Yeast Flora Research Articles

Enumeration and Identification of Yeasts Associated with Commercial Poultry Processing and Spoilage of Refrigerated Broiler Carcasses

Yeasts associated with broiler carcasses taken from various stages of commercial poultry processing operations and broiler carcasses stored at refrigerated temperatures were enumerated and identified. Whole carcass rinses were performed to recover yeasts from carcasses taken from a processing facility and processed carcasses stored at 4°C for up to 14 days. Yeasts in the carcass rinsates were enumerated on acidified potato dextrose agar and identified with the MIDI Sherlock Microbial Identification System. Dendrograms of fatty acid profiles of yeast were prepared to determine the degree of relatedness of the yeast isolates. Findings indicated that as the carcasses are moved through the processing line, significant decreases in the number of yeasts associated with broiler carcasses usually occur, and the composition of the yeast flora of the carcasses is altered. Significant (P < 0.05) increases in the yeast population of the carcasses generally occur during storage at 4°C, however. Furthermore, it was determined that the same strain of yeast may be recovered from different carcasses at different points in the processing line and that the same strain of yeast may be isolated from carcasses processed on different days in the same processing facility.

Lactic acid bacteria and yeasts in kefir grains and kefir made from them

In an investigation of the changes in the microflora along the pathway: kefir grains (A)-->kefir made from kefir grains (B)-->kefir made from kefir as inoculum (C), the following species of lactic acid bacteria (83-90%) of the microbial count in the grains) were identified: Lactococcus lactis subsp. lactis, Streptococcus thermophilus, Lactobacillus delbrueckii subsp. bulgaricus, Lactobacillus helveticus, Lactobacillus casei subsp. pseudoplantarum and Lactobacillus brevis. Yeasts (10-17%) identified were Kluyveromyces marxianus var. lactis, Saccharomyces cerevisiae, Candida inconspicua and Candida maris. In the microbial population of kefir grains and kefir made from them the homofermentative lactic streptococci (52-65% and 79-86%, respectively) predominated. Within the group of lactobacilli, the homofermentative thermophilic species L. delbrueckii subsp. bulgaricus and L. helveticus (70-87% of the isolated bacilli) predominated. Along the pathway A-->B-->C, the streptococcal proportion in the total kefir microflora increased by 26-30% whereas the lactobacilli decreased by 13-23%. K. marxianus var. lactis was permanently present in kefir grains and kefirs, whereas the dominant lactose-negative yeast in the total yeast flora of the kefir grains dramatically decreased in kefir C.

The Oral Yeast Flora: Effect of Upper Removable Orthodontic Appliances

The aim of this study was to test the hypothesis that the oral biology of yeasts in children alters in response to presence of an upper removable appliance (URA). Thirty-four normal controls and 34 patients, undergoing removable orthodontic appliance treatment were studied. Saliva samples were obtained from all subjects along with a foam imprint from the fitting surface of the URA in the case of patients. Samples were cultured on CHROMagar Candida. Colonies that were green–blue in colour were confirmed as Candida albicans by the germ-tube test. Other yeasts were identified by means of commercial identification kits. Oral yeast species were isolated from 32.3% of healthy controls compared with 76.5% of children with a URA ( p< 0.01). C. albicans was the most prevalent species isolated from saliva and appliance in the same individual (13) and from saliva alone in both patient (six) and control (ten) individuals. In eight of 34 patients, but in only one of 34 controls, more than one yeast species was present. The prevalence of oral colonization by Candida spp. is significantly higher amongst wearers of removable orthodontic appliances. Furthermore, the oral biology of yeasts is affected by URA wear so that colonization is more important, and by a wider variety of Candida species, and multispecies are seen. Keywords: orthodontic patients, removable appliance, Candida , CHROMagar Candida.

Use of specific DNA probes for the rapid characterization of yeasts isolated from complex biotopes

We describe a rapid method developed to identify yeast species commonly found during cheese ripening. It is based on the isolation of species-specific sequences and their hybridization with coarse preparations of genomic DNA. Several strategies were followed for the construction of probes: PCR amplification of sequences available in databases, random cloning of genomic DNA fragments, specific RAPD fragments and PCR-amplified ribosomal DNA fragments. After validation, the probes were applied to the characterization of 400 yeast strains isolated from various French cheeses. Since the strains had been previously identified with classical diagnostic tests, we were able to compare molecular and conventional identification. In addition, the specific probes for one of the species, Debaryomyces hansenii, were used successfully in colony hybridization experiments. The probes developed here proved to be very useful for the screening of large yeast collections and for the assessment of biodiversity within the yeast flora in cheese.

Analysis of yeasts derived from natural fermentation in a Tokaj winery.

The diversity of yeast flora was investigated in a spontaneously fermenting sweet white wine in a Tokaj winery. The non-Saccharomyces yeasts dominating the first phase of fermentation were soon replaced by a heterogeneous Saccharomyces population, which then became dominated by Saccharomyces bayanus. Three Saccharomyces sensu stricto strains isolated from various phases of fermentation were tested for genetic stability, optimum growth temperature, tolerance to sulphur dioxide, copper and ethanol as well as for the ability to produce hydrogen sulphide and various secondary metabolites known to affect the organoleptic properties of wines. The analysis of the single-spore cultures derived from spores of dissected asci revealed high stability of electrophoretic karyotypes and various degrees of heterozygosity for mating-types, the fermentation of galactose and the production of metabolic by-products. The production levels of the by-products did not segregate in a 2:2 fashion, suggesting that the synthesis of these compounds is under polygenic control.

Characterisation of yeast flora isolated from an artisanal Portuguese ewes’ cheese

The evolution of the yeast flora was studied for an artisanal semi-hard ewes’ cheese made from raw milk. Mean log 10 yeast counts per gram of cheese body ranged from 2.7 to 6.4, with the higher counts observed after a ripening period of 30 days. The yeast population decreased thereafter and, at the end of curing process, reached values similar to those of the beginning. A total of 344 yeasts strains were randomly isolated from the curd and cheese body during the 60 days long ripening period. Esterase activity was common to almost all isolates (98%) while proteolysis was observed in 12% of the total yeast population. The proportion of strains with positive glucose fermentation increased from 21% in the curd to 75% at the end of the ripening period. A total of 150 isolates representative of the physiological characteristics tested were examined with the API ID 32C system showing different degrees of quality of identification. Only 15% of the strains (23 isolates) were excellently identified being assigned to the species Candida zeylanoides. The most frequent species appeared to be Debaryomyces hansenii (anamorph Candida famata) and Candida intermedia. These two species amounted to 9% of the yeasts in the curd increasing to 86% at the end of the ripening period.

Spoilage yeasts in foods and beverages: characterisation and ecology for improved diagnosis and control

Foods and drinks with high solute concentration (fruit juices and concentrates, marzipan, salted and dry-cured meats, olives and cheeses), and foods containing preservatives (wine, beverages and sauces) were selected in order to characterise the contaminant yeast flora by rapid typing techniques. These included the testing of several types of molecular methods (e.g. RFLP, DNA-fingerprinting, PCR-based techniques), analysis of long-chain fatty acids and of isoenzymes. The PCR-based detection methods enabled a faster detection of emerging specific spoilers at earlier stages of processing. Fatty acid characterisation allowed the assessment of the most frequent types of contamination yeasts and supplied the information for the definition of relevant zymological indicators. A selected group of strains was used for further studies of mechanisms underlying the resistance/tolerance of yeasts towards preservatives (weak acids) and other stress factors (temperature, high sugar and salt concentrations). This study enabled the acquisition of data on the basic biology of yeasts used in the development of differential and selective media for Zygosaccharomyces bailii, Yarrowia lipolytica, Kluyveromyces marxianus and Dekkera sp.

Comparison of the yeast flora of freeze-dried, water-soaked and daily transferred kefir grains (Georgia, Russia)

Yeast flora of three kinds of Georgian kefir grains was investigated. They were freeze-dried grains (FDG), water-soaked grains (WSG), and daily transferred active grains (DTG). Strains of three well-known yeast species, Saccharomyces unisporous, Candida kefyr, and Candida holmii were isolated from DTG. Only Saccharomyces unisporus strains were isolated from FDG and WSG. Furthermore, strains of Candida valida were contaminated at a ratio of 1 : 3 in WSG. These results would indicate that C. kefyr and C. holmii are less tolerant to freeze-drying compared with S. unisporus. Freeze-drying seems to damage the cells of C. kefyr and C. holmii by cold shock and a concentration of lactic acid produced by lactic acid bacteria in the kefir production. C. holmii is nutritionally exacting and grows slowly. The role of this species has not been clear, but some effects will be expected for stimulation of other microorganisms. Preservation of the kefir grain was discussed from the viewpoint of yeast flora.

Cutaneous microenvironment of human immunodeficiency virus (HIV)-seropositive and HIV-seronegative individuals, with special reference to Staphylococcus aureus colonization.

A cross-sectional quantitative study of cutaneous bacterial and yeast flora at seven body sites in 99 human immunodeficiency virus-seropositive and 50 seronegative military personnel was performed. Statistically significant differences in carriage rates were only observed for Staphylococcus aureus on the foreheads of seropositive individuals. Seronegative individuals demonstrated staphylococcal carriage rates 1.3 to 2 times as great as those of historical controls (defined as healthy individuals not receiving any medications) at five of six body sites. We conclude that seropositive military personnel do not exhibit statistically significant elevations in densities and carriage rates of the microorganisms examined (except Staphylococcus aureus), relative to seronegative individuals. Seropositive individuals may be predisposed to staphylococcal carriage. The elevated staphylococcal carriage rates of military personnel undergoing basic training warrants a formal evaluation of the impact of training exercises on cutaneous flora. The information gained may serve to limit the spread of infection during training exercises and battlefield conditions.

Ecology ofCandida-associated Denture Stomatitis

Introduction of a prosthesis into the oral cavity results in profound alterations of the environmental conditions as the prosthesis and the underlying mucosa become colonized with oral microorganisms, including Candida spp. This may lead to denture stomatitis, a non-specific inflammatory reaction against microbial antigens, toxins and enzymes produced by the colonizing microorganisms. The role of Candida in the etiology of denture stomatitis is indicated by an increased number of yeasts on the mucosa and the dentures, increased levels of anti-Candida antibody titres in affected individuals and the clinical improvement of the mucosa due to eradication of the yeast flora. The colonization of the fitting denture surface by Candida depends on several factors including adherence of yeast cells, interaction with oral commensal bacteria, redox potential of the site, and surface properties of the acrylic resin. The pathogenicity of denture plaque can be enhanced by factors stimulating yeast propagation, such as poor oral hygiene, high carbohydrate intake, reduced salivary flow and continuous denture wearing. The more important factors which can modulate the host-parasite relationship and increase the susceptibility to Candida-associated denture stomatitis may be ageing, malnutrition, immunosuppression, radiation therapy, diabetes mellitus, and possibly treatment with antibacterial antibiotics. To control plaque formation on the fitting denture surface and the underlying mucosa it is important to install appropriate oral and denture hygiene measures as well as denture wearing habits. The Candida infection associated with denture stomatitis is a relatively harmless condition in healthy individuals; however, the infection may spread in susceptible individuals.

Resistance of Candida species to antifungal agents used in the treatment of onychomycosis: a review of current problems.

Treatment of Candida infections with fluconazole has resulted in the emergence of drug resistance, a problem particularly apparent in HIV-infected patients. Frequently, the yeast is also cross-resistant to itraconazole and other azoles. In neutropenic patients fluconazole therapy or prophylaxis has caused overgrowth and infection by inherently less susceptible species of Candida, principally C. glabrata and C. krusei. Consequently, the use of intermittent long-term azole therapy to treat onychomycosis could result in changes in the commensal yeast flora of patients--either resistance or pathogen shift. An 'off-study' investigation undertaken in patients receiving either continuous terbinafine or intermittent itraconazole for toenail onychomycosis (L.I.ON. study) showed no evidence of changes in the yeast species present, nor in their sensitivity to itraconazole or fluconazole. Although intermittent itraconazole seems unlikely to cause problems in this respect, the situation with regard to intermittent fluconazole therapy of onychomycosis needs further study.

Prevention of aerobic spoilage of maize silage by a genetically modified killer yeast, Kluyveromyces lactis, defective in the ability to grow on lactic acid.

In this study, we propose a new process of adding a genetically modified killer yeast to improve the aerobic stability of silage. Previously constructed Kluyveromyces lactis killer strain PCK27, defective in growth on lactic acid due to disruption of the gene coding for phosphoenolpyruvate carboxykinase, a key enzyme for gluconeogenesis, inhibited the growth of Pichia anomala inoculated as an aerobic spoilage yeast and prevented a rise in pH in a model of silage fermentation. This suppressive effect of PCK27 was not only due to growth competition but also due to the killer protein produced. From these results, we concluded that strain PCK27 can be used as an additive to prolong the aerobic stability of maize silage. In the laboratory-scale experiment of maize silage, the addition of a killer yeast changed the yeast flora and significantly reduced aerobic spoilage.

Microbial and chemical changes during the spontaneous ensilage of grape pomace

Pilot scale fermentations with grape pomace from two different wineries were investigated during the 24 weeks of the ensiling period, along with laboratory scale experiments in which the environmental temperatures were held constant at 20, 25, 30 and 35 °C. During this period, yeast and lactic acid bacteria (LAB) counts were made, after which the identity of both groups of organisms was studied, as were the major microbial metabolites present. Major microbial and chemical alterations occurred during the first 3 weeks of ensilage, leaving a more stable product differing significantly from the initial substrate. The results obtained indicated that after initial growth, yeast and LAB populations undergo progressive inactivation at environmental temperatures above 20 °C, although LAB seem to adjust better to this specific, post-fermentation environment. Homofermentative species of Lactobacillus were the dominant LAB. The initial yeast flora of non-Saccharomyces species was replaced by a typical wine yeast flora, i.e. predominantly Saccharomyces cerevisiae. At the chemical level, major alterations were due to an alcoholic fermentation and a malolactic conversion within the first 3 weeks.

Examination of aroma production kinetics of different commercial wine yeasts in fermenting muscat ottonel wines with the help of spme head-space sampling and fast gc analysis

Solid phase microextraction (SPME) coupled to fast capillary gas chromatography was used for monitoring the wine fermentation process. This combination offers a simple, quick and sensitive approach suitable for characterization of head-space components of wines during the fermentation process without a complicated sample preparation procedure. In this work this method was used to observe the differences in aroma production between three different commercial yeasts and the indigenous yeast flora.

Fluconazole-resistant Candida species in the oral flora of fluconazole-exposed HIV-positive patients

The purpose of this study was to examine the effect of preceding fluconazole treatment on the oral mycologic flora and on the sensitivity of oral Candida albicans isolates to fluconazole. Saline oral rinses were collected from 89 HIV-positive patients, of whom 48 had been exposed to fluconazole and 41 were fluconazole-naive. The rinses were cultured on Sabouraud's and Pagano Levin agars, and yeasts were identified by standard methods. Fluconazole sensitivity of C. albicans isolates was measured by disk diffusion assay. C. albicans was isolated from 69% of patients who had received fluconazole and from 93% of the patients who were fluconazole-naive ( p < 0.05). Nine other species of yeasts were also isolated, most commonly C. glabrata. Five patients previously exposed to fluconazole harbored fluconazole-resistant C. albicans, whereas no resistance was detected among the patients who were fluconazole-naive ( p < 0.01). Sixteen of the patients who were fluconazole-exposed carried yeasts other than C. albicans, compared with only five patients in the fluconazole-naive group ( p < 0.01). All of the fluconazole-resistant strains were isolated from patients with low CD4 counts (less than 100 cells/ml) and after lengthy fluconazole exposures. Nevertheless, patients in Charlotte, N.C., who had a greater mean fluconazole exposure time (10.25 ± 1.41 months) than patients in Glasgow, UK, (0.65 ± 0.18 months; p < 0.005), did not develop significantly more in vitro resistance or species diversity. This study indicates that long-term fluconazole treatment can have significant effects on the yeast flora of the mouth, particularly in a patient with a CD4 count of less than 100 cells/ml.

Yeast occurrence in Danish feta cheese

The yeast content in feta cheese from three different dairies was examined. Almost all samples contained yeasts in considerable numbers, 104–106cfu g−1. A specific narrow yeast flora was found for each dairy.Torulaspora delbrueckiiwas the dominating species in feta from Dairy A. This strong-fermenting species was found to cause swelling of the cans.Debaryomyces hansenii,Candida sakeandKluyveromyces marxianuswere dominant in samples from Dairy B.Candida butyriandDebaromyces hanseniiwere found in Dairy C.Yarrowia lipolyticawas present in low numbers in cheese samples from all three dairies.Geotrichum candidumwas also commonly found although in low concentrations. Yeast occurrence was due to recontamination. Yeasts were found in high concentrations in production areas.

Studies on Camembert Cheese Manufacture Using Yeast Flora (5)

Studies on Camembert Cheese Manufacture Using Yeast Flora (5)

Reinforcement of the radiative and thermic stresses of the grape vine. Repercussions on yeast surface microflora

&lt;p&gt;All along the ripening period, the radiative and thermic stresses of the grapevine may be reinforced by the use of a reflective soil cover (aluminized film). Such a treatment leads to repercussions on the berries, on the must composition and finally on the wine quality. During such a preliminary experiment, we demonstrated that the temperature increase and/or the reinforcement of the reflected ultraviolet radiations (measured at 254 nm) at the level of grape berries severely impaired the development of yeast cells at their surfaces. By means of an artificial inoculation of grapes at the beginning of the ripening period with a mixture of four different yeast genera (&lt;em&gt;Saccharomyces cerevisiae&lt;/em&gt;, &lt;em&gt;Hanseniaspora uvarum&lt;/em&gt;, &lt;em&gt;Pichiafermentans&lt;/em&gt; and &lt;em&gt;Schizosaccharomyces pombe&lt;/em&gt;), we demonstrated that the repartition of yeast genera amongst this population was affected by the treatment of stocks with the aluminized film: during the experiment presented in this paper, the &lt;em&gt;Saccharomyces&lt;/em&gt; genus was favoured. One may consider by extension similar effects resulting from the reflective properties of some natural soils. Such effects may considerably influence the distribution of wild yeast flora during the spontaneous fermentation of musts. If such an hypothesis is confirmed at a local or regional level, it will represent a first significant piece of the definition of one of the aspects of the" terroir" effect on the characteristics of wines.&lt;/p&gt;

Resistance of Yeast Flora of Labaneh to Potassium Sorbate and Sodium Benzoate

Ten yeast cultures belonging to eight species representing the yeast flora of labaneh were tested for their resistance to potassium sorbate and sodium benzoate. Changes in counts were monitored after 7, 14, and 21 d at 5°C in yeast-free labaneh containing different concentrations of potassium sorbate or sodium benzoate. More than 400 mg/kg of sodium benzoate were needed to limit the counts of Saccharomyces cerevisiae (biovariants 1, 2, and 7) Cryptococcus curvatus, Pichia farinosa, Candida blankii, Debaryomyces hansenii, and Trichosporon brassicae to ≤105 cfu/g after 14 d at 5°C; 150 and 300 mg/kg were needed for Geotrichum candidum and Trichosporon cutaneum, respectively. When potassium sorbate was used, >400 mg/kg were needed for P. farinosa and D. hansenii; 350 mg/kg were needed for S. cerevisiae biovar 1, Cr. curvatus, and C. blankii; 250 and 200 mg/kg were needed for S. cerevisiae biovariants 2 and 7, respectively; and <100 mg/kg were needed for T. brassicae, T. cutaneum, and G. candidum. Less than 150 and <250 mg/kg of potassium sorbate and sodium benzoate, respectively, were needed to limit yeast counts to <105 cfu/g for 7 d at 5°C in two commercial labaneh samples that had initial yeast counts of 4.8 × 102 and 9.0 × 102 cfu/g; 200 and >400 mg/kg were required when the storage was extended to 14 d. In commercial labaneh with 9.6 × 103 cfu/g, >300 and >400 mg/kg of potassium sorbate and sodium benzoate, respectively, were needed to limit the yeast count to ≤105 cfu/g after 7 and 14 d at 5°C.

Use of RAPD and mitochondrial DNA RFLP for typing of Candida zeylanoides and Debaryomyces hansenii yeast strains isolated from cheese

Summary In order to evaluate the genetic diversity of the yeast flora in Northern Spain cheese, we adapted two molecular techniques devised for Saccharomyces, RAPD and RFLP of mitochondrial DNA, to type 27 Candida zeylanoides strains and 28 Debaryomyces hansenii strains isolated from Roncal and Idiazabal cheese at different stages of manufacture in different dairies. RAPD with (GTG)5 primer, although very reproducible, only defined 2 groups of strains for both species. On the other hand, mtDNA RFLP proved to be more discriminating and defined 5 groups of strains for both species. In addition, the strains from the minor RAPD groups in both species corresponded to specific groups defined by mtDNA RFLP. Overall, this analysis has revealed an important genetic diversity, considering the limited sample of different biotopes tested. Nevertheless, the mtDNA RFLP results indicate that a large majority of the strains appeared closely related, suggesting little variability between cheese and dairies. This work has demonstrated that rapid identification methods (the typing of strains can be achieved in two days) can be applied to these poorly characterized yeast species, allowing a rapid evaluation of genetic diversity of these species in different biotopes.