The enzymatic degradation of L-methionine and subsequent formation of volatile sulfur compounds (VSCs) is believed to be essential for flavor development in cheese. L-Methionine-␥-lyase (MGL) can convert Lmethionine to methanethiol (MTL), ␣-ketobutyrate, and ammonia. The mgl gene encoding MGL was cloned from the type strain Brevibacterium linens ATCC 9175 known to produce copious amounts of MTL and related VSCs. The disruption of the mgl gene, achieved in strain ATCC 9175, resulted in a 62% decrease in thiolproducing activity and a 97% decrease in total VSC production in the knockout strain. Our work shows that L-methionine degradation via ␥-elimination is a key step in the formation of VSCs in B. linens.Due to their low detection threshold and diversity, volatile sulfur compounds (VSCs) are of prime importance in the overall flavor of cheese and make a significant contribution to the typical aromas of different cheeses (12,14,33). VSCs arise primarily from the degradation of L-methionine to methanethiol (MTL) by the cheese microflora. This thiol is a common precursor for a variety of other sulfur-bearing compounds including the auto-oxidation products (11), dimethyl disulfide (DMDS), dimethyl trisulfide (DMTS), and S-methylthioesters, primarily arising from chemical reaction of MTL with acyl coenzyme A (acyl-CoA) (22). Numerous studies have therefore been done to control and/or diversify VSC synthesis during the ripening process by the use of properly selected microorganisms (4, 6, 15, 43). Many cheese microorganisms are capable of producing VSCs from L-methionine. Some of them, such as brevibacteria, especially Brevibacterium linens (17), are known to be very good VSC producers while others, such as lactic acid bacteria (LAB), can produce only limited amounts of VSCs (14).The most direct route for MTL biosynthesis, is the L-methionine ␥-elimination that directly produces MTL, ␣-ketobutyrate, and ammonia from L-methionine. This L-methionine ␥-elimination activity is quite high in B. linens and corynebacteria (17) and is also suspected in several other cheese surface bacteria, such as Micrococcus luteus, Arthrobacter sp., and Staphylococcus equorum (8). In contrast, such activity is quite low in LAB (14). In B. linens, the methionine ␥-elimination is catalyzed by a L-methionine-␥-lyase (MGL), a pyridoxal phosphate (PLP)-dependent enzyme for which L-methionine is the best substrate (16). In contrast, in LAB the reaction is catalyzed by a cystathionine ␤-lyase (CBL) and a cystathionine ␥-lyase (CGL) which are only slightly active towards L-methionine (1, 10, 18). In LAB, another pathway for L-methionine conversion to VSCs also exists but produces limited amounts of MTL (7,35).Coryneform bacteria are generally found on the surface of smear cheeses and give the typical sulfur notes to cheeses such as Limburger, Tilsiter, Livarot, Epoisses, and Munster. To date, B. linens is the only food-grade bacterium from which MGL has been purified and characterized (16,26,31,38,39), but neither its protein sequence nor its gene...

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Fourteen isolates of two different bacterial species isolated from the surface of smear-ripened cheeses were found to exhibit many characteristics of the genus Arthrobacter. The isolates were aerobic, Gram-positive, catalase-positive, non-spore-forming and non-motile. The cell-wall peptidoglycan contained lysine, alanine and glutamic acid. rrs sequence analysis indicated that the new isolates Re117T and Ca106T are closely related to the Arthrobacter nicotianae group and showed highest sequence similarity (>98 %) to Arthrobacter nicotianae and Arthrobacter protophormiae. However, DNA–DNA hybridization studies indicated that the strains represented two novel genomic species within the genus Arthrobacter and did not belong to A. nicotianae or A. protophormiae (<43 % DNA–DNA relatedness). On the basis of the phylogenetic and phenotypic distinctiveness of the new isolates, these bacteria should be classified as two novel Arthrobacter species, for which the names Arthrobacter bergerei sp. nov. and Arthrobacter arilaitensis sp. nov. are proposed. Type strains have been deposited in culture collections as Arthrobacter bergerei Ca106T (=CIP 108036T=DSM 16367T) and Arthrobacter arilaitensis Re117T (=CIP 108037T=DSM 16368T).

Aims:  This work aimed at clarifying the physiological responses of Lactobacillus delbrueckii subsp. bulgaricus CFL1 cells after exposure to acidification at the end of fermentation, in relation to their cryotolerance. Methods and Results:  Cells acidified at the end of the fermentation (pH 5·25 for 30 min) had their cryotolerance improved as compared to the reference condition (pH 6·0). By analyzing the cytosolic proteome, it was established that changes occurred in the synthesis of 21 proteins, involved in energy metabolism, nucleotide and protein synthesis and stress response. Acidification also induced a slight decrease in unsaturated to saturated and cyclic to saturated membrane fatty acid ratios. Conclusions:  Lactobacillus bulgaricus CFL1 was able to develop a combined physiological response at both membrane and cytosolic levels. This acid adaptation was referred as a cross‐protection phenomenon as it allowed the cells to become more tolerant to cold stress. Significance and Impact of the Study:  This study increased knowledge concerning the physiological mechanisms that explained the cross‐protection by acid adaptation. It may be useful for improving cryotolerance of lactic acid bacteria, either in cells banks or in an industrial context.