Daqu Fermentation Selects for Heat-Resistant Enterobacteriaceae and Bacilli.

Daqu is a spontaneous solid-state cereal fermentation used as saccharification and starter culture in Chinese vinegar and liquor production. The evolution of microbiota in this spontaneous fermentation is controlled by the temperature profile, which reaches temperatures from 50 to 65°C for several days. Despite these high temperatures, mesophilic (including ) and bacilli are present throughout Daqu fermentation. This study aimed to determine whether Daqu spontaneous solid-state fermentation selects for heat-resistant variants of these organisms. Heat resistance in is mediated by the locus of heat resistance (LHR). One LHR-positive strain of was identified in Daqu, and it exhibited higher heat resistance than the LHR-negative isolated from malted oats. Heat resistance in endospores is mediated by the operon. Out of 10 Daqu isolates of the species , , , , and , 5 did not contain , 3 contained one copy, and 2 contained two copies. The presence and copy number of the operon increased the resistance of spores to treatment with 110°C. To confirm the selection of LHR- and -positive strains during Daqu fermentation, the copy numbers of these genetic elements in Daqu samples were quantified by quantitative PCR (qPCR). The abundance of LHR and the operon in community DNA relative to that of total bacterial 16S rRNA genes increased 3-fold and 5-fold, respectively, during processing. In conclusion, culture-dependent and culture-independent analyses suggest that Daqu fermentation selects for heat-resistant and bacilli. Daqu fermentations select for mobile genetic elements conferring heat resistance in and bacilli. The locus of heat resistance (LHR), a genomic island conferring heat resistance in , and the operon, conferring heat resistance on bacterial endospores, were enriched 3- to 5-fold during Daqu fermentation and maturation. It is therefore remarkable that the LHR and the operon are accumulated in the same food fermentation. The presence of heat-resistant spp. and spp. in Daqu is not of concern for food safety; however, both genomic islands are mobile and transferable to pathogenic bacteria or toxin-producing bacteria by horizontal gene transfer. The identification of the LHR and the operon as indicators of fitness of and bacilli in Daqu fermentation provides insights into environmental sources of heat-resistant organisms that may contaminate the food supply.