Acetobacter oryzifermentans sp. nov., isolated from Korean traditional vinegar and reclassification of the type strains of Acetobacter pasteurianus subsp. ascendens (Henneberg 1898) and Acetobacter pasteurianus subsp. paradoxus (Frateur 1950) as Acetobacter ascendens sp. nov., comb. nov

Latest Trends in Industrial Vinegar Production and the Role of Acetic Acid Bacteria: Classification, Metabolism, and Applications-A Comprehensive Review

Vinegar is one of the most appreciated fermented foods in European and Asian countries. In industry, its elaboration depends on numerous factors, including the nature of starter culture and raw material, as well as the production system and operational conditions. Furthermore, vinegar is obtained by the action of acetic acid bacteria (AAB) on an alcoholic medium in which ethanol is transformed into acetic acid. Besides the highlighted oxidative metabolism of AAB, their versatility and metabolic adaptability make them a taxonomic group with several biotechnological uses. Due to new and rapid advances in this field, this review attempts to approach the current state of knowledge by firstly discussing fundamental aspects related to industrial vinegar production and then exploring aspects related to AAB: classification, metabolism, and applications. Emphasis has been placed on an exhaustive taxonomic review considering the progressive increase in the number of new AAB species and genera, especially those with recognized biotechnological potential.

Acetobacter vaccinii sp. nov., a novel acetic acid bacterium isolated from blueberry fruit (Vaccinium corymbosum L.)

Strain C17-3^T was isolated from blueberry fruits collected from a farmland located in Damyang-gun, Jeollanam-do, Republic of Korea. Phylogenetic analysis based on 16S rRNA gene sequences allocated strain C17-3^T to the genus Acetobacter, where it occupied a rather isolated line of descent with Acetobacter ghanensis 430A^T and Acetobacter lambici LMG 27439^T as the nearest neighbours (98.9 % sequence similarity to both species). The highest average nucleotide identity and digital DNA-DNA hybridization values were 76.3 % and 21.7 % with Acetobacter garciniae TBRC 12339^T; both values were well below the cutoff values for species delineation. Cells are strictly aerobic, Gram-stain-negative rods, catalase-positive and oxidase-negative. The DNA G+C content calculated from the genome sequence was 59.2 %. Major fatty acids were summed feature 8 (C_18 : 1ω6c and/or C_18 : 1ω7c) and C_19 : 0cyclo ω8c. The major isoprenoid quinone was ubiquinone 9. On the basis of the results of phylogenetic analyses, phenotypic features and genomic comparisons, it is proposed that strain C17-3^T represents a novel species of the genus Acetobacter and the name Acetobacter vaccinii sp. nov. is proposed. The type strain is C17-3^T (= KACC 21233^T = LMG 31758^T).

ODFM, an omics data resource from microorganisms associated with fermented foods

ODFM is a data management system that integrates comprehensive omics information for microorganisms associated with various fermented foods, additive ingredients, and seasonings (e.g. kimchi, Korean fermented vegetables, fermented seafood, solar salt, soybean paste, vinegar, beer, cheese, sake, and yogurt). The ODFM archives genome, metagenome, metataxonome, and (meta)transcriptome sequences of fermented food-associated bacteria, archaea, eukaryotic microorganisms, and viruses; 131 bacterial, 38 archaeal, and 28 eukaryotic genomes are now available to users. The ODFM provides both the Basic Local Alignment Search Tool search-based local alignment function as well as average nucleotide identity-based genetic relatedness measurement, enabling gene diversity and taxonomic analyses of an input query against the database. Genome sequences and annotation results of microorganisms are directly downloadable, and the microbial strains registered in the archive library will be available from our culture collection of fermented food-associated microorganisms. The ODFM is a comprehensive database that covers the genomes of an entire microbiome within a specific food ecosystem, providing basic information to evaluate microbial isolates as candidate fermentation starters for fermented food production.

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.

Acetobacter oryzoeni sp. nov., isolated from Korean rice wine vinegar

A Gram-stain-negative, obligately aerobic bacterium, designated strain B6^T, was isolated from rice wine vinegar in the Republic of Korea. Cells were non-motile and oval short rods showing catalase-positive and oxidase-negative activities. Growth was observed at 15-45 °C (optimum, 30 °C) and pH 3.5-8.0 (optimum, pH 5.5-6.5). Strain B6^T contained summed feature 8 (comprising C_{18 : 1}ω7c and/or C_{18 : 1} ω6c), and C_{16 : 0} as major fatty acids and ubiquinone-9 was identified as the sole isoprenoid quinone. The G+C content of the genomic DNA calculated from the whole genome was 53.1 mol%. Strain B6^T was most closely related to Acetobacter pasteurianus LMG 1262^T with very high 16S rRNA gene sequence similarity (100 %) and the strains formed a very close phylogenetic lineage together in phylogenetic trees based on 16S rRNA gene sequences. However, relatedness analyses based on concatenated amino acid sequences of 354 core genes and whole-cell MALDI-TOF profiles showed that strain B6^T may form a distinct phyletic lineage from Acetobacter species. In addition, average nucleotide identity and in silico DNA-DNA hybridization values between strain B6^T and the type strains of Acetobacter species were less than 93.3 and 51.4 %, respectively. The genomic features of strain B6^T were also differentiated from those of closely related Acetobacter type strains. Based on the phenotypic, chemotaxonomic and genomic features, strain B6^T clearly represents a novel species of the genus Acetobacter, for which the name Acetobacter oryzoeni sp. nov. is proposed. The type strain is B6^T (=KACC 21201^T=JCM 33371^T).

Analysis of 1,000+ Type-Strain Genomes Substantially Improves Taxonomic Classification of Alphaproteobacteria

The class Alphaproteobacteria is comprised of a diverse assemblage of Gram-negative bacteria that includes organisms of varying morphologies, physiologies and habitat preferences many of which are of clinical and ecological importance. Alphaproteobacteria classification has proved to be difficult, not least when taxonomic decisions rested heavily on a limited number of phenotypic features and interpretation of poorly resolved 16S rRNA gene trees. Despite progress in recent years regarding the classification of bacteria assigned to the class, there remains a need to further clarify taxonomic relationships. Here, draft genome sequences of a collection of genomes of more than 1000 Alphaproteobacteria and outgroup type strains were used to infer phylogenetic trees from genome-scale data using the principles drawn from phylogenetic systematics. The majority of taxa were found to be monophyletic but several orders, families and genera, including taxa recognized as problematic long ago but also quite recent taxa, as well as a few species were shown to be in need of revision. According proposals are made for the recognition of new orders, families and genera, as well as the transfer of a variety of species to other genera and of a variety of genera to other families. In addition, emended descriptions are given for many species mainly involving information on DNA G+C content and (approximate) genome size, both of which are confirmed as valuable taxonomic markers. Similarly, analysis of the gene content was shown to provide valuable taxonomic insights in the class. Significant incongruities between 16S rRNA gene and whole genome trees were not found in the class. The incongruities that became obvious when comparing the results of the present study with existing classifications appeared to be caused mainly by insufficiently resolved 16S rRNA gene trees or incomplete taxon sampling. Another probable cause of misclassifications in the past is the partially low overall fit of phenotypic characters to the sequence-based tree. Even though a significant degree of phylogenetic conservation was detected in all characters investigated, the overall fit to the tree varied considerably.

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.

Genomic and metabolic features of Tetragenococcus halophilus as revealed by pan-genome and transcriptome analyses

The genomic and metabolic diversity and features of Tetragenococcus halophilus, a moderately halophilic lactic acid bacterium, were investigated by pan-genome, transcriptome, and metabolite analyses. Phylogenetic analyses based on the 16S rRNA gene and genome sequences of 15 T. halophilus strains revealed their phylogenetic distinctness from other Tetragenococcus species. Pan-genome analysis of the T. halophilus strains showed that their carbohydrate metabolic capabilities were diverse and strain dependent. Aside from one histidine decarboxylase gene in one strain, no decarboxylase gene associated with biogenic amine production was identified from the genomes. However, T. halophilus DSM 20339^T produced tyramine without a biogenic amine-producing decarboxylase gene, suggesting the presence of an unidentified tyramine-producing gene. Our reconstruction of the metabolic pathways of these strains showed that T. halophilus harbors a facultative lactic acid fermentation pathway to produce l-lactate, ethanol, acetate, and CO₂ from various carbohydrates. The transcriptomic analysis of strain DSM 20339^T suggested that T. halophilus may produce more acetate via the heterolactic pathway (including d-ribose metabolism) at high salt conditions. Although genes associated with the metabolism of glycine betaine, proline, glutamate, glutamine, choline, and citrulline were identified from the T. halophilus genomes, the transcriptome and metabolite analyses suggested that glycine betaine was the main compatible solute responding to high salt concentration and that citrulline may play an important role in the coping mechanism against high salinity-induced osmotic stresses. Our results will provide a better understanding of the genome and metabolic features of T. halophilus, which has implications for the food fermentation industry.

Tetragenococcus halophilus MJ4 as a starter culture for repressing biogenic amine (cadaverine) formation during saeu-jeot (salted shrimp) fermentation

Biogenic amines (BAs) are frequently present in traditionally fermented salted foods. In this study, a Tetragenococcus halophilus strain (MJ4) with no BA-producing ability was isolated from a fish (anchovy) sauce. Strain MJ4 did not produce BAs from supplied precursors and no BA-producing genes were identified in its genome. Bacterial community analysis showed that in non-inoculated saeu-jeot (shrimp sauce) fermentation, Tetragenococcus predominated after 82 days, while in strain MJ4-inoculated saeu-jeot, Tetragenococcus predominated during the entire fermentation. Strain MJ4 repressed the growth of T. muriaticus, a known BA producer, during fermentation, but metabolite analysis demonstrated that metabolite profiles, including amino acids, were similar regardless of MJ4 inoculation. The metabolite analysis also showed that strain MJ4 clearly repressed the formation of cadaverine during fermentation. This study suggests that the use of strain MJ4 as a starter culture in salted fish fermentation may be a good strategy for the reduction of BA formation.

Roseicella frigidaeris gen. nov., sp. nov., isolated from an air-conditioning system

A Gram-stain-negative, facultatively aerobic bacterial strain, designated DB1506^T, of the family Acetobacteraceae, was isolated from an air-conditioning system in the Republic of Korea. Colonies were pink- to rosy-coloured and cells were non-motile cocci with catalase- and oxidase-positive activities. Growth of strain DB1506^T was observed at 20-37 °C (optimum, 30 °C), pH 5.5-8.5 (pH 7.0) and in the presence of 0-0.5 % (w/v) NaCl (0 %). Strain DB1506^T contained summed feature 8 (comprising C18 : 1ω7c and/or C18 : 1ω6c) and C18 : 1 2-OH as major fatty acids and ubiquinone-10 as the sole isoprenoid quinone. Phosphatidylglycerol, phosphatidylethanolamine, unidentified phospholipids, unidentified aminolipids and unidentified polar lipids were detected as major polar lipids. The G+C content of the genomic DNA calculated from the whole genome sequence was 72.5 mol%. Strain DB1506^T was most closely related to Paracraurococcus ruber NS89^T, Dankookia rubra WS-10^T and Siccirubricoccus deserti SYSU D8009^T with 16S rRNA gene sequence similarities of 96.01, 95.88 and 95.44 %, respectively, but strain DB1506^T formed a clearly distinct phylogenic lineage from them within the family Acetobacteraceae. On the basis of phenotypic, chemotaxonomic and molecular properties, strain DB1506^T represents a novel species of a new genus within the family Acetobacteraceae, for which the name Roseicella frigidaeris gen. nov., sp. nov. is proposed. The type strain is DB1506^T (=KACC 19791^T=JCM 32945^T).