Comparative Genomics of Acetobacterpasteurianus Ab3, an Acetic Acid Producing Strain Isolated from Chinese Traditional Rice Vinegar Meiguichu

The bacteriome-coupled phage communities continuously contract and shift to orchestrate the traditional rice vinegar fermentation

Amounts of microbiome studies have uncovered the microbial communities of traditional food fermentations, while in which the phageome development with time is poorly understood. Here, we conducted a study to decipher both phageome and bacteriome of the traditional rice vinegar fermentation. The vinegar phageomes showed significant differences in the alpha diversity, network density and clustering coefficient over time. Peduoviridae had the highest relative abundance. Moreover, the phageome negatively correlated to the cognate bacteriome in alpha diversity, and undergone constantly contracting and shifting across the temporal scale. Nevertheless, 257 core virial clusters (VCs) persistently occurred with time whatever the significant impacts imposed by the varied physiochemical properties. Glycoside hydrolase (GH) and glycosyltransferase (GT) families genes displayed the higher abundances across all samples. Intriguingly, diversely structuring of toxin-antitoxin systems (TAs) and CRISPR-Cas arrays were frequently harbored by phage genomes. Their divergent organization and encoding attributes underlie the multiple biological roles in modulation of network and/or contest of phage community as well as bacterial host community. This phageome-wide mapping will fuel the current insights of phage community ecology in other traditional fermented ecosystems that are challenging to decipher.

RNA-Seq transcriptomic analysis reveals gene expression profiles of acetic acid bacteria under high-acidity submerged industrial fermentation process

Acetic acid bacteria (AAB) are Gram-negative obligate aerobics in Acetobacteraceae family. Producing acetic acid and brewing vinegars are one of the most important industrial applications of AAB, attributed to their outstanding ability to tolerate the corresponding stresses. Several unique acid resistance (AR) mechanisms in AAB have been revealed previously. However, their overall AR strategies are still less-comprehensively clarified. Consequently, omics analysis was widely performed for a better understanding of this field. Among them, transcriptome has recently obtained more and more attention. However, most currently reported transcriptomic studies were conducted under lab conditions and even in low-acidity environment, which may be unable to completely reflect the conditions that AAB confront under industrialized vinegar-brewing processes. In this study, we performed an RNA-Seq transcriptomic analysis concerning AAB’s AR mechanisms during a continuous and periodical industrial submerged vinegar fermentation process, where a single AAB strain performed the fermentation and the acetic acid concentration fluctuated between ~8% and ~12%, the highest acidity as far we know for transcriptomic studies. Samples were directly taken from the initial (CK), mid, and final stages of the same period of the on-going fermentation. 16S rRNA sequence analysis indicated the participation of Komagataeibacter europaeus in the fermentation. Transcriptomic results demonstrated that more genes were downregulated than upregulated at both mid and final stages. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrich analysis reflected that the upregulated genes mainly carried out tricarboxylic acid cycle and oxidative phosphorylation processes, probably implying a considerable role of acetic acid overoxidation in AR during fermentation. Besides, upregulation of riboflavin biosynthesis pathway and two NAD⁺-dependent succinate-semialdehyde dehydrogenase-coding genes suggested a critical role of succinate oxidation in AR. Meanwhile, downregulated genes were mainly ribosomal protein-coding ones, reflecting that the adverse impact on ribosomes initiates at the transcription level. However, it is ambiguous whether the downregulation is good for stress responding or it actually reflects the stress. Furthermore, we also assumed that the fermentation stages may have a greater effect on gene expression than acidity. Additionally, it is possible that some physiological alterations would affect the AR to a larger extent than changes in gene expression, which suggests the combination of molecular biology and physiology research will provide deeper insight into the AR mechanisms in AAB.

Molecular biology: Fantastic toolkits to improve knowledge and application of acetic acid bacteria

Acetic acid bacteria (AAB) are a group of gram-negative, obligate aerobic bacteria within the Acetobacteraceae family of the alphaproteobacteria class, which are distributed in a wide variety of different natural sources that are rich in sugar and alcohols, as well as in several traditionally fermented foods. Their capabilities are not limited to the production of acetic acid and the brewing of vinegar, as their names suggest. They can also fix nitrogen and produce various kinds of aldehydes, ketones and other organic acids by incomplete oxidation (also referred to as oxidative fermentation) of the corresponding alcohols and/or sugars, as well as pigments and exopolysaccharides (EPS). In order to gain more insight into these organisms, molecular biology techniques have been extensively applied in almost all aspects of AAB research, including their identification and classification, acid resistance mechanisms, oxidative fermentation, EPS production, thermotolerance and so on. In this review, we mainly focus on the application of molecular biological technologies in the advancement of research into AAB while presenting the progress of the latest studies using these techniques.

Impacts of type II toxin-antitoxin systems on cell physiology and environmental behavior in acetic acid bacteria

Acetic acid bacteria (AAB) are a group of Gram-negative and strictly aerobic microorganisms widely used in vinegar industry, especially the species belonging to the genera Acetobacter and Komagataeibacter. The environments inhabited by AAB during the vinegar fermentation, in particular those natural traditional bioprocesses, are complex and dynamically changed, usually accompanied by diverse microorganisms, bacteriophages, and the increasing acetic acid concentration. For this reason, how AAB survive to such harsh niches has always been an interesting research field. Previous omic analyses (e.g., genomics, proteomics, and transcriptomics) have provided abundant clues for the metabolic pathways and bioprocesses indispensable for the acid stress adaptation of AAB. Nevertheless, it is far from fully understanding what factors regulate these modular mechanisms overtly and covertly upon shifting environments. Bacterial toxin-antitoxin systems (TAS), usually consisting of a pair of genes encoding a stable toxin and an unstable antitoxin that is capable of counteracting the toxin, have been uncovered to have a variety of biological functions. Recent studies focusing on the role of TAS in Acetobacter pasteurianus suggest that TAS contribute substantially to the acid stress resistance. In this mini review, we discuss the biological functions of type II TAS in the context of AAB with regard to the acid stress resistance, persister formation and resuscitation, genome stability, and phage immunity. KEY POINTS: • Type II TAS act as regulators in the acid stress resistance of AAB. • Type II TAS are implicated in the formation of acid-tolerant persister cells in AAB. • Type II TAS are potential factors responsible for phage immunity and genome stability.

Classification of acetic acid bacteria and their acid resistant mechanism

Acetic acid bacteria (AAB) are obligate aerobic Gram-negative bacteria that are commonly used in vinegar fermentation because of their strong capacity for ethanol oxidation and acetic acid synthesis as well as their acid resistance. However, low biomass and low production rate due to acid stress are still major challenges that must be overcome in industrial processes. Although acid resistance in AAB is important to the production of high acidity vinegar, the acid resistance mechanisms of AAB have yet to be fully elucidated. In this study, we discuss the classification of AAB species and their metabolic processes and review potential acid resistance factors and acid resistance mechanisms in various strains. In addition, we analyze the quorum sensing systems of Komagataeibacter and Gluconacetobacter to provide new ideas for investigation of acid resistance mechanisms in AAB in the form of signaling pathways. The results presented herein will serve as an important reference for selective breeding of high acid resistance AAB and optimization of acetic acid fermentation processes.

Deciphering the succession patterns of bacterial community and their correlations with environmental factors and flavor compounds during the fermentation of Zhejiang rosy vinegar

Zhejiang Rosy Vinegar (ZRV) is a traditional condiment in Southeast China, produced using semi-solid-state fermentation techniques under an open environment, yet little is known about the functional microbiota involved in the flavor formation of ZRV. In this study, 43 kinds of volatile flavor substances were identified by HS-SPME/GC-MS, mainly including ethyl acetate (relative content at the end of fermentation: 1104.1 mg/L), phenylethyl alcohol (417.6 mg/L) and acetoin (605.2 mg/L). The most abundant organic acid was acetic acid (59.6 g/L), which kept rising during the fermentation, followed by lactic acid (7.0 g/L), which showed a continuously downward trend. Amplicon sequencing analysis revealed that the richness and diversity of bacterial community were the highest at the beginning and then maintained decreasing during the fermentation. The predominant bacteria were scattered in Acetobacter (average relative abundance: 63.7%) and Lactobacillus (19.8%). Both sequencing and culture-dependent analysis showed Lactobacillus dominated the early stage (day 10 to 30), and Acetobacter kept highly abundant from day 40 to the end. Spearman correlation analysis displayed that the potential major groups involved in the formation of flavor compounds were Acetobacter and Lactobacillus, which were also showed strong relationships with other bacteria through co-occurrence network analysis (edges attached to Acetobacter: 61.7%; Lactobacillus: 14.0%). Moreover, structural equation model showed that the contents of ethanol, titratable acid and reducing sugar were the major environmental factors playing essential roles in influencing the succession of bacterial community and their metabolism during the fermentation. Overall, these findings illuminated the dynamic profiles of bacterial community and flavor compounds and the potential functional microbes, which were expected to help us understand the formation of flavor substances in ZRV.

Transcriptome response of Acetobacter pasteurianus Ab3 to high acetic acid stress during vinegar production

Acetic acid accumulation is a universal limiting factor to the vinegar manufacture because of the toxic effect of acetic acid on the acid producing strain, such as Acetobacter pasteurianus. In this study, we aimed to investigate the genome-wide transcriptional response of A. pasteurianus Ab3 to high acid stress during vinegar production. By comparing the transcriptional landscape of cells harvested from a long-term cultivation with high acidity (70 ± 3 g/L) to that of low acidity (10 ± 2 g/L), we demonstrated that 1005 genes were differentially expressed. By functional enrichment analysis, we found that the expression of genes related to the two-component systems (TCS) and toxin-antitoxin systems (TAS) was significantly regulated under high acid stress. Cells increased the genome stability to withstand the intracellular toxicity caused by the acetic acid accumulation by repressing the expression of transposases and integrases. Moreover, high acid stress induced the expression of genes involved in the pathways of peptidoglycan, ceramide, and phosphatidylcholine biosynthesis as well as the Tol-Pal and TonB-ExbB systems. In addition, we observed that cells increased and diversified the ATP production to resist high acid stress. Transcriptional upregulation in the pathways of pyrroloquinoline quinone (PQQ) synthesis and thiamine metabolism suggested that cells may increase the production of prosthetic groups to ensure the enzyme activity upon high acid stress. Collectively, the results of this study increase our current understanding of the acetic acid resistance (AAR) mechanisms in A. pasteurianus and provide opportunities for strain improvement and scaled-up vinegar production.Key Points• TCS and TAS are responsive to the acid stress and constitute the regulating networks.• Adaptive expression changes of cell envelope elements help cell resist acid stress.• Cells promote genome stability and diversify ATP production to withstand acid stress.

Comparative Genomic Analysis of Closely Related Acetobacter pasteurianus Strains Provides Evidence of Horizontal Gene Transfer and Reveals Factors Necessary for Thermotolerance

Acetobacter pasteurianus is an industrial strain used for the vinegar production. Many A. pasteurianus strains with different phenotypic characteristics have been isolated so far. To understand the genetic background underpinning these phenotypes, a comparative genomic analysis of A. pasteurianus strains was conducted. Based on bioinformatics and experimental results, we report the following. (i) The gene repertoire related to the respiratory chains showed that several horizontal gene transfer events occurred after the divergence of these strains, indicating that the respiratory chain in A. pasteurianus has the diversity to adapt to its environment. (ii) There is a clear difference in thermotolerance even between 12 closely related strains. NBRC 3279, NBRC 3284, and NBRC 3283, in particular, which have only 55 mutations in total, showed differences in thermotolerance. The Na⁺/H⁺ antiporter gene nhaK2 was mutated in the thermosensitive NBRC 3279 and NBRC 3284 strains and not in the thermotolerant NBRC 3283 strain. The Na⁺/H⁺ antiporter activity of the three strains and expression of nhaK2 gene from NBRC 3283 in the two thermosensitive strains showed that these mutations are critical for thermotolerance. These results suggested that horizontal gene transfer events and several mutations have affected the phenotypes of these closely related strains.IMPORTANCE Acetobacter pasteurianus, an industrial vinegar-producing strain, exhibits diverse phenotypic differences such as respiratory activity related to acetic acid production, acetic acid resistance, or thermotolerance. In this study, we investigated the correlations between genome sequences and phenotypes among closely related A. pasteurianus strains. The gene repertoire related to the respiratory chains showed that the respiratory components of A. pasteurianus has a diversity caused by several horizontal gene transfers and mutations. In three closely related strains with clear differences in their thermotolerances, we found that the insertion or deletion that occurred in the Na⁺/H⁺ antiporter gene nhaK2 is directly related to their thermotolerance. Our study suggests that a relatively quick mutation has occurred in the closely related A. pasteurianus due to its genetic instability and that this has largely affected its phenotype.

Characterization and comparative analysis of toxin-antitoxin systems in Acetobacter pasteurianus

Bacterial toxin-antitoxin (TA) systems play important roles in diverse cellular regulatory processes. Here, we characterize three putative type II TA candidates from Acetobacter pasteurianus and investigate the profile of type II TA systems in the genus Acetobacter. Based on the gene structure and activity detection, two-pairs loci were identified as the canonical hicAB and higAB TA systems, respectively, and DB34_01190-DB34_01195 as a putative new one without a canonical TA architecture. Physiologically, the expression of the three pairs conferred E. coli with additional plasmid maintenance and survival when under acetic acid stress. Chromosomal TA systems can be horizontally transferred within an ecological vinegar microbiota by co-option, and there was a tendency for toxin module loss. The antitoxin retention in the genome is suggested to have a broad role in bacterial physiology. Furthermore, A. pasteurianus strains, universally domesticated and used for industrial vinegar fermentation, showed a higher number of type II TA loci compared to the host-associated ones. The amount of TA loci per genome showed little positive relationship to insertion sequences, although its prevalence was species-associated, to the extent of even being strain-associated. The TA system is a candidate of studying the resistant mechanistic network, the TAs-dependent translatome affords a real-time profile to explore stress adaptation of A. pasteurianus, promoting industrial development.

Producing Acetic Acid of Acetobacter pasteurianus by Fermentation Characteristics and Metabolic Flux Analysis

The acetic acid bacterium Acetobacter pasteurianus plays an important role in acetic acid fermentation, which involves oxidation of ethanol to acetic acid through the ethanol respiratory chain under specific conditions. In order to obtain more suitable bacteria for the acetic acid industry, A. pasteurianus JST-S screened in this laboratory was compared with A. pasteurianus CICC 20001, a current industrial strain in China, to determine optimal fermentation parameters under different environmental stresses. The maximum total acid content of A. pasteurianus JST-S was 57.14 ± 1.09 g/L, whereas that of A. pasteurianus CICC 20001 reached 48.24 ± 1.15 g/L in a 15-L stir stank. Metabolic flux analysis was also performed to compare the reaction byproducts. Our findings revealed the potential value of the strain in improvement of industrial vinegar fermentation.

New insights into the mechanisms of acetic acid resistance in Acetobacter pasteurianus using iTRAQ-dependent quantitative proteomic analysis

Acetobacter pasteurianus is the main starter in rice vinegar manufacturing due to its remarkable abilities to resist and produce acetic acid. Although several mechanisms of acetic acid resistance have been proposed and only a few effector proteins have been identified, a comprehensive depiction of the biological processes involved in acetic acid resistance is needed. In this study, iTRAQ-based quantitative proteomic analysis was adopted to investigate the whole proteome of different acidic titers (3.6, 7.1 and 9.3%, w/v) of Acetobacter pasteurianus Ab3 during the vinegar fermentation process. Consequently, 1386 proteins, including 318 differentially expressed proteins (p<0.05), were identified. Compared to that in the low titer circumstance, cells conducted distinct biological processes under high acetic acid stress, where >150 proteins were differentially expressed. Specifically, proteins involved in amino acid metabolic processes and fatty acid biosynthesis were differentially expressed, which may contribute to the acetic acid resistance of Acetobacter. Transcription factors, two component systems and toxin-antitoxin systems were implicated in the modulatory network at multiple levels. In addition, the identification of proteins involved in redox homeostasis, protein metabolism, and the cell envelope suggested that the whole cellular system is mobilized in response to acid stress. These findings provide a differential proteomic profile of acetic acid resistance in Acetobacter pasteurianus and have potential application to highly acidic rice vinegar manufacturing.