Enterococcus faecium is frequently isolated from fermented food; in particular, they positively contribute to the aroma compound 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 compound profiles revealed a different VOC composition for the three strains. In addition, resting cell experiments of E. faecium performed in the presence of leucine, citrate, or pyruvate as 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 relative to Com12 or to citrate defective IQ110 strains. In addition, in the experimental conditions tested, a defective citrate-fermenting phenotype for the Com12 strain was found, while its leucine degradation and pyruvate metabolism were conserved. In conclusion, rational selection of E. faecium strains could be performed based on genotypic and phenotypic analyses. 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.

中文翻译：

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粪肠球菌中亮氨酸和柠檬酸盐产生的挥发性有机化合物的多样性。

粪肠球菌通常从发酵食品中分离出来。特别是，它们为传统奶酪中的香气化合物的产生做出了积极贡献。柠檬酸发酵是这些细菌的理想特性，但是该特性在粪肠球菌菌株之间分布不均。在本研究中，分析了三种选定的屎肠球菌菌株IQ110（cit-），GM70（cit + I型）和Com12（cit + II型）在牛奶中产生香气化合物的过程。通过固相微萃取结合气相色谱质谱法（SPME-GC-MS）测定最终产物和挥发性有机化合物（VOC）。香气化合物谱的主成分分析（PCA）显示了三种菌株的VOC组成不同。此外，在亮氨酸存在下，进行了粪肠球菌的静息细胞实验，柠檬酸盐或丙酮酸盐作为香气化合物的前体使我们能够确定所研究菌株之间的代谢差异。相对于Com12或柠檬酸盐缺陷的IQ110菌株，GM70（I型柠檬酸+ I型）显示出活跃的柠檬酸盐代谢，双乙酰基和乙酰丙酮生成水平增加。此外，在测试的实验条件下，发现Com12菌株的柠檬酸盐发酵表型有缺陷，而其亮氨酸降解和丙酮酸代谢得以保留。总之，可以基于基因型和表型分析对粪肠球菌进行合理选择。这将导致表现良好的菌株，例如GM70，可以对风味产生积极的影响，在佐剂培养中具有典型的二乙酰，乙酰丁酸，3-甲基丁醛和3-甲基丁醇的风味。丙酮酸酯作为芳香化合物的前体，使我们能够确定所研究菌株之间的代谢差异。相对于Com12或柠檬酸盐缺陷的IQ110菌株，GM70（I型柠檬酸+ I型）显示出活跃的柠檬酸盐代谢，双乙酰基和乙酰丙酮生成水平增加。此外，在测试的实验条件下，发现Com12菌株的柠檬酸盐发酵表型有缺陷，而其亮氨酸降解和丙酮酸代谢得以保留。总之，可以基于基因型和表型分析对粪肠球菌进行合理选择。这将导致表现良好的菌株，例如GM70，可以对风味产生积极的影响，在佐剂培养中具有典型的二乙酰，乙酰丁酸，3-甲基丁醛和3-甲基丁醇的风味。丙酮酸酯作为芳香化合物的前体，使我们能够确定所研究菌株之间的代谢差异。相对于Com12或柠檬酸盐缺陷的IQ110菌株，GM70（I型柠檬酸+ I型）显示出活跃的柠檬酸盐代谢，双乙酰基和乙酰丙酮生成水平增加。此外，在测试的实验条件下，发现Com12菌株的柠檬酸盐发酵表型有缺陷，而其亮氨酸降解和丙酮酸代谢得以保留。总之，可以基于基因型和表型分析对粪肠球菌进行合理选择。这将导致表现良好的菌株，例如GM70，可以对风味产生积极的影响，在佐剂培养中具有典型的二乙酰，乙酰丁酸，3-甲基丁醛和3-甲基丁醇的风味。