Propionate is an important antimicrobial short-chain carboxylic acid (SCCA) that prolongs the shelf life of food products. L-rhamnose is metabolized to 1,2-propanediol (1,2-PD) by the deoxyhexose pathway, this intermediate can be further converted to propionate by the 1,2-PD pathway. Loigolactobacillus coryniformis subsp. coryniformis DSM 20001 has been recently identified as a propionate producer, but the mechanism behind the conversion has not been revealed. The aim of this study was to investigate L. coryniformis L-rhamnose utilization and to demonstrate the antimicrobial activity of fermentates.

L. coryniformis was grown in bioreactors in the presence of L-rhamnose under anaerobic conditions at 30ᵒC and pH 6.5 for 78 h. Expression of key genes related to deoxyhexose metabolism was monitored using qPCR assays. The antimicrobial properties of individual SCCAs, fermentates, and synthetic SCCA (sSCCA) mixtures mimicking the composition of fermentates against common food pathogens including Escherichia coli, Salmonella enterica, Klebsiella oxytoca, Staphylococcus aureus, and Candida albicans were investigated using 2-fold dilution assays at pH 4.5.

qPCR analysis demonstrated that genes encoding enzymes responsible for L-rhamnose utilization were expressed during the initial phase of fermentation, whereas genes involved in 1,2 PD utilization were transcribed later. From 40 mM rhamnose, up to 16 mM propionate was produced in addition to lactate (26 mM), formate (2 mM), and acetate (4 mM). Among the tested only propionate exhibited the ability to suppress the growth of the tested pathogens. The comparison with sSCCA indicated that propionate contributed to the inhibitory effect of fermentates against the tested Enterobacteriaceae.

In conclusion, our data suggested conversion of L-rhamnose to 1,2 PD and then to propionate. Our findings showed that in the L. coryniformis fermentates, propionate was the relevant SCCA that conferred antimicrobial properties against pathogens.