Characterization of lactic acid bacteria growth-stimulating yeasts from pickled cucumber and evaluation of their impact on microbial community dynamics during cucumber fermentation.

Most yeasts can promote the growth of lactic acid bacteria (LAB), but the influence of pickled vegetable indigenous yeasts on the growth of LAB during vegetable fermentation is still obscure. Here, eight indigenous yeasts were isolated from pickled cucumbers, and the simulated intestinal fluid (PD) and gastric fluid (GD) tolerances, as well as enzyme-producing activities, were characterized. The growth-promoting activities of eight yeasts on Lactobacillus plantarum (Lp-GR3), Lactobacillus pentosus and Leuconostoc mesenteroides, were evaluated. The influence of Pichia kudriavzevii-Y4 (Pku-Y4) and Debaryomyces hansenii-X1 (Dha-X1) on cucumber fermentation was further investigated by high-throughput sequencing. All eight yeasts demonstrated good PD and GD tolerance; however, they showed diverse β-glucosidases, esterase and lipase activities between strains. The yeasts could promote the growth of three LAB in a chemically defined medium, and Pku-Y4 showed the strongest growth-promoting effect on all of three LAB. Furthermore, cucumbers were fermented by inoculating Dha-X1, Pku-Y4 and Lp-GR3, respectively. Dha-X1- and Pku-Y4-inoculated fermentation showed similar dynamic changes in pH, total acidity and organic acid, with spontaneous fermentation. Although LAB counts showed a similar trend with spontaneous fermentation, the LAB counts of Dha-X1- and Pku-Y4-inoculated fermentation were elevated at the end of fermentation by up to 107-108 CFU mL-1. High-throughput sequencing analysis showed that Dha-X1- and Pku-Y4-incubating fermentation exhibited a similar community dynamic pattern to that of Lp-GR3, whereas the growth of pathogenic bacteria such as Enterobacteriaceae, Pseudomonas and Fusobacterium were effectively inhibited. The results demonstrated that indigenous yeasts had promotional effects on Lactobacillus proliferation during cucumber fermentation.