Raman microspectroscopy has increased in popularity in the field of microbiology because it allows a spectral fingerprinting of bacterial pathogens at an unrivaled speed, which is important for the early treatment of infectious diseases such as tuberculosis. An indispensable prerequisite for the success of this method is a profound knowledge, how the spectral profiles depend on the age of the bacteria. We therefore followed the growth of two rapidly growing Mycobacterium tuberculosis relatives, the pigmented Mycobacterium aurum, and the non-pigmented Mycobacterium smegmatis, by means of Raman microspectroscopy. Both species showed remarkable temporal changes in the single-bacteria Raman spectra: In the signatures of M. aurum, pigment-associated Raman signals could be detected not until 72 h of growth and also remained highly variable thereafter. The Raman spectra of M. smegmatis exhibited lipid signals presumably arising from mycolic acids, which are a hallmark feature of mycobacteria, but only after the bacteria reached the late stationary growth phase (>48 h). A principal component analysis thus classified the Raman spectra according to the cultivation age. In summary, these findings have to be reckoned with in future studies dealing with the identification of mycobacteria via Raman microspectroscopy. Graphical abstract Changes in the chemical composition of bacterial cells over growth time may influence the results of Raman spectroscopic studies of bacteria.
Keywords: Analytical methods; Growth pigments; Microscopy; Mycobacteria; Raman spectroscopy.