Hubert Verachtert

Isolation of Adherent Polycyclic Aromatic Hydrocarbon (PAH)-Degrading Bacteria Using PAH-Sorbing Carriers

ABSTRACT Two different procedures were compared to isolate polycyclic aromatic hydrocarbon (PAH)-utilizing bacteria from PAH-contaminated soil and sludge samples, i.e., (i) shaken enrichment cultures in liquid mineral medium in which PAHs were supplied as crystals and (ii) a new method in which PAH degraders were enriched on and recovered from hydrophobic membranes containing sorbed PAHs. Both techniques were successful, but selected from the same source different bacterial strains able to grow on PAHs as the sole source of carbon and energy. The liquid enrichment mainly selected for Sphingomonasspp., whereas the membrane method exclusively led to the selection ofMycobacterium spp. Furthermore, in separate membrane enrichment set-ups with different membrane types, three repetitive extragenic palindromic PCR-related Mycobacterium strains were recovered. The new Mycobacterium isolates were strongly hydrophobic and displayed the capacity to adhere strongly to different surfaces. One strain, Mycobacterium sp. LB501T, displayed an unusual combination of high adhesion efficiency and an extremely high negative charge. This strain may represent a new bacterial species as suggested by 16S rRNA gene sequence analysis. These results indicate that the provision of hydrophobic sorbents containing sorbed PAHs in the enrichment procedure discriminated in favor of certain bacterial characteristics. The new isolation method is appropriate to select for adherent PAH-degrading bacteria, which might be useful to biodegrade sorbed PAHs in soils and sludge.

Yeast flocculation: what brewers should know.

For many industrial applications in which the yeast Saccharomyces cerevisiae is used, e.g. beer, wine and alcohol production, appropriate flocculation behaviour is certainly one of the most important characteristics of a good production strain. Yeast flocculation is a very complex process that depends on the expression of specific flocculation genes such as FLO1, FLO5, FLO8 and FLO11. The transcriptional activity of the flocculation genes is influenced by the nutritional status of the yeast cells as well as other stress factors. Flocculation is also controlled by factors that affect cell wall composition or morphology. This implies that, during industrial fermentation processes, flocculation is affected by numerous parameters such as nutrient conditions, dissolved oxygen, pH, fermentation temperature, and yeast handling and storage conditions. Theoretically, rational use of these parameters offers the possibility of gaining control over the flocculation process. However, flocculation is a very strain-specific phenomenon, making it difficult to predict specific responses. In addition, certain genes involved in flocculation are extremely variable, causing frequent changes in the flocculation profile of some strains. Therefore, both a profound knowledge of flocculation theory as well as close monitoring and characterisation of the production strain are essential in order to gain maximal control over flocculation. In this review, the various parameters that influence flocculation in real-scale brewing are critically discussed. However, many of the conclusions will also be useful in various other industrial processes where control over yeast flocculation is desirable.

Biodegradation and toxicity of benzothiazoles

Abstract Benzothiazoles appear in the environment mainly as a result of their production and use as rubber vulcanization accelerators. This review focuses on the potential environmental hazard these substances pose and their biological removal from and transformation in aquatic and terrestrial ecosystems. Benzothiazole degradation by mixed and pure cultures and the toxic impact on microorganisms are discussed. It is concluded that biological purification of wastewaters from 2-mercaptobenzothiazole (MBT) production can be achieved, provided the MBT influent concentration is 100 mg/l at maximum. So far, no attempts have been made to study the molecular mechanisms of benzothiazole degradation, mainly due to the unavailability of axenic cultures capable of benzothiazole breakdown. However, the toxic effects of MBT towards microorganisms have been studied and can be attributed to its metal chelating properties and/or its interference with membrane-bound (co)enzymes in particular.

Bioflavoring and beer refermentation

Various techniques are used to adjust the flavors of foods and beverages to new market demands. Although synthetic flavoring chemicals are still widely used, flavors produced by biological methods (bioflavors) are now more and more requested by consumers, increasingly concerned with health and environmental problems caused by synthetic chemicals. Bioflavors can be extracted from plants or produced with plant cell cultures, microorganisms or isolated enzymes. This Mini-Review paper gives an overview of different systems for the microbial production of natural flavors, either de novo, or starting with selected flavor precursor molecules. Emphasis is put on the bioflavoring of beer and the possibilities offered by beer refermentation processes. The use of flavor precursors in combination with non-conventional or genetically modified yeasts for the production of new products is discussed.

Comparative study of starch degradation and amylase production by ascomycetous yeast species

Summary A large number of starch-assimilating yeast species belonging to the genera Brettanomyces, Bullera, Candida, Cryptococcus, Filobasidium, Leucosporidium, Phaffia, Rhodasporidium, Rhodotorula, Sporobolomyces, Sterigmatomyces, Torulopsis and Trichosporon , were studied. Their ability to degrade different starch preparations and the concomitant release of amylolytic enzymes into the medium were compared quantitatively. Limited hydrolysis of the polysaccharide substrates characterized most of the strains tested. However, significant differences in amylase secretion occurred. Highly active amylolytic species of the genera Candida, Cryptococcus and Trichosporon assimilating both soluble and unmodified starch types, are described for the first time. High levels of extracellular amylolytic activity were detected in culture supernatants of these species, except for Cryptococcus flavus . Using a gel diffusion technique with a medium containing Phadebas-substrate, α-amylase activity was demonstrated in crude enzyme preparations of Cryptococcus flavus , Candida homilentoma, C. silvanorum, C. tsukubaensis and Trichosporon pullulans .

Brettanomyces yeasts--From spoilage organisms to valuable contributors to industrial fermentations.

Ever since the introduction of controlled fermentation processes, alcoholic fermentations and Saccharomyces cerevisiae starter cultures proved to be a match made in heaven. The ability of S. cerevisiae to produce and withstand high ethanol concentrations, its pleasant flavour profile and the absence of health-threatening toxin production are only a few of the features that make it the ideal alcoholic fermentation organism. However, in certain conditions or for certain specific fermentation processes, the physiological boundaries of this species limit its applicability. Therefore, there is currently a strong interest in non-Saccharomyces (or non-conventional) yeasts with peculiar features able to replace or accompany S. cerevisiae in specific industrial fermentations. Brettanomyces (teleomorph: Dekkera), with Brettanomyces bruxellensis as the most commonly encountered representative, is such a yeast. Whilst currently mainly considered a spoilage organism responsible for off-flavour production in wine, cider or dairy products, an increasing number of authors report that in some cases, these yeasts can add beneficial (or at least interesting) aromas that increase the flavour complexity of fermented beverages, such as specialty beers. Moreover, its intriguing physiology, with its exceptional stress tolerance and peculiar carbon- and nitrogen metabolism, holds great potential for the production of bioethanol in continuous fermentors. This review summarizes the most notable metabolic features of Brettanomyces, briefly highlights recent insights in its genetic and genomic characteristics and discusses its applications in industrial fermentation processes, such as the production of beer, wine and bioethanol.

Screening and evaluation of the glucoside hydrolase activity in Saccharomyces and Brettanomyces brewing yeasts

Aims:  The aim of this study was to select and examine Saccharomyces and Brettanomyces brewing yeasts for hydrolase activity towards glycosidically bound volatile compounds.

The effect of nutrient supplementation on the biofiltration removal of butanal in contaminated air

Butanal is one of the odorous compounds produced in the animal-rendering and food-processing industries and also in sewage-treatment plants. It shows the necessity for complementing such plants with systems for off-gas treatment. Biofiltration using simple packing material was tested for the removal of butanal. Excellent results were obtained when the filters operated at optimal humidity and were supplemented with inorganic nutrients. Without nutrients, butyric acid was detected in the effluent gas, which may explain the lower efficiency of filters without nutrients. Under optimal conditions an elimination of around 90 g·m−3·h−1 was reached.

Determination of by-products formed during the ethanolic fermentation, using batch and immobilized cell systems of Zymomonas mobilis and Saccharomyces bayanus

SummaryRecent research has indicated that the bacterium Zymomonas mobilis might be an interesting alternative for the production of ethanol from sugar, when compared to Saccharomyces. Especially in immobilized cell systems the bacterium showed a higher productivity. One of the factors affecting productivity might be a lower efficiency of glucose to ethanol conversion. Therefore the amounts and the nature of the by-products of the ethanolic yeast and bacterial fermentations were studied using batch systems and immobilized cell systems. Glycerol, succinate, acetate, lactate, acetoin, and butanediol syntheses were studied. It was found that Zymomonas produced fewer by-products than Saccharomyces and that the by-product profile was different. Glycerol was a characteristic yeast product while lactate was typical for the bacterium. There results were verified for the batch systems as well as for the immobilized systems. Mixed cultures of yeast and bacterium resulted in the highest productivity using immobilized systems. The system using small stones coated with agar and gelatin was proved to be superior to a previous system using carrageenan gel beads. The higher productivity shown by the bacterium cannot be explained, however, only by the shifting of less sugar to by-product synthesis.

Microbial transformations of 2-substituted benzothiazoles.

Abstract. The occurrence of benzothiazoles in the environment seems to be restricted to aquatic compartments and is mainly associated with the manufacture and use of the rubber additive 2-mercaptobenzothiazole (MBT) and its derivatives. Although data on benzothiazole biotransformations in natural environments at ppb and ppt levels are scarce, the unsubstituted benzothiazole (BT) and 2-hydroxybenzothiazole (OBT) are generally considered to be biodegradable, whereas 2-methylthiobenzothiazole is recalcitrant. The fungicide 2-thiocyanomethylthiobenzothiazole is assumed to be hydrolysed to MBT, which is then further methylated. At higher concentration levels, similar conclusions can generally be drawn. In addition, BT, MBT, 2-aminobenzothiazole and benzothiazole-2-sulphonate can be biodegraded, although side- and end-products may form. For BT and MBT, threshold concentration were reported above which inhibitory effects on biological treatment processes occur. Due to the limited availability of axenic bacterial cultures capable of benzothiazole mineralization, only the initial steps of the degradation pathways have been elucidated so far.