Juan C Vázquez-Ucha, Ana C Almeida-Santos, Andrea García-Pose, Bárbara Duarte, Alejandro Beceiro, Guido Werner, Jennifer K Bender, Antónia Read, Mariana Romanholo, Carla Novais, Luisa Peixe, Ana R Freitas
This study reports the first optrA-positive Efs causing human infections in Portugal, shifting from previously linezolid resistance mechanisms (23S rRNA mutations). Infections by distinct clones suggest independent acquisition events, while transferable resistance genes' presence in the gut implicates food/environmental sources. Linezolid-resistant optrA-carrying Efs infections are an ongoing clinical reality, alerting the urgent need for proactive surveillance and containment strategies.
OBJECTIVE: The emergence of transferable linezolid resistance genes as optrA represents an increasing concern. This study characterizes the first optrA-positive E. faecalis (Efs) isolates associated with human infections in Portugal, while highlighting undetected colonization as a reservoir for dissemination within healthcare settings.
METHODS: Between July 2024-February 2025, three Efs isolates, classified as linezolid-resistant, were identified from different patients at a central hospital in Porto, Portugal. Two isolates were available for phenotypic/molecular testing and whole-genome sequencing. Clonality, antimicrobial susceptibility, filter-mating, and screening for AMR genes, bacteriocins, virulence and plasmid replicases were performed. Core-SNP and cgMLST analyses compared these strains to publicly available Efs genomes, and the genetic contexts carrying optrA were characterized.
RESULTS: All patients had no prior exposure to linezolid. Available isolates were multidrug-resistant but vancomycin susceptible. One expressed linezolid resistance (MIC=8 mg/L),the other displayed a borderline MIC (4 mg/L,EUCAST=ECOFF), highlighting potential underdiagnoses. They belonged to novel complex types, ST16-CT4478 and ST585-CT4489. Both carried optrA, located either chromosomally or on a 64-kb plasmid, within genetic platforms resembling those reported in livestock-associated E. faecalis isolates. A prior survey (October/2023-February/2024) at the same hospital found 2% colonized with linezolid-resistant enterococci (mostly optrA-carrying Efs), indicating likely emergence.
CONCLUSION: This study reports the first optrA-positive Efs causing human infections in Portugal, shifting from previously linezolid resistance mechanisms (23S rRNA mutations). Infections by distinct clones suggest independent acquisition events, while transferable resistance genes' presence in the gut implicates food/environmental sources. Linezolid-resistant optrA-carrying Efs infections are an ongoing clinical reality, alerting the urgent need for proactive surveillance and containment strategies.