| Summary: | The use of non-<i>Saccharomyces</i> yeast species generally involves sequential or co-inoculation of a <i>Saccharomyces cerevisiae</i> strain to complete fermentation. While most studies have focused on characterising the impact that <i>S. cerevisiae</i> has on the growth and metabolic activity of these non-<i>Saccharomyces</i> species, microbial interactions work reciprocally. Antagonism or competition of non-<i>Saccharomyces</i> species against <i>S. cerevisiae</i> has been shown to impact subsequent fermentation performance. To date, it remains unclear whether these negative interactions are strain specific. Hence, characterisation of strain-specific responses to co-inoculation would enable the identification of specific <i>S. cerevisiae</i> strain/non-<i>Saccharomyces</i> combinations that minimise the negative impacts of sequential fermentation on fermentation performance. The competitive fitness response of 93 <i>S. cerevisiae</i> strains to several non-<i>Saccharomyces</i> species was simultaneously investigated using a barcoded library to address this knowledge gap. Strain-specific fitness differences were observed across non-<i>Saccharomyces</i> treatments. Results obtained from experiments using selected <i>S. cerevisiae</i> strains sequentially inoculated after <i>Metschnikowia pulcherrima</i> and <i>Torulaspora delbrueckii</i> were consistent with the competitive barcoded library observations. The results presented in this study indicate that strain selection will influence fermentation performance when using non-<i>Saccharomyces</i> species, therefore, appropriate strain/yeast combinations are required to optimise fermentation.
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