Brendan J. Kelly, MD, MS, is an Assistant Professor of Medicine, Epidemiology & Microbiology at the University of Pennsylvania, where he works as an infectious diseases physician and molecular epidemiologist. He directs the microbiome transmission (MOTION) lab, which serves as the core laboratory for the Center for the Study of Antibiotic Resistance (C-STAR) at the University of Pennsylvania. Dr. Kelly's research focuses on the mechanisms of colonization resistance and the application of microbiome-based therapeutics to address the public health threat posed by antimicrobial resistance. Specifically, Dr. Kelly seeks to understand properties of microbial ecosystems that protect against bacterial pathogen colonization and infection, how microbial ecosystems are altered by antibiotics, and how they recover from infection and antibiotic treatment. He has performed large, clinical-translational studies to better understand two infections that occur commonly during critical illness: bacterial pneumonia and C. difficile colitis. He has also investigated the use of fecal microbiota transplant (FMT) to aid recovery of the intestinal microbiota after C. difficile treatment. During the COVID-19 pandemic, Dr. Kelly led several large, translational studies to understand the molecular evolution of SARS-CoV-2. Dr. Kelly's work has been supported by the National Institute of Allergy and Infectious Diseases (NIAID) and the Centers for Disease Control and Prevention (CDC).