Marta Goretti

Antimicrobial and antiviral activity of hydrolysable tannins.

Hydrolysable tannins (HTs), secondary metabolites widely distributed in the plant kingdom, are generally multiple esters of gallic acid with glucose. HTs have been shown to be effective antagonists against viruses, bacteria and eukaryotic microorganisms. The present review examines the antimicrobial and antiviral activity of HTs, the mechanism(s) of action, and some structure-activity relationships.

Psychrophilic yeasts from worldwide glacial habitats: diversity, adaptation strategies and biotechnological potential

Glacial habitats (cryosphere) include some of the largest unexplored and extreme biospheres on Earth. These habitats harbor a wide diversity of psychrophilic prokaryotic and eukaryotic microorganisms. These highly specialized microorganisms have developed adaptation strategies to overcome the direct and indirect life-endangering influence of low temperatures. For many years Antarctica has been the geographic area preferred by microbiologists for studying the diversity of psychrophilic microorganisms (including yeasts). However, there have been an increasing number of studies on psychrophilic yeasts sharing the non-Antarctic cryosphere. The present paper provides an overview of the distribution and adaptation strategies of psychrophilic yeasts worldwide. Attention is also focused on their biotechnological potential, especially on their exploitation as a source of cold-active enzymes and for bioremediation purposes.

In vitro antimycotic activity of a Williopsis saturnus killer protein against food spoilage yeasts

The in vitro antimycotic activity of a purified killer protein (KT4561) secreted by a strain of Williopsis saturnus was tested against 310 yeast strains belonging to 21 food spoilage species of 14 genera (Candida, Debaryomyces, Dekkera, Hanseniaspora, Issatchenkia, Kazachstania, Kluyveromyces, Pichia, Rhodotorula, Saccharomyces, Schizosaccharomyces, Torulaspora, Yarrowia and Zygosaccharomyces). Minimum inhibitory concentration (MIC) determinations showed that over 65% of the target strains were susceptible to concentrations < or = 32 microg/ml of KT4561. Three conventional food-grade antimicrobial agents were used as controls: 41, 33 and 40% of the target strains were sensitive to < or = 512 mg/ml of ethyl 3-hydroxybenzoate (E214), potassium sorbate (E202) or potassium metabisulphite (E224), respectively. The susceptibility of food spoilage yeasts towards KT4561, E214, E202 and E224 was species- and strain-dependent. In most cases KT4561 exhibited MIC values several orders of magnitude lower (100 to 100,000 times) than those observed for E214, E202 and E224. With only a few exceptions, the activity of KT4561 was pH-, ethanol-, glucose- and NaCl-independent. The present study demonstrates the potential of this yeast killer protein as a novel and natural control agent against food spoilage yeasts.

Molecular characterization of Prototheca strains isolated from Italian dairy herds

One hundred sixty-one Prototheca spp. strains isolated from composite milk and barn-surrounding environmental samples (bedding, feces, drinking, or washing water, surface swabs) of 24 Italian dairy herds were characterized by genotype-specific PCR analysis. Overall, 97.2% of strains isolated from composite milk samples were characterized as Prototheca zopfii genotype 2, confirming its role as the main mastitis pathogen, whereas Prototheca blaschkeae was only sporadically isolated (2.8%). Regarding environmental sampling, 84.9% of isolates belonged to P. zopfii genotype 2, 13.2% to P. blaschkeae, and 1.9% to P. zopfii genotype 1. The data herein contradict previous hypotheses about the supposed exclusive role of P. zopfii genotype 2 as the causative agent of protothecal mastitis and, on the contrary, confirm the hypothesis that such pathology could be caused by P. blaschkeae in a few instances.

Influence of abiotic variables on culturable yeast diversity in two distinct Alpine glaciers

The influence of some abiotic variables (pH, dry weight, organic carbon, nitrogen and phosphorous) on culturable yeast diversity in two distinct, but adjacent Alpine glaciers (Glacier du Géant, France, and Miage Glacier, Italy) was investigated. In all, 682 yeast strains were isolated and identified by D1/D2 and ITS sequencing as belonging to species of the genera Aureobasidium, Candida, Bulleromyces, Cryptococcus, Cystofilobasidium, Dioszegia, Guehomyces, Holtermanniella, Leucosporidiella, Mrakia, Mrakiella, Rhodotorula, Sporidiobolus, Sporobolomyces and Udenyomyces. Overall, the most represented genera were Cryptococcus (55% of isolates), Rhodotorula (17%) and Mrakia (10%). About 10% of strains, presumably belonging to new species (yet to be described), were preliminarily identified at the genus level. Principal component analysis (PCA) revealed that organic carbon, nitrogen and phosphorous are apparently mostly related to culturable yeast abundance and diversity. In this context, the hypothesis that the frequency of isolation of certain species may be correlated with some organic nutrients (with special emphasis for phosphorous) is discussed.

Biotransformation of Acyclic Monoterpenoids by Debaryomyces sp., Kluyveromyces sp., and Pichia sp. Strains of Environmental Origin

Sixty yeast strains, which belong to 32 species of the genera Debaryomyces, Kluyveromyces, and Pichia, and which were isolated from plant‐, soil‐ or insect‐associated habitats, were screened for their ability to biotransform the acyclic monoterpenes geraniol and nerol. The aptitude to convert both compounds (from 2.6 to 30.6, and from 2.7 to 29.1%/g cell DW (=dry weight), resp.) was apparently a broad distributed character in such yeasts. Depending upon the substrate used, the production of linalool, α‐terpineol, β‐myrcene, D‐limonene, (E)‐β‐ocimene, (Z)‐β‐ocimene, or carene was observed. Linalool was the main product obtained from geraniol, whereas linalool and α‐terpineol were the main products obtained through the conversion of nerol. Yet, differently from nerol, the aptitude to exhibit high bioconversion yields of geraniol to linalool was an apparently genus‐related character, whereas the ability to produce other monoterpenes was a both genus‐ and habitat‐related character. The possible pathways of bioconversion of geraniol or nerol to their derivatives were proposed/discussed.

Production of Flavours and Fragrances via Bioreduction of (4R)-(-)-Carvone and (1R)-(-)-Myrtenal by Non-Conventional Yeast Whole-Cells

As part of a program aiming at the selection of yeast strains which might be of interest as sources of natural flavours and fragrances, the bioreduction of (4R)-(−)-carvone and (1R)-(−)-myrtenal by whole-cells of non-conventional yeasts (NCYs) belonging to the genera Candida, Cryptococcus, Debaryomyces, Hanseniaspora, Kazachstania, Kluyveromyces, Lindnera, Nakaseomyces, Vanderwaltozyma and Wickerhamomyces was studied. Volatiles produced were sampled by means of headspace solid-phase microextraction (SPME) and the compounds were analysed and identified by gas chromatography–mass spectroscopy (GC-MS). Yields (expressed as % of biotransformation) varied in dependence of the strain. The reduction of both (4R)-(−)-carvone and (1R)-(−)-myrtenal were catalyzed by some ene-reductases (ERs) and/or carbonyl reductases (CRs), which determined the formation of (1R,4R)-dihydrocarvone and (1R)-myrtenol respectively, as main flavouring products. The potential of NCYs as novel whole-cell biocatalysts for selective biotransformation of electron-poor alkenes for producing flavours and fragrances of industrial interest is discussed.

Large-scale screening of the in vitro susceptibility of Prototheca zopfii towards polyene antibiotics

A large scale screening of the in vitro susceptibility of 105 strains of Prototheca zopfii to a panel of polyene antibiotics (amphotericin B, nystatin, primaricin and filipin) was conducted. Strains studied were isolated from dairy-associated environments in five different localities. Groups 1-4 included strains recovered from four separate regions of Italy, while group 5 included isolates from Belgium. Amphotericin B and primaricin exhibited the highest activity, with th MIC90 ranging from 4 and 8 microg/ml, respectively. On the other hand, the MIC90 of nystatin and filipin were from two to four times higher. Two strains were resistant to all four polyenes tested. The above results are compared with those in the literature and the importance of carrying out large-scale screening surveys to assess polyene susceptibility patterns within the species P. zopfii is discussed.

Response surface methodology as optimization strategy for asymmetric bioreduction of (4S)-(+)-carvone by Cryptococcus gastricus

Response surface methodology was applied in optimizing the asymmetric bioreduction of (4S)-(+)-carvone to dihydrocarvone (with low incidence of unsought side reactions) by using whole-cells of Cryptococcus gastricus. A factorial design (2(5)) including five independent variables was performed: X(1)=incubation time; X(2)=pH; X(3)=amount of whole-cells; X(4)=concentration of (4S)-(+)-carvone; X(5)=concentration of cofactor-recycling system. The utilization of glucose and glycerol as cofactor-recycling systems was checked. On the basis of the results of factorial design, three independent variables (X(1), X(3) and X(4)) out of five were further selected for performing a central composite design (CCD). First and second order polynomial equations obtained by CCD were used to select the optimal values of independent variables in order to maximize the bioreduction yield of (4S)-(+)-carvone and, at the same time, to minimize the occurrence of side reactions (i.e. further reduction of dihydrocarvone to dihydrocarveol).

Catechins and Proanthocyanidins: Naturally Occurring O-Heterocycleswith Antimicrobial Activity

Pharmacological interest in drug discovery has increased in recent years. Accordingly, an increasingpart of the scientific body has thus far studied several thousands of compounds of natural and syntheticorigin to ascertain their biological activity. The occurrence in plant tissues of secondary metabolitescharacterized by a O-heterocyclic structure and exhibiting antimicrobialproperties is a well-known phenomenon. Among them, catechins and proanthocyanidins are two classesof compounds that exhibit antiviral or antimicrobial properties towards prokaryotic and eukaryotic microorganisms.Yet, despite the profusion of studies published so far, the real potentialities and limitations presentedby the use of the above-mentioned class of molecules as antiviral or antimicrobial agents have not beencritically evaluated. The present chapter represents an overview of the recent literature regarding theantiviral and antimicrobial properties exhibited by this class of compounds. Their mode of action as wellas their synergy with currently used antibiotic molecules are also reviewed. In addition, their potentialitiesand the causes that have hampered their pharmaceutical exploitation so far are discussed.