APPLIED MICROBIAL AND CELL PHYSIOLOGY Saccharomyces cerevisiae EC-1118 enhances the survivability of probiotic Lactobacillus rhamnosus HN001 in an acidic environment Phebe Lixuan Lim 1 & Mingzhan Toh 1 & Shao Quan Liu 1,2 Received: 6 January 2015 /Revised: 18 March 2015 /Accepted: 20 March 2015 # Springer-Verlag Berlin Heidelberg 2015 Abstract The present study attempted to partially character- ize and elucidate the viability-enhancing effect of a yeast strain Saccharomyces cerevisiae EC-1118 on a probiotic strain Lactobacillus rhamnosus HN001 under acidic conditions using a model system (non-growing cells). The yeast was found to significantly enhance (P <0.05) the viability of the probiotic strain under acidic conditions (pH 2.5 to 4.0) by 2 to 4 log cycles, and the viability-enhancing effects were ob- served to be influenced by pH, and probiotic and yeast con- centrations. Microscopic observation and co-aggregation as- say revealed that the viability-enhancing effect of the yeast could be attributed to direct cell-cell contact co-aggregation mediated by yeast cell surface and/or cell wall components or metabolites. Furthermore, non-viable yeast cells killed by thermal means were observed to enhance the viability of the probiotic strain as well, suggesting that the surface and/or cell wall component(s) of the yeast contributing to co-aggregation was heat-stable. Cell-free yeast supernatant was also found to enhance the viability of the probiotic strain, indicating the presence of protective yeast metabolite(s) in the supernatant. These findings laid the foundation for further understanding of the mechanism(s) involved and for developing novel micro- bial starter cultures possibly without the use of live yeast for ambient-stable high-moisture probiotic foods. Keywords Lactobacillus rhamnosus . Saccharomyces cerevisiae . Acidic environment . Viability . Co-aggregation . Metabolites Introduction The increasing scientific and commercial interests in probiotics have been spurred by mounting evidence of their therapeutic health effects over the past two decades (Caselli et al. 2013). Probiotics have been shown to improve lactose metabolism, reduce serum cholesterol, possess anti-mutagenic and anti-carcinogenic properties, and balance the gener- ation of pro- and anti-inflammatory cytokines, thereby creating healthy interactions between the host and the microbes in the gut (Klaenhammer and Kullen 1999; Lee and Salminen 2009). Majority of the commercial probiotics are strains of Lactobacillus and Bifidobacterium, though not necessarily re- stricted to these genera, which have been characterized down to the strain level and validated to ensure that they fulfill safety and functional criteria (Sanders 2008; Vasiljevic and Shah 2008). Selection criteria for probiotics include being of host origin, capable of surviving the gastrointestinal tract and col- onizing the gut, having biological activity toward their targets and suitability for the manufacture and survival in commercial products (Conway 1996; Saarela et al. 2000). The viability of probiotics at sufficient levels at the point of consumption is essential as it can affect their efficacy. Although recent evi- dences have shown that cell components derived from non- viable probiotics can exert certain health benefits, the need for viable probiotics is still imperative to ensure that probiotics can exert its optimal beneficial effects (Lahtinen 2012; Sashihara et al. 2006). * Shao Quan Liu chmLsq@nus.edu.sg 1 Food Science and Technology Programme, Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore 2 National University of Singapore (Suzhou) Research Institute, No. 377 Linquan Street, Suzhou Industrial Park, Suzhou, Jiangsu 215123, China Appl Microbiol Biotechnol DOI 10.1007/s00253-015-6560-y