Measurement of the partial pressure of dissolved CO2 for estimating the concentration of Streptococcus thermophilus in coculture with Lactobacillus bulgaricus

Summary The use of mixed cultures in the food industry is hindered by the lack of rapid and specific measurement techniques. The coculture of Streptococcus thermophilus and Lactobacillus bulgaricus, used for producing starters for yoghurt production, is a simple model of a mixed culture.After verifying that Streptococcus thermophilus specifically degraded urea, we attempted to correlate the cell concentration of the species first with urease activity and then with the rate of CO₂ production from urea in the medium. The measurement was performed in real time with a specific electrode for dissolved CO₂. The results obtained with the method have the same uncertainy as those obtained by cell counts. The estimation is valid between 10⁷ and 10⁹ cells/ml.     

Four-year growth dynamics of beech-spruce model ecosystems under CO2 enrichment on two different forest soils

To elucidate how atmospheric CO₂ enrichment, enhanced nutrient supply and soil quality interact to affect regrowth of temperate forests, young Fagus sylvatica and Picea abies trees were grown together in large model ecosystems. Identical communities were established on a nutrient-poor acidic and on a more fertile calcareous soil and tree growth, leaf area index, fine root density and soil respiration monitored over four complete growing seasons. Biomass responses to CO₂ enrichment and enhanced N supply at the end of the experiment reflected compound interest effects of growth stimulation during the first two to three seasons rather than persistent stimulation over the whole duration of the experiment. Whereas biomass of Picea was enhanced in elevated CO₂ on both soils, Fagus responded negatively to CO₂ on acidic but positively on calcareous soil. Biomass of both species profited from enhanced N supply on the poor acidic soil only. Leaf area index on both soils was greater in high N supply as a consequence of a stimulation early in the experiment, but was unaffected by CO₂ enrichment. Fine root density on acidic soil was increased in high N supply, but this did not stimulate soil respiration rate. In contrast, elevated CO₂ stimulated both fine root density and soil CO₂ efflux on calcareous soil, especially towards the end of the experiment. Our experiment suggests that future species dominance in beech-spruce forests is likely to change in response to CO₂ enrichment, but this response is subject to complex interactions with environmental factors other than CO₂, particularly soil type.     

Effects of Proteus vulgaris growth on the establishment of a cheese microbial community and on the production of volatile aroma compounds in a model cheese

Aims:  To investigate the impact of Proteus vulgaris growth on a multispecies ecosystem and on volatile aroma compound production during cheese ripening.
Methods and Results:  The microbial community dynamics and the production of volatile aroma compounds of a nine-species cheese ecosystem were compared with or without the presence of P. vulgaris in the initial inoculum. Proteus vulgaris was able to colonize the cheese surface and it was one of the dominant species, representing 37% of total isolates at the end of ripening with counts of 9·2 log₁₀ CFU g⁻¹. In the presence of P. vulgaris , counts of Arthrobacter arilaitensis , Brevibacterium aurantiacum and Hafnia alvei significantly decreased. Proteus vulgaris influenced the production of total volatile aroma compounds with branched-chain aldehydes and their corresponding alcohols being most abundant.
Conclusions:  Proteus vulgaris was able to successfully implant itself in a complex cheese ecosystem and significantly contributed to the organoleptic properties of cheese during ripening. This bacterium also interacted negatively with other bacteria in the ecosystem studied.
Significance and Impact of the Study:  This is the first time that the impact of a Gram-negative bacterium on cheese microbial ecology and functionality has been described.     

Soil moisture dynamics of calcareous grassland under elevated CO2

Water relations of nutrient-poor calcareous grassland under long-term CO₂ enrichment were investigated. Understanding CO₂ effects on soil moisture is critical because productivity in these grasslands is water limited. In general, leaf conductance was reduced at elevated CO₂, but responses strongly depended on date and species. Evapotranspiration (measured as H₂O gas exchange) revealed only small, non-significant reductions at elevated CO₂, indicating that leaf conductance effects were strongly buffered by leaf boundary layer and canopy conductance (leaf area index was not or only marginally increased under elevated CO₂). However, these minute and non-significant responses of water vapour loss accumulated over time and resulted in significantly higher soil moisture in CO₂-enriched plots (gravimetric spot measurements and continuous readings using a network of time-domain reflectometry probes). Differences strongly depended on date, with the smallest effects when soil moisture was very high (after heavy precipitation) and effects were largest at intermediate soil moisture. Elevated CO₂ also affected diurnal soil moisture courses and rewetting of soils after precipitation. We conclude that ecosystem-level controls of the water balance (including soil feedbacks) overshadow by far the physiological effects observed at the leaf level. Indirect effects of CO₂ enrichment mediated by trends in soil moisture will have far-ranging consequences on plant species composition, soil bacterial and faunal activity as well as on soil physical structure and may indirectly also affect hydrology and trace gas emissions and atmospheric chemistry. Received: 21 December 1997 / Accepted: 3 August 1998     

Trapping of γ-Decalactone by Adsorption on Hydrophobic Sorbents : Application to the bioconversion of methyl ricinoleate by the yeast Sporidiobolus salmonicolor

Trapping by adsorption on hydrophobic porous polymers was used to selectively remove -decalactone from the complex bioconversion medium. Several sorbents were tested : activated carbon, and three porous polystyrene-type polymers (Porapak Q, Chromosorb 105, and Resin SM4). The sorption isotherms were determined in water and in a model bioconversion medium without microorganisms. In the last part of the study, the application to the production of -decalactone by Sporidiobolus salmonicolor was carried out. Hydrophobic porous adsorbents are compatible with the bioconversion and protect the yeast from the toxic effect of -decalactone.     

Dual influence of the carbon source and <Emphasis Type="SmallCaps">l</Emphasis>-methionine on the synthesis of sulphur compounds in the cheese-ripening yeast<Emphasis Type="Italic"> Geotrichum candidum</Emphasis>

The effect of the carbon source and l-methionine on the ability of Geotrichum candidum to produce volatile sulphur compounds (VSC) was studied. This yeast was cultivated in a synthetic medium supplemented with various carbon sources and l-methionine at different concentrations. Both glycerol and glucose significantly increased VSC production by G. candidum. Unlike the effect on the l-methionine- and 4-methylthio-2-oxobutyric acid-demethiolating activities, the supply of a carbon source had a dramatic effect on the activity of aminotransferase, a key enzyme in l-methionine catabolism. An increase in the initial concentration of l-methionine resulted in a rise in the production of sulphur compounds (VSC, 4-methylthio-2-oxobutyric acid) but had limited effect on l-methionine-catabolising enzyme activities. Evidence for the existence of a dual effect of the carbon source and l-methionine on VSC biosynthesis was obtained in this study.     

Production of volatile compounds by cheese-ripening yeasts: requirement for a methanethiol donor for S-methyl thioacetate synthesis by Kluyveromyces lactis

Five cheese-ripening yeasts (Geotrichum candidum, Saccharomyces cerevisiae, Kluyveromyces lactis, Yarrowia lipolytica and Debaryomyces hansenii) were compared with respect to their ability to generate volatile aroma compounds. K. lactis produced a variety of esters - ethylacetate (EA) being the major one - and relatively limited amounts of volatile sulphur compounds (VSCs). Conversely, G. candidum produced significant amounts of VSCs [with the thioester S-methyl thioacetate (MTA) being the most prevalent] and lower quantities of non-sulphur volatile compounds than K. lactis. We suspect that K. lactis is able to produce and/or accumulate acetyl CoA - a common precursor of MTA and EA - but that it produces limited amounts of methanethiol (MTL); both acetyl CoA and MTL are precursors for MTA synthesis. When supplemented with exogenous MTL, MTA production greatly increased in K. lactis cultures whereas it was unchanged in G. candidum cultures, suggesting that MTL is a limiting factor for MTA synthesis in K. lactis but not in G. candidum. Our results are discussed with respect to L-methionine catabolism.     

Production of halogenated compounds byBjerkandera adusta

The white-rot fungusBjerkandera adusta produces volatile chlorinated phenyl compounds. The main compounds identified were 3-chloro-4-methoxybenzaldehyde (3-chloro-p-anisaldehyde), 3-chloro-4-methoxybenzyl alcohol (3-chloro-p-anisyl alcohol), 3,5-dichloro-4-methoxybenzaldehyde (3,5-dichloro-p-anisaldehyde), and 3,5-dichloro, 4-methoxybenzyl alcohol (3,5-dichloro-p-anisyl alcohol).p-Anisaldehyde, veratraldehyde and the corresponding alcohols,p-anisyl alcohol and veratryl alcohol were produced simultaneously. Even with a very low concentration of chloride in the medium (< 10^–5 m), chlorinated aromatic compounds were still observed. Addition of bromide to the culture medium led to the production of brominated compounds: 3-bromo-4-methoxybenzaldehyde, 3-bromo-4-methoxybenzyl alcohol, 3,5-dibromo-4-methoxybenzaldehyde and 3-bromo-5-chloro-4-methoxybenzaldehyde. These brominated compounds have not previously been reported as natural products. Although iodo-aromatic compounds were not produced by supplementation of the medium with iodide, isovanillin was found in the culture broth under these conditions. This compound may be formed by substitution of the iodine intermediate by a hydroxyl group on the third carbon of the ring. Diiodomethane or chloroiodomethane were also found. It is the first time that the production of halomethane has been related to the production of halogenated aromatic compounds. All the strains tested have these capabilities.     

The arthrobacter arilaitensis Re117 genome sequence reveals its genetic adaptation to the surface of cheese

Arthrobacter arilaitensis is one of the major bacterial species found at the surface of cheeses, especially in smear-ripened cheeses, where it contributes to the typical colour, flavour and texture properties of the final product. The A. arilaitensis Re117 genome is composed of a 3,859,257 bp chromosome and two plasmids of 50,407 and 8,528 bp. The chromosome shares large regions of synteny with the chromosomes of three environmental Arthrobacter strains for which genome sequences are available: A. aurescens TC1, A. chlorophenolicus A6 and Arthrobacter sp. FB24. In contrast however, 4.92% of the A. arilaitensis chromosome is composed of ISs elements, a portion that is at least 15 fold higher than for the other Arthrobacter strains. Comparative genomic analyses reveal an extensive loss of genes associated with catabolic activities, presumably as a result of adaptation to the properties of the cheese surface habitat. Like the environmental Arthrobacter strains, A. arilaitensis Re117 is well-equipped with enzymes required for the catabolism of major carbon substrates present at cheese surfaces such as fatty acids, amino acids and lactic acid. However, A. arilaitensis has several specificities which seem to be linked to its adaptation to its particular niche. These include the ability to catabolize D-galactonate, a high number of glycine betaine and related osmolyte transporters, two siderophore biosynthesis gene clusters and a high number of Fe(3+)/siderophore transport systems. In model cheese experiments, addition of small amounts of iron strongly stimulated the growth of A. arilaitensis, indicating that cheese is a highly iron-restricted medium. We suggest that there is a strong selective pressure at the surface of cheese for strains with efficient iron acquisition and salt-tolerance systems together with abilities to catabolize substrates such as lactic acid, lipids and amino acids.     

Analysis of amino acid requirements ofClostridium thermocellum

Summary Clostridium thermocellum is a species able to convert cellulose to ethanol on complex medium. A systematic study of changes in the concentration of amino acids in the medium during growth of the bacterium has permitted us to obtain a chemically defined medium more effective than that hitherto used and to obtain some information about the use of amino acids by this bacterium.