Abstract
ABSTRACTTo successfully complete malolactic fermentation (MLF), Oenococcus oeni must overcome wine stress conditions of low pH, high ethanol, and the presence of SO2. Failure to complete MLF may result in detrimental effects to the quality and stability of the resulting wines. Research efforts to date have focused on elucidating the mechanisms and genetic features that confer the ability to withstand low pH and high ethanol concentrations on O. oeni; however, the responses to SO2 stress are less well defined. This study focused on characterizing the transcriptional response of O. oeni to SO2 challenge during cultivation in a continuous system at wine-like pH (3.5). This experimental design allowed the precise discrimination of transcriptional changes linked to SO2 stress from responses associated with growth stage and cultivation parameters. Differential gene expression analysis revealed major transcriptional changes following SO2 exposure and suggested that this compound primarily interacts with intracellular proteins, DNA, and the cell envelope of O. oeni. The molecular chaperone hsp20, which has a demonstrated function in the heat, ethanol, and acid stress response, was highly upregulated, confirming its additional role in the response of this species to SO2 stress. This work also reports the first nanopore-based complete genome assemblies for O. oeni.IMPORTANCE Malolactic fermentation is an indispensable step in the elaboration of most wines and is generally performed by Oenococcus oeni, a Gram-positive heterofermentative lactic acid bacterium species. While O. oeni is tolerant to many of the wine stresses, including low pH and high ethanol concentrations, it has high sensitivity to SO2, an antiseptic and antioxidant compound regularly used in winemaking. Understanding the physiological changes induced in O. oeni by SO2 stress is essential for the development of more robust starter cultures and methods for their use. This study describes the main transcriptional changes induced by SO2 stress in the wine bacterium O. oeni and provides foundational understanding on how this compound interacts with the cellular components and the induced protective mechanisms of this species.
Highlights
To successfully complete malolactic fermentation (MLF), Oenococcus oeni must overcome wine stress conditions of low pH, high ethanol, and the presence of SO2
More specific responses to acid stress involve the induction of genes encoding alanine carboxypeptidase, which is involved in the maintenance of bacterial cell wall integrity, malate dehydrogenase/malate permease that contribute to cytoplasmic deacidification, and the gene hsp18, encoding the heat shock protein Lo18 [13, 14], a membrane-associated heat shock protein from the alpha crystallin family known as gene hsp20 [15]
This study investigated the transcriptional changes linked to SO2 stress in the wine bacterium O. oeni and provides evidence for how this compound interacts with the cellular components of this species
Summary
To successfully complete malolactic fermentation (MLF), Oenococcus oeni must overcome wine stress conditions of low pH, high ethanol, and the presence of SO2. Malolactic fermentation (MLF) is defined as the decarboxylation of L-malic acid into L-lactic acid and CO2 [1] It is considered an indispensable step in elaborating most wines due to the chemical changes associated with this process, including reduction of acidity, enhancement of organoleptic properties, and increased microbiological stability [2]. Understanding the physiological changes induced in O. oeni under stressful wine conditions (low pH, high ethanol, and SO2) is essential for the development of more robust starter cultures and methods for their use. Studies focused on understanding the response of O. oeni to low extracellular pH have shown that malic acid utilization and the consequent consumption of protons creates a membrane potential that powers ATP generation via membrane-bound ATPases [6,7,8]. More specific responses to acid stress involve the induction of genes encoding alanine carboxypeptidase, which is involved in the maintenance of bacterial cell wall integrity, malate dehydrogenase/malate permease that contribute to cytoplasmic deacidification, and the gene hsp, encoding the heat shock protein Lo18 [13, 14], a membrane-associated heat shock protein from the alpha crystallin family known as gene hsp20 [15]
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