Revealing the Metabolic Activity of Persisters in Mycobacteria by Single-Cell D2O Raman Imaging Spectroscopy.

The metabolic activity of bacterial cells largely differentiates even within a clonal population. Such metabolic divergence among cells is thought to play an important role for phenotypic adaptation to ever-changing environmental conditions, such as antibiotic persistence. It has long been thought that persisters are in a state called dormancy, in which cells are metabolically inactive and do not grow. However, recent studies suggest that some types of persisters are not necessarily dormant, triggering a debate about the mechanisms of persisters. Here, we combined single-cell Raman imaging spectroscopy and D2O labeling to analyze metabolic activities of bacterial persister cells. Metabolically active cells uptake deuterium through metabolic processes and give distinct C-D Raman bands, which are direct indicators of metabolic activity. Using this imaging method, we characterized the metabolic activity of Mycobacterium smegmatis, a fast-growing model for Mycobacterium tuberculosis. We found that persister cells of M. smegmatis show certain metabolic activity and active cell growth in the presence of the antibiotic rifampicin. Interestingly, persistence is not correlated with growth rate prior to antibiotic exposure. These results show that dormancy is not responsible for the persistence of M. smegmatis cells against rifampicin, suggesting that the mechanism of persistence largely varies depending on the type of antibiotics and bacteria. Our results successfully demonstrate the potential of our perfusion-based single-cell D2O Raman imaging system for the analysis of the metabolic activity and growth of bacterial persister cells.