Johan Elf is Professor of Physical Biology at Uppsala University, where his research combines quantitative microscopy, molecular biology, microfluidics, and computation to understand and exploit bacterial cell physiology. He pioneered single-molecule tracking in living cells, enabling direct measurements of molecular search, diffusion, and binding kinetics in bacteria, and later developed optical pooled screening to connect dynamic cellular phenotypes with genetic identity at scale. His group also established rapid phenotypic antibiotic susceptibility testing based on growth measurements of individual bacteria, a technology translated through Astrego Diagnostics and acquired by Sysmex. The resulting PA-100 system was recognized with the £8 million Longitude Prize in 2024. Elf has received three ERC grants and is a Wallenberg Scholar. He is a member of the Royal Swedish Academy of Sciences, EMBO, and the Royal Swedish Academy of Engineering Sciences. His current work focuses on single-cell infection diagnostics, antimicrobial resistance, and high-content functional genomics.