Diversity of volatile organic compound production from leucine and citrate in Enterococcus faecium

Abstract

Enterococcus faecium is frequently isolated from fermented food; in particular, they positively contribute to the aroma compounds generation in traditional cheese. Citrate fermentation is a desirable property in these bacteria, but this feature is not uniformly distributed among E. faecium strains. In the present study three selected E. faecium strains, IQ110 (cit-), GM70 (cit+ type I) and Com12 (cit+ type II) were analyzed in their production of aroma compounds in milk. End products and volatile organic compounds (VOCs) were determined by solid phase micro-extraction combined with gas chromatography-mass spectrometry (SPME GC-MS). Principal component analysis (PCA) of aroma compounds profiles revealed a different VOCs composition for the three strains. In addition, resting cells experiments of E. faecium performed in the presence of leucine, citrate or pyruvate aroma compound precursors, allowed us to determine metabolic differences between the studied strains. GM70 (cit+ type I) showed an active citrate metabolism, with increased levels of diacetyl and acetoin generation relatively to Com12 or to citrate defective IQ110 strains. In addition, in our experimental conditions, we found a defective citrate fermenting phenotype for the Com12 strain, while its leucine degradation and pyruvate metabolism are conserved. In conclusion, rational selection of E. faecium strains could be performed based on genotypic and phenotypic analyzes. This would result in a performing strain, such as GM70, that could positively contribute to flavor, with typical notes of diacetyl, acetoin, 3-methyl butanal and 3-methyl butanol in an adjuvant culture.