Viable But Not Culturable (VBNC) state of Brettanomyces bruxellensis in wine: new insights on molecular basis of VBNC behaviour using a transcriptomic approach

The spoilage potential of Brettanomyces bruxellensis in wine is strongly connected with the aptitude of this yeast to enter in a Viable But Non Culturable (VBNC) state when exposed to the harsh wine conditions. In this work, we characterized the VBNC behaviour of seven strains of B. bruxellensis representing a regional intraspecific biodiversity, reporting conclusive evidence for the assessment of VBNC as a strain-dependent character. The VBNC behaviour was monitored by fluorescein diacetate staining/flow cytometry for eleven days after addition of 0.4, 0.6, 0.8, 1 and 1.2 mg/L of molecular SO2 (entrance in the VBNC state) and after SO2 removal (exit from the VBNC state). Furthermore, one representative strain was selected and RNA-seq analysis performed after exposure to 1.2 mg/L SO2 and during the recovery phase. 30 and 1634 genes were identified as differentially expressed following VBNC entrance and 'resuscitation', respectively. The results reported strongly suggested that the entrance in the SO2-induced VBNC state in B. bruxellensis is associated with both, sulfite toxicity and oxidative stress response, confirming the crucial role of genes/proteins involved in redox cell homeostasis. Among the genes induced during recovery, the expression of genes involved in carbohydrate metabolism and encoding heat shock proteins, as well as enriched categories including amino acid transport and transporter activity was observed. The evidences of a general repression of genes involved in DNA replication suggest the occurrence of a true resuscitation of cell rather than a simple regrowth.