Plasmids for the Modern Microbe.

Worldwide, antimicrobial-resistant infections were associated with approximately 4.7 million deaths in 2021, and over 8 million annual deaths associated with antimicrobial-resistant bacteria are projected to occur globally by 2050 (1). Bacteria develop resistance to antibiotics primarily through 2 mechanisms: mutation of their chromosomal DNA (genome) or acquisition of extrachromosomal DNA, such as plasmids (2). Plasmids are circular pieces of DNA that replicate independently of the host chromosome and can be transferred horizontally between bacteria through a process called conjugation (2). Genes encoded within a plasmid act as tools in a toolbox, helping bacteria survive harsh environments. Many bacteria commonly encountered in clinical settings have developed plasmid toolboxes stocked with genes that enable antimicrobial resistance (AMR). A study in the December 4, 2025, issue of Science offers a look at how plasmid-mediated AMR may have evolved under the evolutionary pressures of the antibiotic era (3). Researchers from the United Kindom sequenced 765 plasmids from 368 isolates of pathogenic Enterobacteriaceae cultured from human infections between the years 1917 and 1954, known as the Murray Collection (4), and used BLAST and other tools to query public sequence data from modern isolates irrespective of host. In doing so, they were able to evaluate the modern fates of these plasmids after 70 years or more of evolutionary pressure from antibiotic use.