Christiansen Jason K, Hughes Joanne E, Welker Dennis L, Rodríguez Beatriz T, Steele James L, Broadbent Jeff R
Department of Nutrition and Food Sciences, Utah State University, 8700 Old Main Hill, Logan, UT 84322-8700, USA.
Appl Environ Microbiol. 2008 Jan;74(2):416-23. doi: 10.1128/AEM.01174-07. Epub 2007 Nov 9.
The conversion of amino acids into volatile and nonvolatile compounds by lactic acid bacteria in cheese is thought to represent the rate-limiting step in the development of mature flavor and aroma. Because amino acid breakdown by microbes often entails the reversible action of enzymes involved in biosynthetic pathways, our group investigated the genetics of amino acid biosynthesis in Lactobacillus helveticus CNRZ 32, a commercial cheese flavor adjunct that reduces bitterness and intensifies flavor notes. Most lactic acid bacteria are auxotrophic for several amino acids, and L. helveticus CNRZ 32 requires 14 amino acids. The reconstruction of amino acid biosynthetic pathways from a draft-quality genome sequence for L. helveticus CNRZ 32 revealed that amino acid auxotrophy in this species was due primarily to gene absence rather than point mutations, insertions, or small deletions, with good agreement between gene content and phenotypic amino acid requirements. One exception involved the phenotypic requirement for Asp (or Asn), which genome predictions suggested could be alleviated by citrate catabolism. This prediction was confirmed by the growth of L. helveticus CNRZ 32 after the addition of citrate to a chemically defined medium that lacked Asp and Asn. Genome analysis also predicted that L. helveticus CNRZ 32 possessed ornithine decarboxylase activity and would therefore catalyze the conversion of ornithine to putrescine, a volatile biogenic amine. However, experiments to confirm ornithine decarboxylase activity in L. helveticus CNRZ 32 by the use of several methods were unsuccessful, which indicated that this bacterium likely does not contribute to putrescine production in cheese.
奶酪中乳酸菌将氨基酸转化为挥发性和非挥发性化合物的过程被认为是成熟风味和香气形成的限速步骤。由于微生物对氨基酸的分解通常需要参与生物合成途径的酶的可逆作用，我们小组研究了瑞士乳杆菌CNRZ 32中氨基酸生物合成的遗传学，该菌株是一种商业奶酪风味添加剂，可减少苦味并增强风味。大多数乳酸菌对几种氨基酸是营养缺陷型的，而瑞士乳杆菌CNRZ 32需要14种氨基酸。从瑞士乳杆菌CNRZ 32的草图质量基因组序列重建氨基酸生物合成途径表明，该物种的氨基酸营养缺陷主要是由于基因缺失，而不是点突变、插入或小缺失，基因含量与表型氨基酸需求之间具有良好的一致性。一个例外涉及对天冬氨酸（或天冬酰胺）的表型需求，基因组预测表明，柠檬酸分解代谢可以缓解这种需求。在缺乏天冬氨酸和天冬酰胺的化学限定培养基中添加柠檬酸后，瑞士乳杆菌CNRZ 32的生长证实了这一预测。基因组分析还预测，瑞士乳杆菌CNRZ 32具有鸟氨酸脱羧酶活性，因此会催化鸟氨酸转化为腐胺，一种挥发性生物胺。然而，使用几种方法来证实瑞士乳杆菌CNRZ 32中鸟氨酸脱羧酶活性的实验均未成功，这表明这种细菌可能不会在奶酪中产生腐胺。
