Phenolic acids are among the most abundant phenolic compounds in edible parts of plants. Lactic acid bacteria (LAB) metabolize phenolic acids, but the enzyme responsible for reducing hydroxycinnamic acids to phenylpropionic acids (HcrB) was only recently characterized in Lactobacillus plantarum. In this study, heterofermentative LAB species were screened for their hydroxycinnamic acid metabolism. Data on strain-specific metabolism in combination with comparative genomic analyses identified homologs of HcrB as putative phenolic acid reductases. Par1 and HcrF both encode putative multidomain proteins with 25% and 63% amino acid identity to HcrB, respectively. Of these genes, par1 in L. rossiae and hcrF in L. fermentum were overexpressed in response to hydroxycinnamic acids. The deletion of par1 in L. rossiae led to the loss of phenolic acid metabolism. The strain-specific metabolism of phenolic acids was congruent with the genotype of lactobacilli; however, phenolic acid reductases were not identified in strains of Weissella cibaria that reduced hydroxycinnamic acids to phenylpropionic acids. Phylogenetic analysis of major genes involved in hydroxycinnamic acid metabolism in strains of the genus Lactobacillus revealed that Par1 was found to be the most widely distributed phenolic acid reductase, while HcrB was the least abundant, present in less than 9% of Lactobacillus spp. In conclusion, this study increased the knowledge on the genetic determinants of hydroxycinnamic acid metabolism, explaining the species- and strain-specific metabolic variations in lactobacilli and providing evidence of additional enzymes involved in hydroxycinnamic acid metabolism of lactobacilli. IMPORTANCE The metabolism of secondary plant metabolites, including phenolic compounds, by food-fermenting lactobacilli is a significant contributor to the safety, quality, and nutritional quality of fermented foods. The enzymes mediating hydrolysis, reduction, and decarboxylation of phenolic acid esters and phenolic acids in lactobacilli, however, are not fully characterized. The genomic analyses presented here provide evidence for three novel putative phenolic acid reductases. Matching comparative genomic analyses with phenotypic analysis and quantification of gene expression indicates that two of the three putative phenolic acid reductases, Par1 and HcrF, are involved in reduction of hydroxycinnamic acids to phenylpropionic acids; however, the activity of Par2 may be unrelated to phenolic acids and recognizes other secondary plant metabolites. These findings expand our knowledge on the metabolic potential of lactobacilli and facilitate future studies on activity and substrate specificity of enzymes involved in metabolism of phenolic compounds.

The risks associated with the presence of hidden allergens in food have increased the need for rapid, sensitive, and reliable methods for tracing food allergens in commodities. Conventional enzyme immunosorbent assay (ELISA) has usually been performed in a centralized lab, requiring considerable time and sample/reagent consumption and expensive detection instruments. In this study, a microfluidic ELISA platform combined with a custom-designed optical sensor was developed for the quantitative analysis of the proteins wheat gluten and Ara h 1. The developed microfluidic ELISA biosensor reduced the total assay time from hours (up to 3.5 h) to 15–20 min and decreased sample/reagent consumption to 5–10 μL, compared to a few hundred microliters in commercial ELISA kits, with superior sensitivity. The quantitative capability of the presented biosensor is a distinctive advantage over the commercially available rapid methods such as lateral flow devices (LFD) and dipstick tests. The developed microfluidic biosensor demonstrates the potential for sensitive and less-expensive on-site determination for rapidly detecting food allergens in a complex sample system.

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