2,3-Butanediol fermentation promotes growth of Serratia plymuthica at low pH but not survival of extreme acid challenge

Screening and isolation of acid-tolerant bacteria using a novel pH shift culture method

Acid-tolerant bacteria, which multiply under neutral pH and can survive under acidic pH conditions, have a potential role in various applications under acidic conditions. Despite higher biomass productivity, their isolation and utilisation are not sufficiently developed compared to those of acidophiles. It takes considerable effort to distinguish the acid-tolerant bacteria from the rest of the bacterial community using conventional screening methods. Thus, we developed a novel screening method for acid-tolerant bacteria, which involves shifting the pH between acidic and neutral conditions. With this method, the bacterium Enterobacter sp. AC06 was isolated. Based on comparisons with the results reported in previous studies, the strain can be classified as acid-tolerant bacteria. The decreases in the live cell concentrations were 3.87 and 6.16 log cycles after 3 h acid treatment under pH 3.0 and 2.5, respectively. These results suggest that it is possible to isolate acid-tolerant bacteria using the pH shift culture method. In summary, this is the first study on bacterial screening based on acid tolerance. Our novel method potentially contributes to the understanding and utilisation of acid-tolerant bacteria by enhancing screening efficiency. Furthermore, our novel concept shift culture is potentially valuable for screening previously uncultured bacteria tolerant to various selective stress conditions.

Comparative metabolic expressions of fermented soybeans according to different microbial starters

The quality of fermented soybeans can be determined by diverse metabolites produced by microorganisms. Mass spectrometry-based metabolomic approach was applied to investigate the differences in volatile and non-volatile metabolite profiles of fermented soybeans by different microorganisms [e.g., molds, yeasts, lactic acid bacteria (LAB), and other bacteria]. The partial least squares-discriminant analysis (PLS-DA) for volatile metabolites profiles indicated that the fungi group (mold/yeast) was clearly discriminated from the bacteria group (bacteria/LAB). The metabolic pathways related to the formation of volatile metabolites also differed according to microorganisms. In particular, the formation of branched-chain aliphatic alcohols and esters increased in the fungi group, while that of volatiles derived from fatty acids was superior in the bacteria group. In addition, we could determine the microorganism-specific metabolites using a correlation network analysis. This study can provide the fundamental knowledge on the metabolic differences according to the type of microorganisms in fermented soybeans.

Biological Production of (S)-acetoin: A State-of-the-Art Review

Acetoin is an important four-carbon compound that has many applications in foods, chemical synthesis, cosmetics, cigarettes, soaps, and detergents. Its stereoisomer (S)-acetoin, a high-value chiral compound, can also be used to synthesize optically active drugs, which could enhance targeting properties and reduce side effects. Recently, considerable progress has been made in the development of biotechnological routes for (S)-acetoin production. In this review, various strategies for biological (S)- acetoin production are summarized, and their constraints and possible solutions are described. Furthermore, future prospects of biological production of (S)-acetoin are discussed.

Distinctive Formation of Volatile Compounds in Fermented Rice Inoculated by Different Molds, Yeasts, and Lactic Acid Bacteria

Rice has been fermented to enhance its application in some foods. Although various microbes are involved in rice fermentation, their roles in the formation of volatile compounds, which are important to the characteristics of fermented rice, are not clear. In this study, diverse approaches, such as partial least squares-discriminant analysis (PLS-DA), metabolic pathway-based volatile compound formations, and correlation analysis between volatile compounds and microbes were applied to compare metabolic characteristics according to each microbe and determine microbe-specific metabolites in fermented rice inoculated by molds, yeasts, and lactic acid bacteria. Metabolic changes were relatively more activated in fermented rice inoculated by molds compared to other microbes. Volatile compound profiles were significantly changed depending on each microbe as well as the group of microbes. Regarding some metabolic pathways, such as carbohydrates, amino acids, and fatty acids, it could be observed that certain formation pathways of volatile compounds were closely linked with the type of microbes. Also, some volatile compounds were strongly correlated to specific microbes; for example, branched-chain volatiles were closely link to Aspergillus oryzae, while Lactobacillus plantarum had strong relationship with acetic acid in fermented rice. This study can provide an insight into the effects of fermentative microbes on the formation of volatile compounds in rice fermentation.

Selective and Efficient Elimination of Vibrio cholerae with a Chemical Modulator that Targets Glucose Metabolism

Vibrio cholerae, a Gram-negative bacterium, is the causative agent of pandemic cholera. Previous studies have shown that the survival of the seventh pandemic El Tor biotype V. cholerae strain N16961 requires production of acetoin in a glucose-rich environment. The production of acetoin, a neutral fermentation end-product, allows V. cholerae to metabolize glucose without a pH drop, which is mediated by the production of organic acid. This finding suggests that inhibition of acetoin fermentation can result in V. cholerae elimination by causing a pH imbalance under glucose-rich conditions. Here, we developed a simple high-throughput screening method and identified an inducer of medium acidification (iMAC). Of 8364 compounds screened, we identified one chemical, 5-(4-chloro-2-nitrobenzoyl)-6-hydroxy-1,3-dimethylpyrimidine-2,4(1H,3H)-dione, that successfully killed glucose-metabolizing N16961 by inducing acidic stress. When N16961 was grown with abundant glucose in the presence of iMAC, acetoin production was completely suppressed and concomitant accumulation of lactate and acetate was observed. Using a beta-galactosidase activity assay with a single-copy palsD::lacZ reporter fusion, we show that that iMAC likely inhibits acetoin production at the transcriptional level. Thin-layer chromatography revealed that iMAC causes a significantly reduced accumulation of intracellular (p)ppGpp, a bacterial stringent response alarmone known to positively regulate acetoin production. In vivo bacterial colonization and fluid accumulation were also markedly decreased after iMAC treatment. Finally, we demonstrate iMAC-induced bacterial killing for 22 different V. cholerae strains belonging to diverse serotypes. Together, our results suggest that iMAC, acting as a metabolic modulator, has strong potential as a novel antibacterial agent for treatment against cholera.

Formate hydrogen lyase mediates stationary-phase deacidification and increases survival during sugar fermentation in acetoin-producing enterobacteria

Two fermentation types exist in the Enterobacteriaceae family. Mixed-acid fermenters produce substantial amounts of lactate, formate, acetate, and succinate, resulting in lethal medium acidification. On the other hand, 2,3-butanediol fermenters switch to the production of the neutral compounds acetoin and 2,3-butanediol and even deacidify the environment after an initial acidification phase, thereby avoiding cell death. We equipped three mixed-acid fermenters (Salmonella Typhimurium, S. Enteritidis and Shigella flexneri) with the acetoin pathway from Serratia plymuthica to investigate the mechanisms of deacidification. Acetoin production caused attenuated acidification during exponential growth in all three bacteria, but stationary-phase deacidification was only observed in Escherichia coli and Salmonella, suggesting that it was not due to the consumption of protons accompanying acetoin production. To identify the mechanism, 34 transposon mutants of acetoin-producing E. coli that no longer deacidified the culture medium were isolated. The mutations mapped to 16 genes, all involved in formate metabolism. Formate is an end product of mixed-acid fermentation that can be converted to H₂ and CO₂ by the formate hydrogen lyase (FHL) complex, a reaction that consumes protons and thus can explain medium deacidification. When hycE, encoding the large subunit of hydrogenase 3 that is part of the FHL complex, was deleted in acetoin-producing E. coli, deacidification capacity was lost. Metabolite analysis in E. coli showed that introduction of the acetoin pathway reduced lactate and acetate production, but increased glucose consumption and formate and ethanol production. Analysis of a hycE mutant in S. plymuthica confirmed that medium deacidification in this organism is also mediated by FHL. These findings improve our understanding of the physiology and function of fermentation pathways in Enterobacteriaceae.

Role of 1-acyl-sn-glycerol-3-phosphate acyltransferase in psychrotrophy and stress tolerance of Serratia plymuthica RVH1

A mutant with a transposon insertion just upstream of the lysophosphatidic acid acyltansferase gene plsC was isolated in a screen for mutants affected in growth at low temperature of the psychrotroph Serratia plymuthica RVH1. This mutant had lost its ability to grow at 4 °C and was severely affected in growth at 10 °C, but showed only slightly reduced growth at 30 °C. Fatty acid analysis of membrane extracts showed that the ratio of C16:1/C18:1 fatty acids was six-to sevenfold reduced in the mutant, although the ratio of unsaturated to saturated fatty acids was unaffected. The homeoviscous adaptation ability of the mutant was also unaffected. Growth and fatty acid composition were mostly restored by overexpressing plsC on a plasmid. Supplementation of C16:1 (palmitoleic acid) into the growth medium partially rescued low temperature growth, indicating that a balanced ratio of the two main unsaturated fatty acids is required for psychrotrophy. The mutant was significantly more strongly inactivated by high pressure treatment at 250 MPa, but not at higher pressures. It also showed reduced growth at low pH, but not at increased NaCl concentrations. This work provides novel information on the role of membrane fatty acid composition in stress tolerance.

Acetoin synthesis acquisition favors Escherichia coli growth at low pH

Some members of the family Enterobacteriaceae ferment sugars via the mixed-acid fermentation pathway. This yields large amounts of acids, causing strong and sometimes even lethal acidification of the environment. Other family members employ the 2,3-butanediol fermentation pathway, which generates comparatively less acidic and more neutral end products, such as acetoin and 2,3-butanediol. In this work, we equipped Escherichia coli MG1655 with the budAB operon, encoding the acetoin pathway, from Serratia plymuthica RVH1 and investigated how this affected the ability of E. coli to cope with acid stress during growth. Acetoin fermentation prevented lethal medium acidification by E. coli in lysogeny broth (LB) supplemented with glucose. It also supported growth and higher stationary-phase cell densities in acidified LB broth with glucose (pH 4.10 to 4.50) and in tomato juice (pH 4.40 to 5.00) and reduced the minimal pH at which growth could be initiated. On the other hand, the acetoin-producing strain was outcompeted by the nonproducer in a mixed-culture experiment at low pH, suggesting a fitness cost associated with acetoin production. Finally, we showed that acetoin production profoundly changes the appearance of E. coli on several diagnostic culture media. Natural E. coli strains that have laterally acquired budAB genes may therefore have escaped detection thus far. This study demonstrates the potential importance of acetoin fermentation in the ecology of E. coli in the food chain and contributes to a better understanding of the microbiological stability and safety of acidic foods.