Extreme Osmotolerance and Halotolerance in Food-Relevant Yeasts and the Role of Glycerol-Dependent Cell Individuality

Abstract

Osmotolerance or halotolerance are used to describe resistance to sugars and salt, or only salt, respectively. Here, a comprehensive screen of more than 600 different yeast isolates revealed that osmosensitive species were equally affected by NaCl and glucose. However, the relative toxicity of salt became increasingly prominent in more osmoresistant species. We confirmed that growth inhibition by glucose in a laboratory strain of Saccharomyces cerevisiae occurred at a lower water activity (Aw) than by salt (NaCl), and pre-growth in high levels of glucose or salt gave enhanced cross-resistance to either. Salt toxicity was largely due to osmotic stress but with an additive enhancement due to effects of the relevant cation. Almost all of the yeast isolates from the screen were also noted to exhibit hetero-resistance to both salt and sugar, whereby high concentrations restricted growth to a small minority of cells within the clonal populations. Rare resistant colonies required growth for up to 28 days to become visible. This cell individuality was more marked with salt than sugar, a possible further reflection of the ion toxicity effect. In both cases, heteroresistance in S. cerevisiae was strikingly dependent on the GPD1 gene product, important for glycerol synthesis. In contrast, a tps1Δ deletant impaired for trehalose showed altered MIC but no change in heteroresistance. Effects on heteroresistance were evident in chronic (but not acute) salt or glucose stress, particularly relevant to growth on low Aw foods. The study reports diverse osmotolerance and halotolerance phenotypes and heteroresistance across an extensive panel of yeast isolates, and indicates that Gpd1-dependent glycerol synthesis is a key determinant enabling growth of rare yeast subpopulations at low Aw, brought about by glucose and in particular salt.

The Preservative Sorbic Acid Targets Respiration, Explaining the Resistance of Fermentative Spoilage Yeast Species

“…It has been argued that population heterogeneity (preservative heteroresistance) among individual cells or spores within spoilage-yeast or -mold populations has major implications for food spoilage (35)(36)(37)(38). Accordingly, it may take only a few, preservativehyperresistant cells to initiate spoilage.…”

Vinification without Saccharomyces: Interacting Osmotolerant and “Spoilage” Yeast Communities in Fermenting and Ageing Botrytised High-Sugar Wines (Tokaj Essence)

“…The Zygosaccharomyces and Candida species identified in the Essence samples can cope with this condition because they tolerate high osmotic pressures (low water activity). In a recent comparative examination of over 600 hundred strains, Z. rouxii proved to be the most osmotolerant yeast among 151 species [ 35 ]. Its strains were able to grow at glucose concentrations as high as 5.5 M. The other Zygosaccharomyces species whose strains were detected in the Essence wines proved to be much less osmotolerant in those tests: the minimal inhibitory concentrations determined for Z. bailii, Z. bisporus and Z. lentus were 3.5–4.25 M, 3.25–4.5 M and 3.5–4 M, respectively.…”

“…The most frequently occurring yeasts with (at least occasional) wine spoilage effects belong to the species Brettanomyces bruxellensis (Dekkera bruxellensis) [ 16 , 17 ], Zygosaccharomyces bailii ( Saccharomyces bailiii ), Z. rouxii ( Saccharomyces rouxii , Saccharomyces osmophilus ), hybrids/chimeras of various Zygosaccharomyces species (e.g., [ 18 , 19 , 20 , 21 , 22 ]), Candida lactis-condensi ( Torula lactis-condensi , Torulopsis lactis-condensi , Starmerella lactis-condensi ) [ 23 ], Candida zemplinina ( Saccharomyces bacillaris , Torulopsis bacillaris , Starmerella bacillaris ) [ 10 , 23 , 24 ], Hanseniaspora osmophila ( Kloeckeraspora osmophila , Kloeckera corticis ) [ 25 ], Pichia anomala ( Candida pelliculosa , Hansenula anomala ) [ 26 ], Pichia membranifaciens ( Candida valida ) [ 27 , 28 , 29 ], Rhodotorula mucilaginosa [ 30 ], Saccharomycodes ludwigii [ 31 , 32 ], Kregervanrija fluxuum ( Pichia fluxorum ) [ 29 ] and Candida apicola [ 12 ]. Several of these species are osmotolerant or even osmophilic (e.g., [ 6 , 24 , 33 , 34 , 35 ]) and pose a threat to the stability of aging sweetened wines and wines containing higher levels of residual sugar, as well as to other high-sugar beverages, fruit juice concentrates, sugar confectionery products, honey, dried fruit and jams (e.g., [ 36 , 37 , 38 , 39 , 40 ]).…”

Back to the Salt Mines: Genome and Transcriptome Comparisons of the Halophilic Fungus Aspergillus salisburgensis and Its Halotolerant Relative Aspergillus sclerotialis

“…Histidine kinase 7A/B [96] from Hortaea werneckii , the mitogen activated protein kinase (KSS1) [97] from Saccharomyces cerevisiae , both involved in signal transduction, and the cell wall integrity sensor (MID2) [98] were found in the halotolerant, but not in the halophile genome. Interestingly, the irritation of the cell wall and membrane by shrinking triggers induction of the HOG signaling pathway (e.g., in S. cerevisiae [99,100]). Inversely, A. salisburgensis has a homologue of the tyrosine-protein phosphatase 3 (PTP3), which is involved in the repression of the HOG1 gene [101] (Supplementary Table S1).…”