The fate of genetically modified yeast in the environment

Schoeman, Heidi (2005-03)

Dissertation (PhD(Agric))--University of Stellenbosch, 2005.

Thesis

ENGLISH ABSTRACT: Considerable efforts have been made to improve strains of the wine yeast Saccharomyces cerevisiae through the use of genetic engineering. Although the process is well defined, globally there is much resistance towards the use of genetically modified organisms (GMOs), primarily because little is known about their environmental fate and their potential effect on naturally occurring organisms. The public concern is mainly focused on the uncertainty associated with the impact of the deliberate or accidental release of a GMO into the environment. As a consequence, thére is an urgent need to assess the potential risks involved with the use of this new technology. For the eventual global acceptance of any GMO, it is imperative that the consumer must be convinced that it is ultimately safe for human consumption and the environment. In order to achieve this, certain risk assessment procedures must be performed on each and every GMO that is planned to be released into the environment. Although some of the genetically modified (GM) yeasts that have been developed comply with the strict legislation of most countries and have been cleared by regulatory authorities for commercial use, GM yeasts have not, as yet, been used for the commercial production of GM bread, beer or wine. Nevertheless, the use of GM yeasts in the market appears imminent and there is an urgent need to assess and address the perceived health and environmental risks associated with GM foods. The overall objective of this research was to evaluate key environmental issues concerning the use of GM yeasts. The focus was on comparing the behaviour of specific parental and GM yeast strains in model systems in order to determine whether the GM strains may have any selective advantage, which could lead to their spreading. Specifically, it involved monitoring of the growth behaviour of selected GM yeasts within a vineyard microbial community and in fermentations, as well as the interaction of these yeasts with sand and glass surfaces in an aqueous environment. The GM yeasts under investigation were recombinant strains of a well-known, industrial strain of S. cerevisiae VIN13 expressing an a-amylase (designated GMY1); an endo-p-1,4-glucanase and endo-p-xylanase (designated GMY2); and a pectate lyase and polygalacturonase (designated GMY3). The GM yeasts were mist-inoculated onto individually-contained blocks consisting of one-year old grapevines in a secluded glasshouse environment. Specifically, the numbers and dynamics of GM yeast survival, as well as the effect of an introduced GM yeast on the yeast community dynamics and numbers, were investigated. Overall, it was found that the most prevalent wild yeasts isolated from the grapevines were Rhodo torula, Yarrowia lipolytica, Pichia and Candida spp. VIN13 and the GM yeasts did not affect the overall ecological balance of the microflora on the grapevines. Wild strains of S. cerevisiae were seldom isolated from the grapevines. With a few exceptions, the overall detection of GM yeasts was numerically limited. Co-inoculation of (VIN13+GMY1) and (GMY1+GMY2) revealed detection approximately in the same ratio at which they were inoculated, with small differences in the order of GMY2>GMY1 >GMY3. GM yeasts were rarely isolated from bark and soil samples. Spontaneous fermentation of the grapes harvested from the different treated blocks indicated that the GM yeasts survived on the berries, that the natural fermenting ability of VIN13 was conserved in the recombinant strains, and that the GM yeasts did not have any competitive advantage. The soil environment forms an important part of the biosphere and the transport and attenuation of a GM yeast in this matrix will to a large extent affect their ultimate fate in the environment. In soil, microorganisms either occur as suspended cells in pore water or as biofilms on soil surfaces. Although less extensive than a typical soil yeast, Cryptococcus, epifluorescent staining of biofilms confirmed that VIN13 and GMY1 were capable of existing in a biofilm mode on sand granules and glass. Data on effluent numbers detected in flow cells indicated that GMY1 had no advantage due to the genetic modification and had the same reproductive success as VIN13. These strains either had no difference in biofilm density or GMY1 was less dense than VIN13. When co-inoculated, GMY1 had no negative influence on the mobility of Cryptococcus through a sand column, as well as the ability of Cryptococcus to form biofilms. Furthermore, it was found that GMY1 did not incorporate well into a stable biofilm community on glass, but did not disrupt the biofilm community either. This is the first report of the assessment of the fate of GM strains of VIN13 that are suitable for the wine and baking industry. The investigation of the GM yeasts in this study under different scenarios is a good start to an extensive and necessary risk assessment procedure for the possible use of these GM yeasts in the industry. This study could lead to the provision of much-needed scientific and technical information to both industry and regulating bodies. The outcome of this research is also intended to serve as a basis for information sharing with public interest groups.

AFRIKAANSE OPSOMMING: Aansienlike pogings is reeds aangewend om rasse van die wyngis, Saccharomyces cerevisiae, deur middel van genetiese manipulering te verbeter. Alhoewel hierdie proses goed gedefinieerd is, is daar wêreldwyd heelwat teenkanting teen die gebruik van geneties gemanipuleerde organismes (GMO's). Dit is hoofsaaklik te wyte daaraan dat so min bekend is oor hul lot in die omgewing en hul potensiële effek op die organismes wat natuurlik voorkom. Die publiek is veral besorg oor die onsekerheid verbonde aan die bestemde of toevallige vrylating van 'n GMO in die omgewing. Gevolglik is daar 'n dringende behoefte om die potensiële risiko's in die gebruik van hierdie nuwe tegnologie te bepaal. Dit is van uiterste belang dat die verbruiker oortuig sal word van die veiligheid vir menslike gebruik en die omgewing voordat enige GMO uiteindelik wêreldwyd aanvaarbaar sal word. Om dit te kan bereik sal sekere risiko-bepalende prosedures toegepas moet word op ieder en elke GMO wat beplan word om vry gelaat te word in die omgewing. Alhoewel sommige van die geneties gemanipuleerde (GM) giste aan die streng wetgewing van die meeste lande voldoen en deur die owerhede vir kommersiële gebruik goedgekeur is, word GM-giste nog steeds nie vir die produksie van GM brood, bier of wyn gebruik nie. Ten spyte hiervan, blyk die gebruik van GM-giste onafwendbaar te wees en is daar dus 'n dringende behoefte om die voorspelde gesondheids- en omgewingsrisiko's wat met GM voedsel geassosieer word, aan te spreek. Die oorhoofse doel van hierdie navorsing was om belangrike omgewingskwessies aangaande die gebruik van GM-giste te evalueer. Die fokus was op die vergelyking van die gedrag van spesifieke oorspronklike gisrasse en GM-gisrasse in modelsisteme sodat daar bepaal kon word of die GM-gisrasse enige selektiewe voordele het wat moontlik tot hulonbeheerde verspreiding in die natuur sou kon lei. Dit het spesifiek die monitering van die groei van geselekteerde GMgiste binne 'n mikrobiese gemeenskap op wingerd en in fermentasies behels, asook die interaksie van hierdie giste met grond en glas oppervlaktes in 'n wateromgewing. Die GM-giste wat in hierdie studie gebruik is, was rekombinante rasse van 'n bekende industriële ras van S. cerevisiae, VIN13, wat geneties gemodifiseerd was om 'n a-amylase (aangedui as GMG1); 'n endo-p-1,4-glukanase en 'n endo-B-xilanase (aangedui as GMG2); en 'n pektaatliase en 'n poligalaktorinase (aangedui as GMG3) uit te druk. Die GM-giste is op afsonderlike blokke van eenjaaroue wingerdplante binne-in 'n beskutte kweekhuis gesproei-inokuleer. Daar was spesifiek na die selgetalle en dinamika van die oorlewende GM-giste gelet, asook wat die invloed was van die inokulasie van 'n GM gis op die selgetalle van die natuurlike gisgemeenskap. Daar is bevind dat die wildegiste Rhodotorula, Yarrowia Iipolytica, Pichia en Candida spp die gereeldste van die wingerd geïsoleer is. VIN13 en die GM-giste het nie die ekologiese balans van die natuurlike mikrobiese populasie op die wingerd versteur nie. Wilde rasse van S. cerevisiae is selde geïsoleer vanaf die wingerd. In die meeste gevalle is daar bevind dat wanneer GM-giste opgespoor is, hulle in lae selgetalle voorgekom het. Waar giste saam geïnokuleer was, was die opsporing van (VIN 13+GMY1) en (GMY1 +GMY2) ongeveer in dieselfde verhouding as waarin hul geïnokuleer was, terwyl klein verskille in die orde van GMY2>GMY1 >GMY3 opgemerk is. GM-giste is selde vanaf bas- en grond-monsters geïsoleer. Spontane fermentasies van druiwe wat geoes vanaf die verskillende behandelde blokke is, het daarop gedui dat die GM-giste wel op die druiwe oorleef, dat die natuurlike vermoë van VIN13 om te kan fermenteer in die gemodifiseerde gisrasse behoue gebly het en dat die GM-giste geensins deur die genetiese modifikasies bevoordeel was nie. Grond is 'n belangrike deel van die biosfeer en die verspreiding en aanhegting van 'n GM-gis in hierdie matriks sal sy algehele lot in die omgewing tot 'n groot mate beïnvloed. In die grond kom mikroorganismes as gesuspendeerde selle in poriewater of as biofilms op die oppervlaktes van grond voor. Alhoewel biofilmvorming van VIN13 en GMG1 swakker was as in die geval van 'n tipiese grondgis, Cryptococcus, het epifluoresserende kleuring van hierdie S. cerevisiaegiste bevestig dat VIN13 en GMG1 in staat was om as biofilms op sandkorrels en glas te oorleef. Gebaseer op seltellings in vloeiseluitlaat, kon daar afgelei word dat GMG1 geen selektiewe voordeel geniet het as gevolg van die genetiese modifikasie nie en dat die gis net so reproduktief was as VIN13. Hierdie gisrasse het geen verskil in biofilmdigtheid getoon nie of die biofilmvorming van GMG1 was minder dig as die van VIN13. Wanneer GMG1 saam met Cryptococcus geïnokuleer was, het GMG1 geen negatiewe invloed op die beweeglikheid van Cryptococcus deur 'n sandkolom gehad nie en die vermoë van Cryptococcus om biofilms te vorm is ook nie beïnvloed nie. Daar is verder ook bevind dat GMG1 nie goed binne-in 'n gestabiliseerde biofilmgemeenskap op glas geïnkorporeer het nie, maar dat die gis ook nie die biofilmgemeenskap versteur het nie. Hierdie studie verteenwoordig die eerste ondersoek ooit oor die lot, oorlewing en groeigedrag van GM-wyngiste in biologies-afgesonderde wingerd-, fermentasie-, modelgrond- en modelwater-ekosisteme. Die bestudering van hierdie GM-giste onder verskillende omgewingstoestande in afgeslote ekosisteme lê 'n stewige basis vir verdere ondersoeke en die ontwikkeling van omvattende en noodsaaklike risikobepalingsprosedures betreffende die moontlike toekomstige gebruik van GM-giste in die industrie. Hierdie studie baan die weg tot die verkryging van noodsaaklike wetenskaplike en tegniese inligting oor die veiligheidsaspekte rakende GM-wyngiste en dit kan van groot waarde vir die industrie, owerhede en verbruikerspubliek wees.

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