Metabolite profiles of lactic acid bacteria in grass silage

Abstract

The metabolite production of lactic acid bacteria (LAB) on silage was investigated. The aim was to compare the production of antifungal metabolites in silage with the production in liquid cultures previously studied in our laboratory. The following metabolites were found to be present at elevated concentrations in silos inoculated with LAB strains: 3-hydroxydecanoic acid, 2-hydroxy-4-methylpentanoic acid, benzoic acid, catechol, hydrocinnamic acid, salicylic acid, 3-phenyllactic acid, 4-hydroxybenzoic acid, (trans, trans)-3,4-dihydroxycyclohexane-1-carboxylic acid, p-hydrocoumaric acid, vanillic acid, azelaic acid, hydroferulic acid, p-coumaric acid, hydrocaffeic acid, ferulic acid, and caffeic acid. Among these metabolites, the antifungal compounds 3-phenyllactic acid and 3-hydroxydecanoic acid were previously isolated in our laboratory from liquid cultures of the same LAB strains by bioassay-guided fractionation. It was concluded that other metabolites, e.g., p-hydrocoumaric acid, hydroferulic acid, and p-coumaric acid, were released from the grass by the added LAB strains. The antifungal activities of the identified metabolites in 100 mM lactic acid were investigated. The MICs against Pichia anomala, Penicillium roqueforti, and Aspergillus fumigatus were determined, and 3-hydroxydecanoic acid showed the lowest MIC (0.1 mg ml(-1) for two of the three test organisms).

FIG. 1.

Reconstructed total ion mass chromatograms from GC-MS analysis of water-extracted silage harvested after 13 days. The top silage sample was inoculated with MiLAB393 at day 0, whereas the bottom sample was an uninoculated control silage. Samples were analyzed by GC-MS in full scan mode after trimethylsilylation, and the denoted compounds were identified by using a mass spectral database. The compounds indicated in the top chromatogram were quantified relative the internal standard 3-hydroxyundecanoic acid (*) by using selected ions as described in Materials and Methods. The numbering refers to compounds quantified and presented in Table 1.