Improving ecological fitness and environmental stress tolerance of the biocontrol yeast Candida sake by manipulation of intracellular sugar alcohol and sugar content

Exploring yeast-based microbial interactions: The next frontier in postharvest biocontrol

Fresh fruits and vegetables are susceptible to a large variety of spoilage agents before and after harvest. Among these, fungi are mostly responsible for the microbiological deteriorations that lead to economically significant losses of fresh produce. Today, synthetic fungicides represent the first approach for controlling postharvest spoilage in fruits and vegetables worldwide. However, the emergence of fungicide-resistant pathogen biotypes and the increasing awareness of consumers toward the health implications of hazardous chemicals imposed an urgent need to reduce the use of synthetic fungicides in the food supply; this phenomenon strengthened the search for alternative biocontrol strategies that are more effective, safer, nontoxic, low-residue, environment friendly, and cost-effective. In the last decade, biocontrol with antagonistic yeasts became a promising strategy to reduce chemical compounds during fruit and vegetable postharvest, and several yeast-based biocontrol products have been commercialized. Biocontrol is a multipartite system that includes different microbial groups (spoilage mold, yeast, bacteria, and nonspoilage resident microorganisms), host fruit, vegetables, or plants, and the environment. The majority of biocontrol studies focused on yeast-mold mechanisms, with little consideration for yeast-bacteria and yeast-yeast interactions. The current review focused mainly on the unexplored yeast-based interactions and the mechanisms of actions in biocontrol systems as well as on the importance and advantages of using yeasts as biocontrol agents, improving antagonist efficiency, the commercialization process and associated challenges, and future perspectives.

Microbial trehalose boosts the ecological fitness of biocontrol agents, the viability of probiotics during long-term storage and plants tolerance to environmental-driven abiotic stress

Despite the impressive gain in agricultural production and greater availability of food, a large portion of the world population is affected by food shortages and nutritional imbalance. This is due to abiotic stresses encountered by plants as a result of environmental-driven perturbations, loss of viability of starter cultures (probiotics) for functional foods during storage as well as the vulnerability of farm produce to postharvest pathogens. The use of compatible solutes (e.g., trehalose, proline, etc.) has been widely supported as a solution to these concerns. Trehalose is one of the widely reported microbial- or plant-derived metabolites that help microorganisms (e.g., biocontrol agents, probiotics and plant growth-promoting bacteria) and plants to tolerate harsh environmental conditions. Due to its recent categorization as generally regarded as safe (GRAS), trehalose is an essential tool for promoting nutrition-sensitive agriculture by replacing the overuse of chemical agents (e.g., pesticides, herbicides). Therefore, the current review evaluated the progress currently made in the application of trehalose in sustainable agriculture. The challenges, opportunities, and future of this biometabolite in food security were highlighted.

Pretreatment of the Antagonistic Yeast, Debaryomyces hansenii, With Mannitol and Sorbitol Improves Stress Tolerance and Biocontrol Efficacy

The effect of exogenous mannitol and sorbitol on the viability of the antagonist yeast, Debaryomyces hansenii, when exposed to oxidative and high-temperature stress was determined. Results indicated that both the 0.1 M mannitol (MT) and 0.1 M sorbitol (ST) treatments improved the tolerance of D. hansenii to subsequent oxidative and high-temperature stress. MT or ST cells had a significantly higher level of cell survival, elevated the gene expression of catalase 1 (CAT1) and copper-zinc superoxide dismutase (SOD1), as well as the corresponding enzyme activity. Treated cells also exhibited a lower accumulation of intracellular reactive oxygen species (ROS), and a higher content of intracellular mannitol and sorbitol relative to non-treated, control yeast cells, when exposed to a subsequent oxidative (30 mM H₂O₂) or heat (40.5°C) stress for 30 min. Additionally, MT and ST yeast exhibited a higher growth rate in kiwifruit wounds, and a greater ability to inhibit postharvest blue mold (Penicillium expansum) and gray mold (Botrytis cinerea) infections. The present study indicates that increased antioxidant response induced by mannitol and sorbitol in D. hansenii can enhance stress tolerance and biocontrol performance.

Endogenous arabitol and mannitol improve shelf life of encapsulated Metarhizium brunneum

Successful commercialization of microbial biocontrol agents, such as Metarhizium spp., is often constrained by poor drying survival and shelf life. Here, we hypothesized that culture age would influence endogenous arabitol, erythritol, mannitol and trehalose contents in M. brunneum mycelium and that elevated levels of these compounds would improve drying survival and shelf life of encapsulated mycelium coupled with enhanced fungal virulence against T. molitor larvae. We found that culture age significantly influenced endogenous arabitol and mannitol contents in mycelium with highest concentrations of 0.6 ± 0.2 and 2.1 ± 0.2 µg/mg after 72 h, respectively. Drying survival of encapsulated mycelium was independent of culture age and polyol content with 41.1 ± 4.4 to 55.0 ± 6.2%. Best shelf life was determined for biomass harvested after 72 h at all investigated storage temperatures with maximum values of 59.5 ± 3.3% at 5 °C followed by 54.5 ± 1.6% at 18 °C and 19.4 ± 1.3% at 25 °C after 6 months. Finally, high fungal virulence against T. molitor larvae of 83.3 ± 7.6 to 98.0 ± 1.8% was maintained during storage of encapsulated mycelium for 12 months with larval mortalities being independent of culture age and polyol content. In conclusion, our findings indicate beneficial effects of endogenous polyols in improving shelf life of encapsulated mycelium and this may spur the successful development of microbial biocontrol agents in the future.

Prototipo de formulación a base de Rhodotorula mucilaginosa para el control de Botrytis cinerea en Rosas

Los sistemas productivos de Rosas de corte para exportación poseen retos importantes debido a la presencia de diversos agentes fitopatógenos, siendo Botrytis cinerea uno de los más relevantes debido a su persistencia y número de hospederos alternativos. Los mercados internacionales son muy exigentes en el manejo ambientalmente sostenible de los cultivos, por lo que se ha hecho presión para la implementación de estrategias de control biológico de enfermedades. La levadura filosferica Rhodotorula mucilaginosa (Lv20) con potencial biocontrolador contra B. cinérea, fue empleada en este estudio con el objeto de generar un prototipo de formulación en base sólida con el fin de lograr una estabilidad de la actividad y viabilidad celular a través del tiempo. El empleo de mezclas de polímeros sintéticos y de origen natural permitió mantener la viabilidad de esta cepa durante 90 días a unos niveles de 1,90x109 células.mL-1 a una temperatura de 25°C en una formulación líquida. Así mismo, el prototipo de formulación, empleando manitol como agente nucleador en una formulación sólida de tipo granulada,  logró una viabilidad celular de 1.2x108 células.gr-1 a los 90 días de almacenamiento a 4°C, logrando mantener una actividad biocontroladora igual a la cepa fresca sin formular o recién formulada.  Estos resultados obtenidos permiten sugerir que los prototipos de formulación empleando como principio activo la levadura R. mucilaginosa, son una alternativa promisoria para el control de B. cinerea en la post cosecha de rosas variedad véndela.Palabras clave: levaduras, moho gris, formulación sólida, biopolimeros, control biológico, postcosecha.

Stress tolerance and biocontrol performance of the yeast antagonist, Candida diversa, change with morphology transition

As an eco-friendly management method, biological control of postharvest diseases, utilizing antagonistic yeasts, is a research topic receiving considerable attention. Detailed knowledge on the biology of yeast antagonists is crucial when considering their potential application and development as biocontrol products. Changes in the growth form, such as single-cell to pseudohyphae, have been associated with the mode of action in postharvest biocontrol yeasts. In this study, the antagonistic yeast, Candida diversa, reversibly shifted from a single-cell morphology on yeast peptone dextrose (YPD) medium with 2 % agar to a pseudohyphal morphology on YPD with 0.3 % agar. The tolerance of the pseudohyphal form to heat and oxidative stresses, as well as the biocontrol efficacy against Botrytis cinerea on apple and kiwifruit stored at 25 and 4 °C, was significantly higher as compared to the single-cell form. This study provides new information on the ability of C. diversa to change its morphology and the impact of the morphology shift on stress tolerance and biocontrol performance.

Responses of yeast biocontrol agents to environmental stress

Biological control of postharvest diseases, utilizing wild species and strains of antagonistic yeast species, is a research topic that has received considerable attention in the literature over the past 30 years. In principle, it represents a promising alternative to chemical fungicides for the management of postharvest decay of fruits, vegetables, and grains. A yeast-based biocontrol system is composed of a tritrophic interaction between a host (commodity), a pathogen, and a yeast species, all of which are affected by environmental factors such as temperature, pH, and UV light as well as osmotic and oxidative stresses. Additionally, during the production process, biocontrol agents encounter various severe abiotic stresses that also impact their viability. Therefore, understanding the ecological fitness of the potential yeast biocontrol agents and developing strategies to enhance their stress tolerance are essential to their efficacy and commercial application. The current review provides an overview of the responses of antagonistic yeast species to various environmental stresses, the methods that can be used to improve stress tolerance and efficacy, and the related mechanisms associated with improved stress tolerance.

Stress tolerance and virulence of insect-pathogenic fungi are determined by environmental conditions during conidial formation

The virulence to insects and tolerance to heat and UV-B radiation of conidia of entomopathogenic fungi are greatly influenced by physical, chemical, and nutritional conditions during mycelial growth. This is evidenced, for example, by the stress phenotypes of Metarhizium robertsii produced on various substrates. Conidia from minimal medium (Czapek's medium without sucrose), complex medium, and insect (Lepidoptera and Coleoptera) cadavers had high, moderate, and poor tolerance to UV-B radiation, respectively. Furthermore, conidia from minimal medium germinated faster and had increased heat tolerance and were more virulent to insects than those from complex medium. Low water-activity or alkaline culture conditions also resulted in production of conidia with high tolerance to heat or UV-B radiation. Conidia produced on complex media exhibited lower stress tolerance, whereas those from complex media supplemented with NaCl or KCl (to reduce water activity) were more tolerant to heat and UV-B than those from the unmodified complex medium. Osmotic and nutritive stresses resulted in production of conidia with a robust stress phenotype, but also were associated with low conidial yield. Physical conditions such as growth under illumination, hypoxic conditions, and heat shock before conidial production also induced both higher UV-B and heat tolerance; but conidial production was not decreased. In conclusion, physical and chemical parameters, as well as nutrition source, can induce great variability in conidial tolerance to stress for entomopathogenic fungi. Implications are discussed in relation to the ecology of entomopathogenic fungi in the field, and to their use for biological control. This review will cover recent technologies on improving stress tolerance of entomopathogenic fungi for biological control of insects.

Prospects and limitations of microbial pesticides for control of bacterial and fungal pomefruit tree diseases

The tree constitutes an ecosystem in which microorganisms play an essential role in its functionality. Interactions that microorganisms establish with plants may be beneficial or detrimental and are of extreme importance in the exploitation of trees in agriculture as crop production systems. Fruit trees, especially pomefruit trees including apple, pear and several ornamentals are of great economic importance but its production is affected by several diseases. Fungal and bacterial fruit tree diseases are mainly controlled with chemical fungicides and bactericides, but health and environmental concerns about the use of chemical pesticides have result in strong regulatory actions and have stimulated the development of beneficial microorganisms as microbial pesticides. Up to now, several microorganisms have been registered in different countries and in the EU as biocontrol agents (BCA) covering mainly fire blight, soil-borne fungal diseases and postharvest fruit fungal rot. The key aspects in the success of this technology for disease control are related to biosafety and environmental impact of biocontrol agents, the traceability and fate in the environment and food chain, the improvement by physiological, genetic engineering or the use of mixtures or formulations as well as the industrial production and development of delivery systems for treatment application to trees.

Impact of osmotic/matric stress and heat shock on environmental tolerance induction of bacterial biocontrol agents against Fusarium verticillioides

Bacillus amyloliquefaciens and Microbacterium oleovorans reduced the Fusarium verticillioides count and significantly decreased fumonisin B(1) and B(2) levels in maize grains. The aim of this study was to determine the effect of water stress tolerance and heat shock survival upon cells of the biocontrol agents B. amyloliquefaciens and M. oleovorans. The a(w) of solid and liquid media and tryptic soy medium was modified to 0.99, 0.98, 0.97 and 0.96 by addition of ionic solute NaCl and non-ionic solutes such as glycerol and glucose. The non-ionic solute polyethylene glycol 600 (PEG 600) was used to modify matrically solid media. Bacterial incubation was at 30 °C. After 24, 48 and 72 h of incubation, samples from liquid media were spread-plate on nutrient agar medium and incubated for 24 h to determine the number of viable cells. The bacterial cells were harvested by centrifugation and heat treatment carried out in a water bath at 45 °C for 30 min. The viability of cells from different incubation times in liquid media showed statistically significant differences. Cells of B. amyloliquefaciens grown in liquid media amended with glycerol showed better tolerance at low a(w) and high survival under heat stress. These results could have important implications for optimizing and improving formulations.

Increasing survival and efficacy of a bacterial biocontrol agent of fire blight of rosaceous plants by means of osmoadaptation

The efficacy of Pseudomonas fluorescens EPS62e in the biocontrol of Erwinia amylovora, the causal agent of fire blight of apple and pear, depends on the colonization of plant surfaces after spray application. A procedure to increase cell survival in the phyllosphere was developed consisting of saline stress and osmolyte amendment to the growth medium during inoculum preparation. Hyperosmotic stress induced the synthesis of the osmolytes trehalose, N-acetylglutaminylglutamine amide and glucosyl-glycerol, but decreasing growth rate. Amendment of the growth medium with glycine betaine increased growth rate and cell yield and promoted its intracellular accumulation. Under controlled environment conditions, osmoadaptation increased by 10- to 100-fold cell survival to desiccation and to low relative humidity conditions on plant surfaces, in comparison with the nonosmoadapted controls. In the field, cell survival increased 100-1000 times in immature fruit upon osmoadaptation but was not significantly affected in flowers where active colonization occurs. The efficacy in the control of fire blight infections was increased 30-50% upon osmoadaptation on immature fruits but was not affected in blossoms. The method of osmoadaptation may be useful for improving the fitness and efficacy of biological control agents of phyllosphere pathogens under limiting humidity conditions.

Effect of intracellular trehalose in Cryptococcus laurentii and exogenous lyoprotectants on its viability and biocontrol efficacy on Penicillium expansum in apple fruit

AIMS

To improve viability and biocontrol efficacy of Cryptococcus laurentii after freeze drying and in subsequent storage.

METHODS AND RESULTS

Viability of C. laurentii was improved after freeze drying and in subsequent storage at 4 or 25 degrees C by using skimmed milk (SM) and sugars (glucose, galactose, sucrose and trehalose) as protectants. Sugars and SM mixed together showed better protection than when they were used separately. Citric acid used as carbon source could induce accumulation of intracellular trehalose in the yeast. The yeast cells with high trehalose level (HT cells) had higher viability than those with low trehalose level (LT cells) after freeze drying and storage for 90 days. After storage for 90 days at 4 degrees C, the HT cells plus SM and sugars as protectant showed a similar biocontrol effect against blue mould rot in apple fruit caused by Penicillium expansum as fresh cells.

CONCLUSIONS

Increasing intracellular trehalose content of C. laurentii and adding exogenous protectant (sugars + SM) could improve its viability and maintain its biocontrol efficacy.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results have a potential value for commercial application of C. laurentii.

Acid tolerance response induced in the biocontrol agent Pantoea agglomerans CPA-2 and effect on its survival ability in acidic environments

The aim of this work was to optimize acid stress conditions for induction of acid tolerance response (ATR) in the biocontrol agent Pantoea agglomerans and study the effect of ATR induced on the ability to survive under acidic conditions. Initially, Pantoea agglomerans was grown in mild acidic conditions (pH 6.0, 5.5, 5.0 and 4.0) in order to induce ATR. The highest ATR was induced at initial pH of 5 using malic or citric acid. A first in vitro experiment was carried out. Thus, basal liquid medium at different pHs (3.0, 3.5, 4.0 and non-acidified) were then inoculated with acid-adapted and non-adapted inocula of P. agglomerans and survivals were examined during incubation at 25 or 4 degrees C. It was found that acid adaptation enhanced the survivals of Pantoea agglomerans CPA-2 cells at pH levels at which the cells were unable to grow (<3.5 and 4.0, at 25 and 4 degrees C, respectively). In contrast, in pH levels at which the cells were able to grow (pH 4.0 at 25 degrees C and non-acidified medium at 25 and 4 degrees C) no-differences were found between adapted and non-adapted cells. In in vivo tests, adapted and non-adapted cells were inoculated in wounds on mandarins and pome fruits. No differences were found between adapted and non-adapted cells and biocontrol efficacy was maintained. The present study demonstrated that exposure of Pantoea agglomerans to mild acidic conditions could induce acid resistance in this biocontrol agent.

Improving low water activity and desiccation tolerance of the biocontrol agent Pantoea agglomerans CPA-2 by osmotic treatments

AIMS

To study the improvement of tolerance to low water activity (aw) and desiccation during spray drying in Pantoea agglomerans cells subjected to mild osmotic stress during growth.

METHODS AND RESULTS

The micro-organism was cultured in an unmodified liquid (control) or in aw-modified media, and viability of these cells was evaluated on unstressed (0.995) and 0.96 aw stressed solid media, in order to check total viability and aw stress tolerance respectively. Significant improvements in viability on unmodified medium were observed with cells grown for 24 h in NaCl 0.98 aw, glycerol 0.98 aw and 0.97 aw and for 48 h in NaCl 0.98 aw and 0.97 aw modified media. Both yield improvements and water stress tolerance were achieved with low aw media. Cells grown for 24 h in NaCl 0.98 aw or for 48 h in NaCl 0.98 aw, 0.97 aw and 0.96 aw, glucose 0.97 aw and glycerol 0.97 aw showed improved aw stress tolerance in comparison with control cells. The best results were obtained with NaCl treatments (0.98 aw and 0.97 aw) which also exhibited better survival rates than control cells during spray-drying process and maintained their efficacy against postharvest fungal pathogens in apples and oranges.

CONCLUSIONS

NaCl treatments are very appropriate for improving P. agglomerans low aw tolerance obtaining high production levels and maintaining biocontrol efficacy.

SIGNIFICANCE AND IMPACT OF THE STUDY

Improving stress tolerance of biocontrol agents could be an efficient way to obtain consistency and maintain efficacy of biological control under practical conditions.

Effects of trehalose on stress tolerance and biocontrol efficacy of Cryptococcus laurentii

AIMS

To investigate the effects of internal trehalose on viability and biocontrol efficacy of antagonistic yeast Cryptococcus laurentii under stresses of low temperature (LT), controlled atmosphere (CA) and freeze drying.

METHODS AND RESULTS

The content of trehalose in C. laurentii was increased by culturing the yeast in trehalose-containing medium. Compared with yeast cells with low trehalose level, the yeast cells with high level of internal trehalose not only obtained higher viability, but also showed higher population and better biocontrol efficacy against Penicillium expansum on apple fruit both at 1 degrees C and in CA condition (5% O(2), 5% CO(2), 1 degrees C). After freeze drying, survival of the yeast with high trehalose level was markedly increased when stored at 25 degrees C for 0, 15 and 30 days. Meanwhile, high integrity of plasma membrane was detected in the freeze-dried yeast with high trehalose level by propidium iodide staining.

CONCLUSIONS

Induced accumulation of internal trehalose could improve viability and biocontrol efficacy of C. laurentii under stresses of LT and CA. Moreover, survival of the yeast was also increased as internal trehalose accumulation after freeze drying, and one of the reasons might be that trehalose gave an effective protection to plasma membrane.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results of this experiment show a promising way to improve the biocontrol performance of antagonistic yeasts under the commercial conditions.

Influence of Bacillus spp. isolated from maize agroecosystem on growth and aflatoxin B(1) production by Aspergillus section Flavi

A total of 59 bacteria of the Bacillus genus were isolated from different components of a maize agroecosystem and their antifungal activity against Aspergillus section Flavi was evaluated. Thirty-three and 46% of these bacteria were able to inhibit Aspergillus flavus Link and A. parasiticus Speare respectively at water activity (a(w)) 0.982; however, when a(w) was 0.955, these percentages were decreased and only three isolates were able to inhibit Aspergillus section Flavi. The majority of bacilli acted as contact antagonists, while a small number of isolates were able to form inhibition zones. In maize meal extract agar, Aspergillus section Flavi growth rate and aflatoxin B(1) (AFB(1)) production were significantly reduced when these strains were paired at a(w) 0.982 with bacilli at all inoculum levels studied. However, two bacilli isolated were able to reduce growth rate and aflatoxin production when a(w) was 0.955. Lag phase increase followed the same general pattern as growth rate reduction. When Aspergillus section Flavi was grown in sterile maize in the presence of three Bacillus strains at a(w) 0.982, the reduction in count (colony-forming units (cfu) g(-1) maize) was less than 30%, except when Aspergillus section Flavi grew with Bacillus amyloliquefaciens UNRCLR. However, levels of detectable AFB(1) were significantly reduced in these interactions at a(w) 0.982.

Studying and modelling the combined effect of temperature and water activity on the growth rate of P. expansum

The effect of solutes, water activity (a(w), 0.890--0.980) and temperature (5--25 degrees C) on the mycelial growth rate of Penicillium expansum was evaluated. The growth rate dropped as the temperature and a(w) of the medium decreased. NaCl was the solute causing the greatest growth rate reduction, followed by glucose, glycerol and sorbitol. Statistical analysis of the results showed a significant effect of solute, a(w), temperature and combinations of two or three of these factors (P<0.0001). Whatever the solutes and a(w) values, the initiation of colony growth required an additional day at 15 degrees C and 5 degrees C as compared to initiation at 25 degrees C. Growth models based on the results obtained with sorbitol and glycerol differed only slightly, with R(2) values of 97.00% and 97.95%, respectively. The response surfaces of both quadratic polynomial models showed that P. expansum should be able to grow at low a(w) (0.890) and that growth at 25 degrees C should be fastest at a(w) values ranging from 0.960 to 0.980. Both models presented a good fit between predicted and observed values.

Accumulation of the compatible solutes, glycine-betaine and ectoine, in osmotic stress adaptation and heat shock cross-protection in the biocontrol agent Pantoea agglomerans CPA-2

AIMS

The effect of modifying the water activity (a(w)) of Pantoea agglomerans growth medium with the ionic solute NaCl on water stress resistance, heat-shock survival and intracellular accumulation of the compatible solutes glycine-betaine and ectoine were determined.

METHODS AND RESULTS

The bacterium was cultured in an unmodified liquid medium or that modified with NaCl to 0.98 and 0.97 a(w), and viability of cells evaluated on a 0.96 a(w)-modified solid media to check water stress tolerance. Cells grown under ionic stress had better water stress tolerance than control cells. These cells also had cross-protection to heat stress (30 min, 45 degrees C). The modified cells accumulated substantial amounts of the compatible solutes glycine-betaine and ectoine in contrast to the control cells, which contained little or none of these two compounds.

CONCLUSIONS

Improvement in osmotic and thermal tolerance of cells of the biocontrol agent P. agglomerans by modifying growth media with the ionic solute NaCl was achieved. The compatible solutes glycine-betaine and ectoine play a critical role in environmental stress tolerance improvement.

SIGNIFICANCE AND IMPACT OF THE STUDY

This approach provides a method for improving the physiological quality of inocula and could have implications for formulation and shelf-life of biocontrol agents.

Osmotically induced trehalose and glycine betaine accumulation improves tolerance to desiccation, survival and efficacy of the postharvest biocontrol agentPantoea agglomeransEPS125

The application of the biocontrol agent Pantoea agglomerans EPS125 to unwounded fruits was practically ineffective for control of postharvest blue mould caused by Penicillium expansum when the treatment and subsequent wounding and pathogen inoculation were separated by periods of unfavourable conditions. This was due to a rapid decrease in viability of the alocthonous introduced biocontrol agent in the intact peel surface. A system for osmoadaptation of the biocontrol agent was developed by combining saline osmotic stress and osmolyte amendment to the growth medium. Osmoadapted cells accumulated trehalose and glycine betaine (GB) intracellularly and showed a higher tolerance to desiccation than non-osmoadapted cells. Osmoadaptation in NaCl plus GB during inoculum preparation increased considerably survival on the peel surface of apple fruits. This effect was significant under low relative humidity (RH) and fluctuating RH conditions, but was not significant at high RH. Osmoadaptation significantly improved blue mould control under conditions where the standard biological control treatments were ineffective. The rot diameter was significantly reduced in apple fruits which were treated with EPS125 and incubated for several days under low, high or fluctuating RH, followed by wounding and inoculation of P. expansum. Growth of EPS125 with NaCl, either with or without the addition of GB, was an effective osmoadaptation treatment for improving blue mould rot control. However, the addition of GB to the NaCl amended growth medium increased 4-5-fold growth rate and OD of the cultures. This is an advantage for mass production of P. agglomerans EPS125 in a NaCl amended growth medium.

In vitro studies on the potential for biological control on Aspergillus section Flavi by Kluyveromyces spp

AIMS

Antagonist activity of Kluyveromyces spp. isolates on Aspergillus section Flavi was studied.

METHODS AND RESULTS

The screening of isolates were made through studies of growth at different water activities and temperatures, index of dominance (I(D)), ecological similarity, antifungal activity and impact on aflatoxin B1 accumulation. High optical density was obtained at 25 and 30 degrees C and 48 h of incubation. Cell growth decreases with decrease in water activity. The predominant interaction was mutual intermingling at a(w) = 0.982 and 0.955, while at a(w) = 0.999 and 0.937 mutual inhibition for contact was exhibited. All isolates were catabolically identical to Aspergillus section Flavi and compete by nutritional source. At high water activities yeasts showed inhibitory activity on Aspergillus strains, inhibition percentages varied between 75 and 100%. The isolates Y9, Y14, Y16, Y22, Y25 and Y33 showed antifungal activity and inhibitory activity on aflatoxin B1 accumulation at all water activities assayed from all Aspergillus section Flavi strains.

CONCLUSIONS

The data show that the isolates selected in a wide range of environmental conditions could exert their roll like biological control agents for Aspergillus section Flavi in storage maize ecosystem.

SIGNIFICANCE AND IMPACT OF THE STUDY

Isolates of Kluyveromyces spp. may have practical value in the postharvest control of storage maize.

Liquid Formulation of the Postharvest Biocontrol Agent Candida sake CPA-1 in Isotonic Solutions

ABSTRACT Viability of the postharvest biocontrol agent Candida sake CPA-1 stored as liquid formulation was evaluated by studying the effect of growth, preservation medium, and temperature. C. sake was grown in molasses medium with unmodified water activity (a(w)) and in the same with a(w) modified to 0.98 with the addition of several solutes. Cells were preserved with isotonic solutions of different substances. Efficacy of liquid formulations stored for different periods was tested against infection by Penicillium expansum on apples. The best growth media were the (unmodified one and those modified to 0.98 a(w) with the addition of glycerol or sorbitol. For all growth media, the best preservation medium was the isotonic solution prepared with trehalose. When the effect of trehalose concentration in the preservation medium was studied, generally, at trehalose concentrations below the isotonic one, C. sake viabilities increased with increased trehalose. However, the best results were obtained when cells were preserved with the trehalose solution which was isotonic with cells. After 7 months of storage at 4 degrees C, cells that were grown in the sorbitol-modified medium and preserved with the isotonic solution of trehalose (0.96 M) maintained their viability and efficacy against P. expansum infection of apples.

Liquid formulation of the biocontrol agent Candida sake by modifying water activity or adding protectants

AIMS

To evaluate the effect of modification of water activity (aw) and the addition of protective substances in the preservation medium of liquid formulations of the biocontrol agent Candida sake stored at 4 and 20 degrees C.

METHODS AND RESULTS

The aw of the preservation medium of C. sake was modified from 0.72 to 0.95 by adding glycerol or polyethylene glycol (PEG). Moreover, several protectant substances at different concentrations were evaluated. Modification of lower aw-levels (0.721-0.901) with glycerol did not maintain the viability of the yeast cells. Higher aw-levels (0.93-0.95) with either glycerol or PEG improved the viability but not at acceptable viability levels. C. sake cells maintained viabilities >60% when sugars, such as trehalose, and polyols, such as glycerol and PEG were used as protectants in liquid formulations. Moreover, liquid formulations of C. sake stored at 4 degrees C showed higher number of viable counts than at 20 degrees C. When different sugars were tested, all of them, except 10% fructose, resulted in a viability higher than 50% of the C. sake formulations. Biocontrol of liquid formulation treatments was similar to fresh cells in controlling Penicillium expansum on wounded apples.

CONCLUSIONS

Sugars such as lactose and trehalose could be considered as good protectants in order to obtain liquid formulations of C. sake cells as they maintain the viability >70% for 4 months at 4 degrees C.

SIGNIFICANCE AND IMPACT OF STUDY

This study shows that a suitable liquid formulation for commercial application can be produced with high viability and conservation of biocontrol efficacy. Moreover, if 10% lactose is the protectant used in the formulation, the economic costs would not be limiting for industrial production.

Improving water stress tolerance of the biocontrol yeast Candida sake grown in molasses-based media by physiological manipulation

The biocontrol agent Candida sake was cultured on either an unmodified molasses-based medium (water activity, a(w) 0.996) or on water stressed media produced by the addition of glycerol, glucose, NaCl, sorbitol, or proline to 0.98, and 0.96 a(w) for 24, 48, and 72 h, to study their impact on subsequent cell viability, and on concentrations of endogenous sugars (trehalose and glucose) and polyols (glycerol, erythritol, arabitol, and mannitol). The viability of cells of different ages cultured on these media was evaluated on NYDA medium with freely available water (a(w) 0.995), and on medium modified with polyethylene glycol to a(w) 0.95. Regardless of solute used, viable counts of cells grown on molasses-based medium (a(w) 0.98) were equal to or higher than those obtained from the medium with water freely available. The amino acid proline stimulated growth at 10% concentration. In contrast, water stress induced by addition of NaCl, glucose, or sorbitol at a(w) 0.96 caused a significant reduction in viable counts. Older cultures were more resistant to water stress. Glycerol and arabitol were the main solutes accumulated by C. sake cells in response to lowered a(w). Intracellular concentration of these polyols depended more on the solute used to adjust the a(w) than on the a(w) itself. Candida sake was more resistant to water stress with higher intracellular concentration of glycerol and erythritol.

Solute stresses affect growth patterns, endogenous water potentials and accumulation of sugars and sugar alcohols in cells of the biocontrol yeast Candida sake

AIM

To evaluate the effect of modifications of water activity (aw 0. 996-0.92) of a molasses medium with different solutes (glycerol, glucose, NaCl, proline or sorbitol) on growth, intracellular water potentials (psi(c)) and endogenous accumulation of polyols/sugars in the biocontrol yeast Candida sake.

METHODS AND RESULTS

Modification of solute stress significantly influenced growth, psi(c) and accumulation of sugars (glucose/trehalose) and polyols (glycerol, erythritol, arabitol and mannitol) in the yeast cells. Regardless of the solute used to modify aw, growth was always decreased as water stress increased. Candida sake cells grew better in glycerol- and proline-amended media, but were sensitive to NaCl. The psi(c) measured using psychrometry showed a significant effect of solutes, aw and time. Cells from the 0.96 aw NaCl treatment presented the lowest psic value (- 5.20 MPa) while cells from unmodified media (aw = 0. 996) had the highest value (- 0.30 MPa). In unmodified medium, glycerol was the predominant reserve accumulated. Glycerol and arabitol were the major compounds accumulated in media modified with glucose or NaCl. In proline media, the concentration of arabitol increased. In glycerol- and sorbitol-amended media, the concentration of glycerol rose. Some correlations were obtained between compatible solutes and psi(c).

CONCLUSIONS AND SIGNIFICANCE

This study demonstrates that subtle changes in physiological parameters significantly affect the endogenous contents of C. sake cells. It may be possible to utilize such physiological information to develop biocontrol inocula with improved quality.