Positron-emission tomography (PET) scans can detect the body’s response to infection by lighting up the body’s inflammatory response using radiolabeled tracers. But many things other than infections cause inflammation. A new radiotracer that marks live bacteria in mice offers more specificity and could eventually help clinicians determine whether antibiotics are working (ACS Cent. Sci. 2020, DOI: 10.1021/acscentsci.9b00743). David Wilson of the University of California, San Francisco, and his colleagues made radiolabeled d-alanine, an amino acid found in bacterial cell walls but not in mammalian cells. The researchers created shoulder injuries in mice to create inflammation, then injected live bacteria into one shoulder and heat-killed microbes into the other. After delivering their 11C-labeled d-alanine probe, PET scans of the areas with living pathogens produced a signal about 3.5 times as high as those with dead microbes. Other PET probes that detect inflammation yielded the same signal in both shoulders. In another experiment, the team repeated the shoulder injuries, then injected antibiotic-resistant Escherichia coli into one joint and antibiotic-sensitive E. coli into the other. After antibiotic treatment, the d-alanine tracer’s signal was completely lost with antibiotic-sensitive bacteria, suggesting the technique could monitor the effectiveness of drugs. “To see that signal decrease after antibiotic treatment, that’s huge,” says Alvaro Ordoñez, who studies imaging of infectious diseases at Johns Hopkins University and was not involved with the study.