Detection of Viable and Total Bacterial Community in the Pit Mud of Chinese Strong-Flavor Liquor Using Propidium Monoazide Combined With Quantitative PCR and 16S rRNA Gene Sequencing

Research progress on the diversity, physiological and functional characteristics of lactic acid bacteria in the Nongxiangxing baijiu microbiome

Nongxiangxing baijiu (NXXB) is one of the main types of Chinese Baijiu. Its unique flavor is formed during fermentation by the combined action of various microorganisms, with lactic acid bacteria (LAB) making a great contribution. Lactate fermentation produces the precursors of key flavor compounds, such as lactic acid, acetic acid, and other flavor compounds, which combine to produce the unique sensory characteristics of NXXB. This review focuses on the diversity of LAB species and their physiological and metabolic characteristics. These characteristics include key species involved at different fermentation stages, metabolite biosynthesis, acid-tolerance mechanisms, interactions with yeasts, and factors influencing LAB community composition. It also discusses current challenges and future research directions for LAB in relation to NXXB production, aiming to advance understanding and potential future applications of these bacteria.

Caproiciproducens converts lactic acid into caproic acid during Chinese strong-flavor Baijiu brewing

Strong-flavor Baijiu, a type of Chinese liquor, is produced through anaerobic solid-state fermentation in a sealed mud pit. Ethyl caproate, the characteristic flavor compound of strong-flavor Baijiu, is influenced by caproic acid-producing bacteria in the pit mud. To better understand the formation of caproic acid, this study investigated the microbial composition and physicochemical parameters of pit mud from different layers (top, middle, and bottom) in Hubei and Sichuan provinces, China. The results revealed that Caproiciproducens plays a key role in caproic acid production by using lactic acid as a substrate, with its abundance increasing with the depth of the pit mud. A strain Caproiciproducens sp. R1, isolated from the pit mud, was shown to produce caproic acid from lactic acid within an initial pH range of 5.5-9.0 and lactic acid concentrations of 1 %-5 % (m/v). In addition, inoculation of strain R1 into Huangshui (a lactic acid-rich liquid from Baijiu production) resulted in 40.72 mM caproic acid production. This study demonstrates that Caproiciproducens plays a crucial role in caproic acid production from lactic acid during the fermentation process of strong-flavor Baijiu.

Uncovering acid resistance genes in lactic acid bacteria and impact of non-viable bacteria on bacterial community during Chinese strong-flavor baijiu fermentation

Chinese strong-flavor baijiu (CSFB) brewing is a spontaneously solid-state fermentation process for approximately 60 days. Numerous microorganisms grow, die, and spark a series of metabolic reactions during fermentation. In this study, the microbial community and structure between total and viable bacteria in zaopei from the 5- and 20-year pits of CSFB are revealed by amplicon sequencing. Metagenome sequencing was applied to investigate acid resistance genes in Lactobacillus and predict carbohydrate active enzyme in zaopei. Besides, SourceTracker was conducted to expose bacterial sources. Results revealed that there was no significant difference in the bacterial community and structure between the total and viable bacteria; Lactobacillus was the most dominant bacterium in zaopei of two types of pits. Meanwhile, acid resistance genes argR, aspA, ilvE, gshA, DnaK, and cfa were genes that sustained Lactobacillus survival in the late stages of fermentation with high contents of acid and ethanol, and glycosyltransferases were identified as the predominated enzymes during the CSFB fermentation which catalyzed the process of lactic acid generation via Embden-Meyerhof-Parnas pathway and Hexose Monophosphate Pathway. Moreover, the environment contributed most bacteria to zaopei of the 5- and 20-year pits. These findings will provide a deeper understanding of the microbial community structure of viable and total bacteria and the reason for the dominance of Lactobacillus in the later stages of CSFB fermentation.

Bacterial Communities Found in Pit-Wall Mud and Factors Driving Their Evolution

Pit-wall mud (PWM) fosters bacterial communities involved in Baijiu production. PWM varies depending on pit age and height. In this study, we explored the bacterial communities in PWM and factors driving their evolution. The abundance and diversity of bacterial communities were low in new PWM (NPWM). In old PWM (OPWM), similar but diverse bacterial communities were observed at different heights. Lactobacillus was the predominant genus in NPWM, and Caproiciproducens, Aminobacterium, Hydrogenispora, Lactobacillus, Petrimonas, Syntrophomonas, and Sedimentibacter were the dominant genera in OPWM. A decrease was noted in the abundance of Lactobacillus, which indicated evolution. Among all the physicochemical properties, pH had the highest degree of interpretation with an R2 value of 0.965. pH also exerted the strongest effect on bacterial communities. The path coefficients of pH on bacterial community diversity and abundance were 0.886 and 0.810, respectively. Caproiciproducens and Clostridium sensu stricto 12 metabolized lactic acid, inhibiting the growth of Lactobacillus at a suitable pH, which led to the maturation of PWM. Our findings enrich the literature on the evolution of bacterial communities in PM and the maturation of PM.

Comparing the differences of prokaryotic microbial community between pit walls and bottom from Chinese liquor revealed by 16S rRNA gene sequencing

This study aims to explore the prokaryotic microbial community structures and diversity in pit mud from different depths, and provide a theoretical basis for the liquor production and further study of pit mud. The fermented pit muds of strong-flavor liquor from Yun distillery were taken as samples. The high-throughput sequencing approach, followed by bioinformatics analyses, was used to compare the differences in the prokaryotic microbial community between pit walls and bottom represented by samples. A total of 31 bacteria phyla and 2 archaea phyla were detected. The dominant phyla in YJ-S, YJ-Z, and YJ-X (sample name) were Proteobacteria and Firmicutes, while the dominant genera in them were Acinetobacter, Aminobacterium, and Lactobacillus. YJ-Z and YJ-X were the closest in species diversity. In species richness analysis, YJ-X was the highest, followed by YJ-Z, and YJ-S was the lowest; in species uniformity analysis, YJ-S was the highest, followed by YJ-Z, and YJ-X was the lowest. The function predicted by 16S rRNA genome showed that prokaryotic microbial function in pit mud was mainly concentrated in "Carbohydrate transport and metabolism" and "Amino acid transport and metabolism." Significant differences in prokaryotic microbial community and gene function prediction between pit walls and bottom were found in YJ-S, YJ-Z, and YJ-X (p < 0.05).

Recent Methods for the Viability Assessment of Bacterial Pathogens: Advances, Challenges, and Future Perspectives

Viability assessment is a critical step in evaluating bacterial pathogens to determine infectious risks to public health. Based on three accepted viable criteria (culturability, metabolic activity, and membrane integrity), current viability assessments are categorized into three main strategies. The first strategy relies on the culturability of bacteria. The major limitation of this strategy is that it cannot detect viable but nonculturable (VBNC) bacteria. As the second strategy, based on the metabolic activity of bacteria, VBNC bacteria can be detected. However, VBNC bacteria sometimes can enter a dormant state that allows them to silence reproduction and metabolism; therefore, they cannot be detected based on culturability and metabolic activity. In order to overcome this drawback, viability assessments based on membrane integrity (third strategy) have been developed. However, these techniques generally require multiple steps, bulky machines, and laboratory technicians to conduct the tests, making them less attractive and popular applications. With significant advances in microfluidic technology, these limitations of current technologies for viability assessment can be improved. This review summarized and discussed the advances, challenges, and future perspectives of current methods for the viability assessment of bacterial pathogens.

A new method for screening and culture of Clostridium from pit mud under non-anaerobic conditions

Strong-flavor Baijiu (SFB) is produced in complex fermentation in pits under ground. Clostridium producing hexanoic acid plays a key role in the flavor formation of SFB. The screening and culture for Clostridium are very difficult because of its strict anaerobic characteristics. In this study, electric field assisted screening (EFAS) was used to screen Clostridium from pit mud, and electric culture (EC) was used to cultivate Clostridium under non-anaerobic conditions. A strain with a high yield of hexanoic acid was screened and named as Clostridium sp. EFAS6. Under non-anaerobic conditions, it grew rapidly only near the cathode end in the EFAS device because of the low oxidation-reduction potential of that electrode. In the experiment of high-density culture in the EC device, the cell concentration reached 10⁶-10⁷. After energy consumption was calculated, the optimal loading voltage was found to be 10 V. In the application, the broth of Clostridium sp. EFAS6 increased the content of ethyl hexanoic in SFB. Under non-anaerobic conditions, the anaerobe was screened by EFAS and cultivated in high density by EC. The EFAS and EC could also be used for the screening and culture of other anaerobes under non-anaerobic conditions.

Metabolite-Based Mutualistic Interaction between Two Novel Clostridial Species from Pit Mud Enhances Butyrate and Caproate Production

Pit mud microbial consortia play crucial roles in the formation of Chinese strong-flavor baijiu's key flavor-active compounds, especially butyric and caproic acids. Clostridia, one of the abundant bacterial groups in pit mud, were recognized as important butyric and caproic acid producers. Research on the interactions of the pit mud microbial community mainly depends on correlation analysis at present. Interaction between Clostridium and other microorganisms and its involvement in short/medium-chain fatty acid (S/MCFA) metabolism are still unclear. We previously found coculture of two clostridial strains isolated from pit mud, Clostridium fermenticellae JN500901 (C.901) and Novisyntrophococcus fermenticellae JN500902 (N.902), could enhance S/MCFA accumulation. Here, we investigated their underlying interaction mechanism through the combined analysis of phenotype, genome, and transcriptome. Compared to monocultures, coculture of C.901 and N.902 obviously promoted their growth, including shortening the growth lag phase and increasing biomass, and the accumulation of butyric acid and caproic acid. The slight effects of inoculation ratio and continuous passage on the growth and metabolism of coculture indicated the relative stability of their interaction. Transwell coculture and transcriptome analysis showed the interaction between C.901 and N.902 was accomplished by metabolite exchange, i.e., formic acid produced by C.901 activated the Wood-Ljungdahl pathway of N.902, thereby enhancing its production of acetic acid, which was further converted to butyric acid and caproic acid by C.901 through reverse β-oxidation. This work demonstrates the potential roles of mutually beneficial interspecies interactions in the accumulation of key flavor compounds in pit mud. IMPORTANCE Microbial interactions played crucial roles in influencing the assembly, stability, and function of the microbial community. The metabolites of pit mud microbiota are the key to flavor formation of Chinese strong-flavor baijiu. So far, researches on the interactions of the pit mud microbial community have been mainly based on the correlation analysis of sequencing data, and more work needs to be performed to unveil the complicated interaction patterns. Here, we identified a material exchange-based mutualistic interaction system involving two fatty acid-producing clostridial strains (Clostridium fermenticellae JN500901 and Novisyntrophococcus fermenticellae JN500902) isolated from pit mud and systematically elucidated their interaction mechanism for promoting the production of butyric acid and caproic acid, the key flavor-active compounds of baijiu. Our findings provide a new perspective for understanding the complicated interactions of pit mud microorganisms.

Analysis of bacterial diversity and genetic evolution of Lacticaseibacillus paracasei isolates in fermentation pit mud

AIMS

Since little is known about the genetic diversity of lactic acid bacteria (LAB) isolates from the fermentation pit mud (FPM), we sought to evaluate the bacterial structure, identify the LAB isolates and investigate the genotype and genetic diversity of the LAB isolates.

METHODS AND RESULTS

Using high-throughput MiSeq sequencing, we identified seven dominant bacterial genera in FPM. Lactobacillus had the highest abundance. We isolated 55 LAB strains. These isolates were all identified as Lacticaseibacillus paracasei. Using an extant multilocus sequence typing (MLST) scheme, isolates were assigned to 18 sequence types (STs) and three clonal complexes. ST1, the largest group, mainly comprised FPM isolates. Niche-specific ST2 to ST18 only contained FPM isolates. Isolates could be divided into four lineages, with most assigned to Lineage 1. Only one FPM isolate was classified as L. paracasei subsp. paracasei. Other isolates could not be classified at the subspecies level using the seven MLST loci.

CONCLUSIONS

Lactobacilli account for a high proportion of bacteria in pit mud. Based on the traditional culture method, L. paracasei was the dominant species, and these isolates exhibit a high ethanol tolerance, high intraspecific diversity and specific genetic profiles.

SIGNIFICANCE AND IMPACT OF THE STUDY

The study described the characterization of FPM bacterial diversity, giving an insight into the genetic diversity of L. paracasei strains present in FPM.

Comparative Genomics Unveils the Habitat Adaptation and Metabolic Profiles of Clostridium in an Artificial Ecosystem for Liquor Production

Pit mud is a typical artificial ecosystem for Chinese liquor production. Clostridium inhabiting pit mud plays essential roles in the flavor formation of strong-flavor baijiu. The relative abundance of Clostridium increased with pit mud quality, further influencing the quality of baijiu.

Research trends in Jiang-flavor baijiu fermentation: From fermentation microecology to environmental ecology

Chinese baijiu is one of the six major distilled spirits worldwide and is widely enjoyed because of its unique flavor. Among typical baijiu, Jiang-flavor baijiu is gaining popularity. However, the fermentation mechanisms of baijiu remain unclear due to its open inoculation environment and complex brewing process. In recent years, advances in high-throughput sequencing and multi-omics technologies have yielded meaningful information regarding fermentation microbiome. Therefore, this paper reviews recent developments in the investigation of the diversity, stability, and metabolism of the Jiang-flavor baijiu microbial community. Furthermore, the importance of protecting the ecology of the production environment is proposed based on the putative contribution of environmental factors to the fermentation microbiome and baijiu characteristics. Finally, this paper discusses current research challenges that need to be addressed, including the limitations of sequencing technologies and difficulties unveiling the mechanisms of microbial interaction between the fermentation microbiome and the environmental ecology. The findings of this review will promote further understanding of the Jiang-flavor baijiu fermentation process and provide valuable information for the research and development of traditional baijiu and other naturally fermented foods. PRACTICAL APPLICATION: Baijiu, a transparent strong alcoholic drink, is the world's largest consumed and the most valuable spirit in the market. However, the fermentation mechanisms of baijiu remain unclear due to its open inoculation environment and complex brewing process. Therefore, if we can summarizes the current advances and research challenges of microbial fermentation in baijiu, it will deepen the reader's understanding of the complex fermentation process and fermentation mechanism in baijiu. Furthermore, based on the putative contribution of environmental factors to the fermentation process, the importance of protecting the ecology of the production environment is proposed in future research trends, which will provide valuable information for the research and development of other traditional naturally fermented foods. This will not only achieve breakthroughs in academic value, but also bring higher practical value to fermented foods.

Detection of viable and total fungal community in zaopei of Chinese strong-flavor baijiu using PMA combined with qPCR and HTS based on ITS2 region

Background

Chinese strong-flavor baijiu (CSFB), one of the three major baijiu types, is the most popular baijiu type among consumers in China. A variety of microbes are involved in metabolizing raw materials to produce ethanol and flavor substances during fermentation, which fundamentally determined the quality of baijiu. It is of great importance to study microbial community of fermented grains (zaopei) during baijiu brewing process for improving its quality. In this study, we firstly used propidium monoazide (PMA) to treat zaopei samples from 5-year pit and 20-year pit for removing the interference of non-viable fungi, and analyzed the diversity of total fungi and viable fungi by quantitative PCR (qPCR) and high-throughput sequencing (HTS) based on ITS2 gene.

Results

The results showed that total fungi and viable fungi displayed no significant differences at OTU, phylum, or genus levels during fermentation within two kinds of pits. A total of 6 phyla, 19 classes, and 118 genera in fungi were found based on OTUs annotation in zaopei samples from 5-year pit and 20-year pit. Besides, non-viable fungi had little effect on the fungal community diversity during the fermentation cycle. It was found that the most dominant viable fungi belonged to Saccharomyces, Kazachstania, Naumovozyma, and Trichosporon, and Naumovozyma was firstly detected in zaopei samples of CSFB. Moreover, based on the variation of flavor substances in zaopei samples, the quality of CSFB produced from older pit was better than that produced from younger pit.

Conclusion

The non-viable fungi had little effect on the fungal diversity, structure, and relative abundance in zaopei samples of CSFB, and Naumovozyma was firstly detected in zaopei samples of CSFB. Our findings can be applied as guidance for improving the quality and stability of CSFB.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-021-02334-8.