Microbes communicate with each other using chemical signal molecules, termed autoinducers (AI) or quorum sensing molecules (QSM). When the signal molecules accumulate a threshold, the communicating microbes begin to alter gene expression, and therefore behavior, in response (Waters and Bassler 2005). This process, termed quorum sensing (QS), initiates many important behaviors of bacteria including bioluminescence, sporulation, toxic factors, biofilm formation and other processes (Llamas et al. 2004; Whitehead et al. 2001). N-acyl homoserine lactones (AHLs) are widely used for regulating QS in gram-negative proteobacteria (Lv et al. 2013). The fungal QS systems was first revealed in fungus Candida albicans, regulated by farnesol as auto-inducers to control filamentation (Hornby et al. 2001). Subsequently, a lot of studies show that farnesol can result in multiple physiological effects on Saccharomyces cerevisiae and C. albicans, including biofilm formation and oxidative stress (Albuquerque and Casadevall 2012). In addition, tyrosol, phenylethanol and tryptophol are all known fungal QSMs (Chen et al. 2004; Lingappa et al. 1969). In S. cerevisiae, phenylethanol and tryptophol as QSMs were found to regulate morphogenesis during nitrogen starvation conditions (Chen and Fink 2006).
In recent years, it is notable that AHLs have been found to induce specific and extensive response from eukaryotes including plants and mammalian cells. Proteome analyzed shows that Medicago truncatula can detect of the two AHLs, 3-oxo-C16:1-HSL and 3-oxo-C12-HSL, resulting in significant changes in accumulation of over 150 proteins, which contribute to plants resistance, protein degradation and modification, and other metabolic procedures (Mathesius et al. 2003). To mammalian cells, AHLs are found to have a function in apoptosis. Tateda et al. (2003) found that 3OC12-HSL effected on macrophage inhibitory protein (MIP)-2 and monocyte chemoattractant protein (MCP)-1 expression.
In past 10 years, the research of the response of fungi to the bacterial QSMs has just begun. Most of the researches focused on the two opportunistic pathogens, C. albicans and Pseudomonas aeruginosa. It has been revealed that 3-oxo-C12-homoserine lactone (HSL) secreted by P. aeruginosa and farnesol generated by C. albicans, both of which contain a 12-carbon backbone, influence morphogenesis by inhibiting the Candida cAMP/PKA pathway (Davis-Hanna et al. 2008). Besides, there are few studies on the QS among other fungus and bacteria. Based on the fact of coevolution and coexistence between bacteria and fungus for millions of years and the similarities between their QSMs, it is reasonable that the existence of more fungus can detect and response to bacterial QS signals.
Saccharomyces cerevisiae, as one of the most ancient eukaryotes and with the characteristics of simple growth requirement, rapid cell division and ease of genetic manipulation, is an excellent candidate for research on the effects of bacterial QS signals on eukaryotes. Moreover, S. cerevisiae is one of the most important ethanol producers in industry, but when alcohols accumulate a threshold, the filamentation, growth, viability and biofilm development in yeast can be effected, as QSMs (farnesol or AHLs) do (Chauhan et al. 2013). Therefore, it is meaningful to research on the response of S. cerevisiae to QS signal molecules for explaining the integrity of coevolution between prokaryotes and eukaryotes, and is profound to reveal the effect of QSMs on the ethanol tolerance in yeast to environmental friendly improve the production of bioethanol.
The aim of this study was to investigate the response of the model fungus S. cerevisiae to bacterial signals, focusing on the influence of bacterial AHLs on growth, morphology and ethanol tolerance of S. cerevisiae. Two typical bacterial AHLs, 3-OC6-HSL and C12-HSL, were selected for this purpose. 3-OC6-HSL presents the short chain AHL with carbon chains of C6 with 3-oxo substitutions, while C12-HSL is a common long chain signal molecule with 12 carbon chains without 3-oxo substitutions. The two AHLs provided good samples for evaluating the effects of diverse bacterial signal molecules on yeast, because of their differences in carbon chain length and functional groups. Our data clearly demonstrated that bacterial QSMs indeed effected on the growth and morphology of S. cerevisiae, and also change its ethanol tolerance, suggesting the interaction between prokaryotic and eukaryotic microbes during coevolution. Moreover, our results could be drew some reference to increase industrial production of alcohol.