No abstract

The conditions under which Brevibacterium linens CNRZ 918, a strain isolated from the surface smear flora of Gruyère de Comté cheese, produced methanethiol from methionine were studied. Demethiolation was estimated from the methanethiol production capacity of resting cells. Methionine was demethiolated mainly during the exponential growth phase of the organism during which time the cells were rod-shaped and had a generation time of 5 h, and the medium became alkaline. At the end of growth (pH 9) the cells were coccoid, and produced only very little methanethiol. The production of methanethiol required the presence of methionine in the culture medium, this reflecting the probable induction of the enzyme systems involved. Glucose favoured growth and inhibited production of methanethiol. Lactate favoured both growth and methanethiol production. Resting rod cells also produced methanethiol from structural analogues of methionine and from methionine-containing peptides. The apparent kinetic constants of the production of methanethiol for rod and coccoid cells were respectively Km = 14 mM and 46 mM, Vmax = 208 nkat g-1 and 25 nkat g-1. The optimum temperature and pH for production were 30 degrees C and pH 8. Azide or malonate favoured the production of methanethiol by resting cells, whereas chloramphenicol had no effect.

Summary -Dairy starter strains Streptococcus thermophilus CNRZ 385 and CNRZ 703 coagulatad low heat skim milk in 3 h while the raference strain CNRZ 302 required more than 10 h. The high acidification rates of these 2 strains (H-strains) were correlated with the presence of a 10-and 7-fold increase in proteinase activity as compared to 13 other randomly chosen strains (L-strains), including the reference strain CNRZ 302. The level of proteinase activity in M17 broth was similar ta the activity of the Prt+ Lactococcus lactis ssp lactis strain CNRZ 1076. Proteinase activity of S thermophilus H-strains was found ta be cell wall-associated and was not released in the absence of CaCI 2 as in the case of L lactis strains. The relevance of these proteinase activities in relation to high acid production rates was confirmed by a proteinase-negative mutant whose growth and proteinase activity were reduced to levels observed for the L-strains. Streptococcus

To explain the competition for nitrogenous nutrients observed in mixed strain cultures of Lactococcus lactis and Leuconostoc mesenteroides, the utilization of peptides as a source of essential amino acids for growth in a chemically defined medium was compared in 12 strains of dairy origin. Both species were multiple amino acid auxotrophs and harboured a large set of intracellular peptidases. Lactococcus lactis can use a wide variety of peptides up to 13 amino acid residues whereas Leuc. mesenteroides assimilated only shorter peptides containing up to seven amino acids. Growth was limited by the transport of peptides and not by their hydrolysis. The nutritional value of peptides varied with the strains and the composition of the peptides, L. lactis being advantaged over Leuc. mesenteroides.

Membrane vesicles of Leuconostoc mesenteroides subsp. dextranicum fused with proteoliposomes prepared from Escherichia coli phospholipids containing beef heart cytochrome c oxidase were used to study the transport of branched-chain amino acids in a strain isolated from a raw milk cheese. At a medium pH of 6.0, oxidation of an electron donor system comprising ascorbate, N,N,N',N'-tetramethyl-p-phenylenediamine, and horse heart cytochrome c resulted in a membrane potential (A*) of-60 mV, a pH gradient of-36 mV, and an L-leucine accumulation of 76-fold (AIJLeu/F = 108 mV). Leucine uptake in hybrid membranes in which a A*, ApH, sodium ion gradient, or a combination of these was imposed artificially revealed that both components of the proton motive force (Ap) could drive leucine uptake but that a chemical sodium gradient could not. Kinetic analysis of leucine (valine) transport indicated three secondary transport systems with Kt values of 1.7 (0.8) mM, 4.3 (5.9) ,uM, and 65 (29) nM, respectively. L-Leucine transport via the high-affinity leucine transport system (K, = 4.3 ,uM) was competitively inhibited by L-valine and L-isoleucine (Ki and Kt values were similar), demonstrating that the transport system translocates branched-chain amino acids. Similar studies with these hybrid membranes indicated the presence of high-affinity secondary transport systems for 10 other amino acids.