Nutritional requirements and survival of the red wine spoilage yeast Brettanomyces bruxellensis

Smith, Brendan Daniel (2016-12)

Thesis (MScAgric)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: Brettanomyces bruxellensis is a red wine spoilage yeast that plagues the wine industry. It facilitates the formation of compounds such as volatile phenols, which impart negative aromas in red wines. Its ability to survive in wine through adaptations to various stressors (e.g. high ethanol and sulphur dioxide concentrations, low pH) have made it the subject of several studies. These studies aim to understand its biology (and in particular its nutrient requirements) and its survival mechanisms in general, as well as to detect and enumerate it accurately and to eliminate it. Nevertheless, literature on the subject is at certain times contradictory with regard to the nutritional needs of this yeast. The survival of this yeast for extended periods of time lead us to question how it is so well adapted to this deficient environment. In other words, what substrates does B. bruxellensis utilise to sustain growth or at least survival in such a nutrientdeficient medium where stronger fermenters (e.g. Saccharomyces cerevisiae) cannot survive? This study investigates the carbon and nitrogen source assimilation of three B. bruxellensis strains in a defined and model synthetic wine medium. In addition, the growth kinetics were determined (as well as the consumption pattern of the various carbon and nitrogen sources). This was performed in conditions similar to bottle ageing (anaerobic) and barrel maturation (semi-anaerobic). Furthermore, the purpose of assimilating these sources is explored with a focus on growth or cell maintenance (i.e. survival with no growth). The data showed that carbon consumption followed a step wise pattern. At first, sugars were consumed, thereby leading to the production of ethanol and biomass concurrently. Upon complete consumption of the sugars, malic acid was consumed together with ethanol, but only when oxygen was present for the latter. These compounds were consumed slowly and resulted in the survival of the cells for a period of 45 days. After this period, the consumption of ethanol allowed for the extended functioning of the cell, however the assimilation of ethanol lead to an increasing degree of imbalance (more and more NADH was produced with little to no conversion to NAD+) in the cell and eventually lead to a slow but steady decline of the population. The formation of 4- Ethylphenol (4-EP) was investigated in order to ascertain its ability to correct the redox potential of the B. bruxellensis cell. The results displayed the partial regeneration of NAD+ during 4-EP formation, however, this metabolic pathway alone is not solely responsible. Unlike for carbon sources, the data showed that the assimilation of nitrogen compounds were strain specific and certain strains required more nitrogen than the others. The sources of prime importance were ammonia and arginine and were assimilated during the exponential growth phase (i.e. during sugar consumption). During the stationary phase, proline was assimilated regardless of the presence/absence of oxygen, possibly to counteract stressors in the cell and ensure survival of the population. This study contributes to an improved understanding of how B. bruxellensis survives in wine and is able to maintain cell function for extended periods of time. This leads to a better understanding of the spoilage yeast B. bruxellensis and will allow for the production of wine in an integrated manner to avoid the proliferation of this microorganism.

AFRIKAANS OPSOMMING: Brettanomyces bruxellensis is ’n gis wat rooiwyn bederf en daardeur die wynbedryf teister. Dit fasiliteer die vorming van verbindings soos vlugtige fenole, wat negatiewe aromas aan rooiwyn oordra. Sy vermoë om in wyn te oorleef deur aanpassings tot verskillende stressors (bv. hoë konsentrasies van etanol en swaeldioksied, lae pH) het daartoe gelei dat dit reeds die onderwerp van verskeie studies was wat gepoog het om die biologie daarvan te verstaan (veral sy voeidingsvereistes) sowel as sy oorlewingsmeganismes in die algemeen, en ook hoe om dit akkuraat te bespeur en te enumerate en dit te elimineer. Nietemin is die literatuur oor die onderwerp by tye teenstrydig met betrekking tot die voedingsbehoeftes van hierdie gis. Die oorlewing van die gis vir lang tydperke noop die vraag hoe dit so goed aangepas is tot hierdie deficient omgewings. Met ander woorde, watter substrate gebruik B. bruxellensis om groei of ten minste oorlewing in so ’n voedingsarme medium te onderhou waar sterker gisters fermenters (bv. Saccharomyces cerevisiae) nie kan oorleef nie? Hierdie studie het die koolstof en stikstof assimilasie van drie B. bruxellensis rasse in ’n gedefinieerde en model sintetiese wynmedium ondersoek. Daarbenewens is die groeikinetika in elke medium bepaal, sowel as die verbruikspatroon van die verskillende koolstof- en stikstofbronne wat verskaf is. Dit is ook ondersoek onder omstandighede wat eenders is aan bottelveroudering (anaërobies) en vatveroudering (semi-anaërobies). Verder is die doel van die assimilering van hierdie bronne ondersoek met ’n fokus op groei of selonderhoud (m.a.w. oorlewing sonder groei). Die data het getoon dat koolstofverbruik ’n stapsgewyse patroon gevolg het. Aanvanklik is suikers verbruik, wat sodoende gelei het tot die gelyktydige produksie van etanol en biomassa. Sodra die suikers volledige verbruik is, is appelsuur tesame met etanol verbruik, maar laasgenoemde slegs wanneer suurstof daarvoor teenwoordig was. Hierdie verbindings is stadig verbruik en het gelei tot die oorlewing vna selle vir ’n tydperk van 45 dae. Hierna het die verbruik van etanol die verlengde funksionering vna die sel toegelaat, hoewel die assimilering van etanol gelei het tot ’n toenemende mate van wanbalans (toenemend NADH is produseer met min of geen omskakeling na NAD+) in die sel en uiteindelik tot ’n stadige maar bestendige afname in die bevolking. Die vorming van 4-etielfenol (4-EP) is ondersoek om die vermoë daarvan om die redokspotensiaal van die B. bruxellensis sel te korrigeer, te bepaal. Die resultate het gedeeltelike regenerasie van NAD+ tydens 4-EP-vorming vertoon, maar hierdie metaboliese pad is egter nie alleen daarvoor verantwoordelik nie. In teenstelling met koolstofbronne toon die data dat die assimilering van stikstofverbindings rasspesifiek is en dat sekere rasse meer stikstof as ander benodig. Die belangrikste bronne was ammoniak en arginien, wat tydens die eksponensiële groeifase geassimileer is (m.a.w. tydens suikerverbruik). Tydens die stasionêre fase is prolien geassimileer, ongeag die teenwoordigheid/afwesigheid van suurstof, moontlik om stressors in die sel teen te werk en die oorlewing van die bevolking te verseker. Hierdie studie maak ’n bydrae tot ’n verbeterde begrip van hoe B. bruxellensis in wyn oorleef en die vermoë het om selfunksionering vir verlengde tydperke te onderhou. Dit lei tot ’n beter begrip van die B. bruxellensis bederfgis en sal die produksie van wyn in ’n geïntegreerde wyse moontlik maak om sodoende die vermenigvuldiging van hierdie mikro-organisme te verhoed.

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