While structural information on biomolecules is mainly obtained from purified in vitro samples, NMR can also be applied in the context of entire cells or organisms. The present study describes maturation processes in living Salmonella enterica serovar Typhimurium, a prevalent cause for human gastroenteritis. In our physiological study, we follow the composition of the O-antigen on the outer bacterial membrane with high-resolution MAS NMR spectroscopy. We detect and characterize an evolution of the O-antigen composition, in particular of the O-acetylation state of the O-antigen, a factor that can play an important role in vaccine development.