demonstrated that the EC121 is capable of adheres to intestinal and bladder human cells, and invades the latter one; it survives to human serum bactericidal effects and produces biofilm. Additionally, the in vivo assay confirmed the EC121 virulence, showing that it should be considered a pathogenic strain. The genetic analyzes highlighted important aspects of EC121 which are typical from strains of sequence type 101, like its involvement in the spread of antimicrobial resistance genes and its relationship with extraintestinal infection from diverse sources. Information concerning the virulence of MDR strains is important for the development of global actions treating the spread of antimicrobial resistance, as well as to elucidate the pathogenesis of strains that were considered as an opportunist.

microbiology↗