Comparative Analysis of Cellular Components of Lactobacillus delbrueckii subsp. bulgaricus ND02 in Normal and Viable but Non-culturable State by Raman Spectroscopy

In this study, scanning electron microscopy (SEM) and single-cell Raman spectroscopy (SCRS) were used to analyze the differences in the cellular morphology and intracellular macromolecular compounds of Lactobacillus delbrueckii subsp. bulgaricus ND02 in the normal and viable but non-cultivable (VBNC) states. The SEM results showed that the VBNC cells, with a wrinkled surface, were shorter and thicker than the normal ones. The Raman spectra of the VBNC cells were significantly different from those of the normal cells, mainly in terms of 12 peaks, such as 1673, 1650, 1459, 1030, and 1005 cm^-1, reflecting changes in intracellular biomacromolecules such as proteins and nucleic acids. It was shown that L. delbrueckii subsp. bulgaricus ND02 could adapt to adverse environments by regulating the metabolism of proteins, nucleic acids and lipids when induced to enter into VBNC state. The results also confirmed that single-cell Raman spectroscopy can allow non-destructive label-free detection of macromolecules inside normal and VBNC cells of L. delbrueckii subsp. bulgaricus ND02.