Development of an enzymatic screening method for d-aspartate-producing lactic acid bacteria

d-aspartate, an amino-acid important for human health, supports anaerobic respiration in several Campylobacter species

Despite being classified as microaerophilic microorganisms, most Campylobacter species can grow anaerobically, using formate or molecular hydrogen (H2) as electron donors, and various nitrogenous and sulfurous compounds as electron acceptors. Herein, we showed that both l-asparagine (l-Asn) and l-aspartic acid (l-Asp) bolster H2-driven anaerobic growth in several Campylobacter species, whereas the d-enantiomer form of both asparagine (d-Asn) and aspartic acid (d-Asp) only increased anaerobic growth in Campylobacter concisus strain 13826 and Campylobacter ureolyticus strain NCTC10941. A gene annotated as racD encoding for a putative d/l-Asp racemase was identified in the genome of both strains. Disruption of racD in Cc13826 resulted in the inability of the mutant strain to use either d-enantiomer during anaerobic growth. Hence, our results suggest that the racD gene is required for campylobacters to use either d-Asp or d-Asn. The use of d-Asp by various human opportunistic bacterial pathogens, including C. concisus, C. ureolyticus, and also possibly select strains of Campylobacter gracilis, Campylobacter rectus and Campylobacter showae, is significant, because d-Asp is an important signal molecule for both human nervous and neuroendocrine systems. To our knowledge, this is the first report of pathogens scavenging a d-amino acid essential for human health.

Profiling of d-alanine production by the microbial isolates of rat gut microbiota

d-alanine (d-Ala) and several other d-amino acids (d-AAs) act as hormones and neuromodulators in nervous and endocrine systems. Unlike the endogenously synthesized d-serine in animals, d-Ala may be from exogenous sources, e.g., diet and intestinal microorganisms. However, it is unclear if the capability to produce d-Ala and other d-AAs varies among different microbial strains in the gut. We isolated individual microorganisms of rat gut microbiota and profiled their d-AA production in vitro, focusing on d-Ala. Serial dilutions of intestinal contents from adult male rats were plated on agar to obtain clonal cultures. Using MALDI-TOF MS for rapid strain typing, we identified 38 unique isolates, grouped into 11 species based on 16S rRNA gene sequences. We then used two-tier screening to profile bacterial d-AA production, combining a d-amino acid oxidase-based enzymatic assay for rapid assessment of non-acidic d-AA amount and chiral LC-MS/MS to quantify individual d-AAs, revealing 19 out of the 38 isolated strains as d-AA producers. LC-MS/MS analysis of the eight top d-AA producers showed high levels of d-Ala in all strains tested, with substantial inter- and intra-species variations. Though results from the enzymatic assay and LC-MS/MS analysis aligned well, LC-MS/MS further revealed the existence of d-glutamate and d-aspartate, which are poor substrates for this enzymatic assay. We observed large inter- and intra-species variation of d-AA production profiles from rat gut microbiome species, demonstrating the importance of chemical profiling of gut microbiota in addition to sequencing, furthering the idea that microbial metabolites modulate host physiology.

Antioxidant Mechanism of Lactiplantibacillus plantarum KM1 Under H2O2 Stress by Proteomics Analysis

Lactiplantibacillus plantarum KM1 was screened from natural fermented products, which had probiotic properties and antioxidant function. The survival rate of L. plantarum KM1 was 78.26% at 5 mM H₂O₂. In this study, the antioxidant mechanism of L. plantarum KM1 was deeply analyzed by using the proteomics method. The results demonstrated that a total of 112 differentially expressed proteins (DEPs) were screened, of which, 31 DEPs were upregulated and 81 were downregulated. The Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis indicated that DEPs participated in various metabolic pathways such as pyruvate metabolism, carbon metabolism, trichloroacetic acid cycle, amino acid metabolism, and microbial metabolism in diverse environments. These metabolic pathways were related to oxidative stress caused by H₂O₂ in L. plantarum KM1. Therefore, the antioxidant mechanism of L. plantarum KM1 under H₂O₂ stress provided a theoretical basis for its use as a potential natural antioxidant.

Mechanism of high D-aspartate production in the lactic acid bacterium Latilactobacillus sp. strain WDN19

D-Aspartate (D-Asp) is a useful compound for a semisynthetic antibiotic and has potentially beneficial effects on humans. Several lactic acid bacteria (LAB) species produce D-Asp as a component of cell wall peptidoglycan. We previously isolated a LAB strain (named strain WDN19) that can extracellularly produce a large amount of D-Asp. Here, we show the factors that contribute to high D-Asp production ability. Strain WDN19 was most closely related to Latilactobacillus curvatus. The D-Asp production ability of strain WDN19 in a rich medium was 13.7-fold higher than that of L. curvatus DSM 20019. A major part of D-Asp was synthesized from L-Asp contained in the medium by aspartate racemase (RacD). During their cultivation, the RacD activity in strain WDN19 was higher than in strain DSM 20019, especially much higher in the early exponential growth phase because of the higher racD transcription and the higher activity of RacD itself of strain WDN19. In a synthetic medium, the extracellular production of D,L-Asp was observed in strain WDN19 but not in strain DSM 20019. The addition of L-asparagine (L-Asn) to the medium increased and gave D,L-Asp production in strains WDN19 and DSM 20019, respectively, suggesting L-Asp synthesis by L-asparaginase (AsnA). The L-Asn uptake ability of the strains was similar, but the AsnA activity in the middle exponential and early stationary growth phases and intracellular D,L-Asp was much higher in strain WDN19. In their genome sequences, only an aspartate aminotransferase gene was found among L-Asp-metabolizing enzymes, except for RacD, but was disrupted in strain WDN19 by transposon insertion. These observations indicated that the high D-Asp production ability of strain WDN19 was mainly based on high RacD and AnsA activities and L-Asp supply. KEY POINTS: • Strain WDN19 was suggested to be a strain of Latilactobacillus curvatus. • Extracellular high d-Asp production ability was not a common feature of L. curvatus. • High d-Asp production was due to high RacD and AnsA activities and l-Asp supply.

Regulation of d-Aspartate Oxidase Gene Expression by Pyruvate Metabolism in the Yeast Cryptococcus humicola

d-Aspartate oxidase (DDO) is a peroxisomal flavoenzyme that catalyzes the oxidative deamination of acidic d-amino acids. In the yeast Cryptococcus humicola strain UJ1, the enzyme ChDDO is essential for d-Asp utilization and is expressed only in the presence of d-Asp. Pyruvate carboxylase (Pyc) catalyzes the conversion of pyruvate to oxaloacetate and is involved in the import and activation of certain peroxisomal flavoenzymes in yeasts. In this study, we analyzed the role of Pyc in the expression of ChDDO gene in C. humicola strain UJ1. PYC gene disruption (∆Chpyc1) in strain UJ1 resulted in growth retardation on glucose and NH₄Cl medium. The growth was restored by supplying oxaloacetate from l-Asp or α-ketoglutarate by a transaminase. On the other hand, the supply of oxaloacetate from d-Asp by ChDDO was not able to prevent growth retardation because of a significant decrease in ChDDO gene expression at the transcriptional level. The addition of pyruvate significantly decreased ChDDO gene transcription in the ∆Chpyc1 strain but increased the same in the wild-type strain, even though the intracellular pyruvate content was similar in both strains. These results suggest that ChDDO gene expression might be regulated by pyruvate metabolism, as well as by the presence of d-Asp.

Complete Genome Sequence of Latilactobacillus sp. Strain WDN19, a High-d-Aspartate-Producing Lactic Acid Bacterium Isolated from a Japanese Pickle

We report here the complete genome sequence of Latilactobacillus sp. strain WDN19, isolated from a Japanese pickle. This strain can produce a large amount of d-aspartate in the culture broth. The genome consists of a circular chromosome (1,967,462 bp; GC content, 41.88%) and a circular plasmid (66,648 bp; GC content, 35.08%).