Influence of Oxygen and pH on Methanethiol Production from l -Methionine by Brevibacterium linens CNRZ 918

Bioactivity of volatile organic compounds produced by Pseudomonas tolaasii

Pseudomonas tolaasii is the main bacterial pathogen of several mushroom species. In this paper we report that strains of P. tolaasii produce volatile substances inducing in vitro mycelia growth inhibition of Pleurotus ostreatus and P. eryngii, and Agaricus bisporus and P. ostreatus basidiome tissue blocks brown discoloration. P. tolaasii strains produced the volatile ammonia but not hydrogen cyanide. Among the volatiles detected by GC-MS, methanethiol, dimethyl disulfide (DMDS), and 1-undecene were identified. The latter, when assayed individually as pure compounds, led to similar effects noticed when P. tolaasii volatiles natural blend was used on mushrooms mycelia and basidiome tissue blocks. Furthermore, the natural volatile mixture resulted toxic toward lettuce and broccoli seedling growth. In contrast, pure volatiles showed different activity according to their nature and/or doses applied. Indeed, methanethiol resulted toxic at all the doses used, while DMDS toxicity was assessed till a quantity of 1.25 μg, below which it caused, together with 1-undecene (≥10 μg), broccoli growth increase.

Na-Stimulated Transport of l-Methionine in Brevibacterium linens CNRZ 918

The transport of l-methionine by the gram-positive species Brevibacterium linens CNRZ 918 is described. The one transport system (K(m) = 55 muM) found is constitutive for l-methionine, stereospecific, and pH and temperature dependent. Entry of l-methionine into cells is controlled by the internal methionine pool. Competition studies indicate that l-methionine and alpha-aminobutyric acid share a common carrier for their transport. Neither methionine derivatives substituted on the amino or carboxyl groups nor d-methionine was an inhibitor, whereas powerful inhibition was shown by l-cysteine, s-methyl-l-cysteine, dl-selenomethionine and dl-homocysteine. Sodium plays important and varied roles in l-methionine transport by B. linens CNRZ 918: (i) it stimulates transport without affecting the K(m), (ii) it increases the specific activity (on a biomass basis) of the l-methionine transport system when present with methionine in the medium, suggesting a coinduction mechanism. l-Methionine transport requires an exogenous energy source, which may be succinic, lactic, acetic, or pyruvic acid but not glucose or sucrose. The fact that l-methionine transport was stimulated by potassium arsenate and to a lesser extent by potassium fluoride suggests that high-energy phosphorylated intermediates are not involved in the process. Monensin eliminates stimulation by sodium. Gramicidin and carbonyl cyanide-m-chlorophenylhydrazone act in the presence or absence of Na. N-Ethylmaleimide, p-chloromercurobenzoate, valinomycin, sodium azide, and potassium cyanide have no or only a partial inhibitory effect. These results tend to indicate that the proton motive force reinforced by the Na gradient is involved in the mechanism of energy coupling of l-methionine transport by B. linens CNRZ 918. Thus, this transport is partially similar to the well-described systems in gram-negative bacteria, except for the role of sodium, which is very effective in B. linens, a species adapted to the high sodium levels of its niche.

Deacidification by Debaryomyces hansenii of smear soft cheeses ripened under controlled conditions: relative humidity and temperature influences

Model smear soft cheeses were prepared from pasteurized milk inoculated with Debaryomyces hansenii (304, GMPA) and Brevibacterium aurantiacum (ATCC 9175) under aseptic conditions. Debaryomyces hansenii growth and curd deacidification were studied in relation to ripening chamber temperature and relative humidity (RH). A total of 9 descriptors, mainly based on kinetic data, were defined to represent D. hansenii growth (2 descriptors), cheese deacidification (5 descriptors), and cheese ripening (2 descriptors). Regardless of the temperature, when the RH was 85%, D. hansenii growth was inhibited due to limitation of carbon substrate diffusions; consequently, cheese deacidification did not take place. Debaryomyces hansenii growth was most prolific when the temperature was 16 degrees C, and the RH was 95%. Kinetic descriptors of lactate consumption and pH increase were maximal at 16 degrees C and 100% RH. Under these 2 ripening conditions, on d 14 (packaging) the creamy underrind represented a third of the cheese; however, at the end of ripening (d 42), cheese was too liquid to be sold. Statistical analysis showed that the best ripening conditions to achieve an optimum between deacidification and appearance of cheeses (thickness of the creamy underrind) were 12 degrees C and 95 +/- 1% RH.

Industrial importance of the genus Brevibacterium

The genus Brevibacterium has long been difficult for taxonomists to classify due to its close morphological similarity to other genera. Since it was proposed in 1953, the genus has often been redefined. The genus is best known for its important role in the ripening of certain cheeses (B. linens) and for its supposed over-production of L: -amino acids. Other interesting industrial applications, including the production of ectoine, have recently been proposed. The general characteristics, the occurrence and the recent taxonomy of Brevibacterium are reviewed here. Furthermore, known and potential industrial applications for Brevibacterium species are briefly discussed.

Behavior of Brevibacterium linens and Debaryomyces hansenii as ripening flora in controlled production of smear soft cheese from reconstituted milk: growth and substrate consumption dairy foods

Experimental cheeses inoculated with Debaryomyces hansenii and Brevibacterium linens were ripened for 76 d under aseptic conditions. Triplicate cheese-making trials were similar as a result of efficient control of the atmosphere. In all trials, D. hansenii grew rapidly during the first 2 d and then slowed, but growth remained exponential until d 10 (generation time around 70 h). Total cell counts were higher than the number of viable cells, and after 10 d they remained around 3 x 10(9) yeast/g of DM. This difference resulted from the nonviability of a fraction of D. hansenii. After d 15, the pH of the rind was close to 7, and B. linens grew exponentially until d 25 (generation time around 70 h). The growth rate subsequently decreased but remained exponential (generation time around 21 d). Cell counts of D. hansenii and B. linens were correlated with the environmental technical conditions. Total D. hansenii counts were also correlated with total B. linens counts. Viable B. linens counts were related to rind lactate, and total counts depended on rind pH, internal lactate, and D. hansenii viable counts. The internal pH of the cheese depended on lactate concentrations, whereas surface pH was related to internal lactose, temperature, and relative humidity. These results suggest a determining role of the diffusion of the carbon sources in the ripening of smear soft cheese.

Aspects of enzymology and biochemical properties of Brevibacterium linens relevant to cheese ripening: a review

Brevibacterium linens is a major surface microorganism that is present in the smear of surface-ripened cheeses. The enzymology and biochemical characteristics of B. linens influence the ripening and final characteristics of smear surface-ripened cheeses. Proteolytic, peptidolytic, esterolytic, and lipolytic activities, which are of particular importance in the ripening process, are discussed in detail. This review also describes the production of volatile compounds, especially sulfur-containing ones, by B. linens, which are thought to be important in respect to the flavor of smear surface-ripened cheeses. The unique orange-colored carotenoids and the factors effecting their production by B. linens are also presented. The catabolism of aromatic amino acids, bacteriocin production, plasmids, and miscellaneous biochemical and physiological properties (peptidoglycan type, antibiotic resistance, insecticide degradation, and biotechnological applications) of B. linens are discussed. The problem associated with the current taxonomical classification of B. linens strains caused by strain variation is evaluated. Finally, the application of B. linens cell extracts or its proteolytic enzymes as cheese ripening accelerants for semi-hard or hard cheese varieties is considered.

Behavior of Listeria monocytogenes during manufacture and ripening of brick cheese

Brick cheese was made by the washed-curd procedure from pasteurized whole milk inoculated to contain ca. 1 x 10(2) to 1 x 10(3) Listeria monocytogenes [strain Scott A, Ohio, V7, or California]/ml. Cheeses were ripened (15 degrees C/95% relative humidity) with a surface smear for 2, 3, or 4 wk to simulate production of mild, aged, or "Limburger-like" brick cheese, respectively, and then stored an additional 20 to 22 wk at 10 degrees C. Populations of strains Scott A, Ohio, V7, and California increased 1.89, 1.72, .83, and .86 orders of magnitude, respectively, following completion of brining ca. 32 h after the start of cheese making. All four L. monocytogenes strains leached from cheese into brine during 24 h and survived in brine at 10 degrees C at least 5 d after removal of cheese. Strains Scott A and Ohio grew rapidly during the initial 2 wk of smear development and attained maximum populations of ca. 6.6 and 6.2, 7.0 and 6.9, and 5.6 and 5.1 log10/g in 4-wk-old slice (pH 6.0 to 6.5), surface (pH 6.5 to 6.9), and interior (pH 5.6 to 6.2) samples of cheese, respectively. Numbers of strains Scott A and Ohio generally decreased 1- to 7-fold during 20 to 22 wk at 10 degrees C. Strains V7 and California failed to grow appreciably in any cheese during or after smear development, despite pH of 6.8 to 7.4 in fully ripened cheese; the strains were never isolated from 2- and 3-wk-old cheese and with direct plating were detected sporadically at levels generally less than or equal to 4.0 log10/g in cheese aged greater than or equal to 4 wk. Cold enrichment of slice, surface, and interior samples of cheeses aged greater than or equal to 4 wk generally yielded positive results for L. monocytogenes; strains V7 and California were detected in all cheeses after 20 to 22 wk at 10 degrees C. At 10 ppm, methyl sulfide, dimethyl disulfide, or methyl trisulfide (compounds commonly produced during ripening of brick and Limburger cheese) failed to inhibit appreciably growth of L. monocytogenes.