Identification and quantification of sophorolipid analogs using ultra-fast liquid chromatography–mass spectrometry

An ultra-fast liquid chromatographic method combined with atmospheric pressure chemical ionization mass detection (UHPLC/APCI-MS) has been developed for the separation and quantification of sophorolipid analogs produced by the yeast Candida bombicola. The sophorolipid mixture was produced by growing the yeast in the presence of glucose and oleic acid under higher aeration. It was found that more than 95% of the analogs are lactonic sophorolipids and all the produced sophorolipids produced were either mono- or di-acetylated. Also observed was a sophorolipid analog with a tri-unsaturated fatty acid, which has not been reported previously.     

Effect of emodin on <Emphasis Type="Italic">Candida albicans</Emphasis> growth investigated by microcalorimetry combined with chemometric analysis

Using the 3114/3115 thermal activity monitor (TAM) air isothermal microcalorimeter, ampoule mode, the heat output of Candida albicans growth at 37°C was measured, and the effect of emodin on C. albicans growth was evaluated by microcalorimetry coupled with chemometric methods. The similarities between the heat flow power (HFP)–time curves of C. albicans growth affected by different concentrations of emodin were calculated by similarity analysis (SA). In the correspondence analysis (CA) diagram of eight quantitative parameters taken from the HFP–time curves, it could be deduced that emodin had definite dose-effect relationship as the distance between different concentrations of it increased along with the dosage and the effect. From the principal component analysis (PCA) on eight quantitative parameters, the action of emodin on C. albicans growth could be easily evaluated by analyzing the change of values of the main two parameters, growth rate constant k ₂ and maximum power output . The coherent results of SA, CA, and PCA showed that emodin at different concentrations had different effects on C. albicans growth metabolism: A low concentration (0–10 μg ml⁻¹) poorly inhibited the growth of C. albicans, and a high concentration (15–35 μg ml⁻¹) could notably inhibit growth of this fungus. This work provided a useful idea of the combination of microcalorimetry and chemometric analysis for investigating the effect of drug and other compounds on microbes.     

Synthesis and characterization of cyclodextrin-based polymers as a support for immobilization of Candida rugosa lipase

The objective of this study was to prepare cross-linked β-cyclodextrin polymers for immobilization of Candida rugosa lipase. The structures of synthesized macrocyclic compounds were characterized by Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA) and scanning electron microscope (SEM) techniques. Properties of the immobilized systems were assessed and their performance on hydrolytic reaction were evaluated and compared with the free enzyme. The influence of activation agents (glutaraldehyde (GA) and hexamethylene diisocyanate (HMDI)) and thermal and pH stabilities of the biocatalyst was evaluated. After the optimization of immobilization process, the physical and chemical characterization of immobilized lipase was performed. Obtained data showed that the immobilized enzyme seemed better and offered some advantages in comparison with free enzyme. It can be observed that the free lipase loses its initial activity within around 80 min at 60 °C, while the immobilized lipases retain their initial activities of about 56% by HMDI and 82% by GA after 120 min of heat treatment at 60 °C.Results showed that the specific activity of the immobilized lipase with glutaraldehyde was 62.75 U/mg protein, which is 28.13 times higher than that of the immobilized lipase with HMDI.     

Molecular modeling of substrate selectivity of Candida antarctica lipase B and Candida rugosa lipase towards c9, t11- and t10, c12-conjugated linoleic acid

Molecular modeling was used to clarify the mechanism of the selectivity of Candida antarctica lipase B and Candida rugosa lipase towards cis9, trans11 (c9, t11-) and trans10, cis12 (t10, c12-) conjugated linoleic acid. Hydrogen bonds network, substrate conformation, binding affinity and water molecules in the binding site were analyzed. Substrate conformation and binding affinity were not correlated with the experimental results of the substrate selectivity. On the contrary, better enzyme preference towards a substrate was correlated with two stronger hydrogen bonds (His-NH-O_a and His-NH-Ser-O_γ) and less water molecules between the substrate the binding pocket. Possible explanation of these was discussed.     

Enhancement of xylitol productivity and yield using a xylitol dehydrogenase gene-disrupted mutant of Candida tropicalis under fully aerobic conditions

The effects of glycerol and the oxygen transfer rate on the xylitol production rate by a xylitol dehydrogenase gene (XYL2)-disrupted mutant of Candida tropicalis were investigated. The mutant produced xylitol near the almost yield of 100% from d-xylose using glycerol as a co-substrate for cell growth and NADPH regeneration: 50 g d-xylose l⁻¹ was completely converted into xylitol when at least 20 g glycerol l⁻¹ was used as a co-substrate. The xylitol production rate increased with the O₂ transfer rate until saturation and it was not necessary to control the dissolved O₂ tension precisely. Under the optimum conditions, the volumetric productivity and xylitol yield were 3.2 g l⁻¹ h⁻¹ and 97% (w/w), respectively.     

Life within a community: benefit to yeast long-term survival

Traditionally, living organisms have often been classified into two main categories: unicellular and multicellular. In recent years, however, the boundary between these two groups has become less strict and clear than was previously presumed. Studies on the communities formed by unicellular microorganisms have revealed that various properties and processes so far mainly associated with metazoa are also important for the proper development, survival and behaviour of muticellular microbial populations. In this review, we present various examples of this, using a yeast colony as representative of a structured organized microbial community. Among other things, we will show how the differentiation of yeast cells within a colony can be important for the long-term survival of a community under conditions of nutrient shortage, how colony development and physiology can be influenced by the environment, and how a group of colonies can synchronize their developmental changes. In the last section, we introduce examples of molecular mechanisms that can participate in some aspects of the behaviour of yeast populations.     

Metal resistance in Candida biofilms

Yeasts are often successful in metal-polluted environments; therefore, the ability of biofilm and planktonic cell Candida tropicalis to endure metal toxicity was investigated. Fifteen water-soluble metal ions, chosen to represent groups 6A to 6B of the periodic table, were tested against this organism. With in vitro exposures as long as 24 h, biofilms were up to 65 times more tolerant to killing by metals than corresponding planktonic cultures. Of the most toxic heavy metals tested, only very high concentrations of Hg2+, CrO4 (2-) or Cu2+ killed surface-adherent Candida. Metal-chelator precipitates could be formed in biofilms following exposure to the heavy metals Cu2+ and Ni2+. This suggests that Candida biofilms may adsorb metal cations from their surroundings and that sequestration in the extracellular matrix may contribute to resistance. We concluded that biofilm formation may be a strategy for metal resistance and/or tolerance in yeasts.     

Ultrastructural and physico-chemical heterogeneities of yeast surfaces revealed by mapping lateral-friction and normal-adhesion forces using an atomic force microscope

Scanning force microscopy has been used to probe the surface of the emerging pathogenic yeast Candida parapsilosis, in order to get insight into its surface structure and properties at submicrometer scales. AFM friction images eventually show patches with a very strong contrast, showing high lateral interaction with the tip. Adhesion force measurement also reveals a high normal interaction with the tip, and patches show extraordinarily high pull off values. The tip eventually sticks completely at the center of the patches. While an extraordinarily high interaction is measured by the tip at those zones, topographic images show extraordinarily flat topography over those zones, both of which characteristics are consistent with a liquid-like area. High resolution friction images show those zones to be surrounded by microfibrillar structures, concentrically oriented, of a mean width of about 25 nm, structures that become progressively less defined as we move away from the center of the patches. No structure can be appreciated inside the zones of maximum contrast. Also some helical or ribbon-like structure can be resolved from friction images. There is not only an ordered disposition of the microfibrillar structures, but also the adhesion force increases radially in the direction towards the center of the patches. These structures responsible for the high adhesion are thought to be incipient-emerging budding zones. Microfibrillar structures are thought to represent the first steps of chitin biosynthesis and cell wall digestion, with chitin polymers being biosynthesized, associated with other macromolecules of the yeast cell wall. They can be also beta glucan helical structures, made visible in the zone of yeast division due to the action of autolysins. The observed gradient in surface adhesion and elastic properties correlates well with that expected from a biochemical point of view. The higher adhesion force measured could be either due to the different macromolecular nature of the patches, or to a mechanical adhesion effect due to the different plasticity of that zone. This work reveals the importance of taking into account the dynamic nature of the cell wall physico-chemical properties. Processes related to the normal cell-cycle, as division, can strongly alter the surface morphology and physico-chemical properties and cause important heterogeneities that might have a profound impact on the adhesion behavior of a single cell, which could not be detected by more macroscopic methods.