Abstract
This work reports a Lactobacillus rossiae strain (L. rossiae D87) isolated from sourdough that synthesizes putrescine - a biogenic amine that raises food safety and spoilage concerns - from arginine via the ornithine decarboxylase (ODC) pathway. The odc and potE genes were identified and sequenced. These genes respectively encode ornithine decarboxylase (Odc), which participates in the decarboxylation of ornithine to putrescine, and the ornithine/putrescine exchanger (PotE), which exchanges ornithine for putrescine. Transcriptional analysis showed that odc and potE form an operon that is regulated transcriptionally by ornithine in a dose-dependent manner. To explore the possible role of the ODC pathway as an acid stress resistance mechanism for this bacterium, the effect of acidic pHs on its transcriptional regulation and on putrescine biosynthesis was analysed. Acidic pHs induced the transcription of the odc-potE genes and the production of putrescine over that seen at neutral pH. Further, putrescine production via the ODC system improved the survival of L. rossiae D87 by counteracting the acidification of the cytoplasm when the cells were subjected to acidic conditions. These results suggest the ODC pathway of L. rossiae D87 provides a biochemical defence mechanism against acidic environments.
Highlights
Lactic acid bacteria (LAB) play an essential role as biopreservatives in the production of fermented foods, preventing spoilage and contamination by pathogenic microorganisms
To monitor the production of putrescine from ornithine over the growth curve, L. rossiae D87 was grown in MRS broth and in MRS + Ort and the concentrations of arginine, ornithine and putrescine quantified by UHPLC at different times
To test whether L. rossiae D87 could deiminate arginine towards ornithine via the arginine deimination (ADI) pathway and its further decarboxylation to putrescine via the ornithine decarboxylase (ODC) pathway, cultures supplemented with arginine instead of ornithine were monitored for the production of putrescine
Summary
Lactic acid bacteria (LAB) play an essential role as biopreservatives in the production of fermented foods, preventing spoilage and contamination by pathogenic microorganisms. LAB are commonly added as starter cultures, but during manufacture and ripening a complex secondary microbiota (non-starter lactic acid bacteria [NSLAB]) develops[1] Both starter and NSLAB microorganisms contribute towards the production of flavour compounds and the physico-chemical changes that confer the desired organoleptic characteristics sought[2,3]. BAs are low molecular weight nitrogenous compounds essential to several physiological functions in humans, e.g., signal transduction at synapses, the immune response, cell growth and gene expression, etc. They can accumulate in foods and animal feed via the microbial decarboxylation of certain amino acids[4,5]. The main function of the amino acid decarboxylative pathways appear to be defence against acidic environments such as those encountered in fermented products[25,27,28,29,30]
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