Production and quality of conidia by Metarhizium anisopliae var. lepidiotum: critical oxygen level and period of mycelium competence

Unraveling the interaction of co-encapsulated Saccharomyces cerevisiae and Metarhizium brunneum in calcium alginate-based attract-and-kill beads

BACKGROUND

Attract-and-kill (AK) beads are biological, microbial insecticides developed as an alternative to synthetic soil insecticides. For wireworm control, beads are based on calcium alginate/starch co-encapsulating the carbon dioxide (CO2) producing yeast Saccharomyces cerevisiae H205 as the attract component, and the entomopathogenic fungus Metarhizium brunneum CB15-III as the kill component. However, the physicochemical processes inside beads during co-cultivation are still unclear. Here we reveal for the first time the spatiotemporal conditions of oxygen and pH inside AK beads measured with microelectrodes and describe the impact of S. cerevisiae on CO2 and conidia formation.

RESULTS

Measurements revealed a steep oxygen gradient already 2 days after co-encapsulation, with an internal hypoxic zone. Encapsulating either S. cerevisiae or M. brunneum already decreased the average pH from 5.5 to 4.7 and 4.6, respectively. However, on day 3, co-cultivation lead to temporal strong acidification of beads down to pH 3.6 which followed the maximum CO2 productivity and coincided with the maximum conidiation rate. Decreasing the yeast load decreased the total CO2 productivity to half, and the conidial production by 93%, while specific productivities normalized to 1% yeast load increased eight-fold and three-fold, respectively, with day 3 being an exception.

CONCLUSION

Our findings indicate a general beneficial interaction between M. brunneum and S. cerevisiae, but also suggest competition for resources. These findings will contribute to develop innovative co-formulations with maximum efficiency to save application rates and costs. © 2024 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Asphyxiation of Metarhizium robertsii during mycelial growth produces conidia with increased stress tolerance via increased expression of stress-related genes

Little is known about the impact of hypoxia and anoxia during mycelial growth on tolerance to different stress conditions of developing fungal conidia. Conidia of the insect-pathogenic fungus Metarhizium robertsii were produced on potato dextrose agar (PDA) medium under normoxia (control = normal oxygen concentrations), continuous hypoxia, and transient anoxia, as well as minimal medium under normoxia. The tolerance of the conidia produced under these different conditions was evaluated in relation to wet heat (heat stress), menadione (oxidative stress), potassium chloride (osmotic stress), UV radiation, and 4-nitroquinoline-1-oxide (=4-NQO genotoxic stress). Growth under hypoxic condition induced higher conidial tolerance of M. robertsii to menadione, KCl, and UV radiation. Transient anoxic condition induced higher conidial tolerance to KCl and UV radiation. Nutritional stress (i.e., minimal medium) induced higher conidial tolerance to heat, menadione, KCl, and UV radiation. However, neither of these treatments induced higher tolerance to 4-NQO. The gene hsp30 and hsp101 encoding a heat shock protein was upregulated under anoxic condition. In conclusion, growth under hypoxia and anoxia produced conidia with higher stress tolerances than conidia produced in normoxic condition. The nutritive stress generated by minimal medium, however, induced much higher stress tolerances. This condition also caused the highest level of gene expression in the hsp30 and hsp101 genes. Thus, the conidia produced under nutritive stress, hypoxia, and anoxia had greater adaptation to stress.

Biocontrol of Xyleborus affinis (Curculionidae: Scolitinae) Females and Progeny by Beauveria bassiana (Hypocreales: Cordycipitaceae) in a Sawdust Artificial Diet Model

The ambrosia beetle Xyleborus affinis, recently reported affecting avocado trees in Mexico, represents one of the most widespread insects worldwide. Previous reports have shown that Xyleborus genera members are susceptible to Beauveria bassiana and other entomopathogenic fungus strains. However, their effect on borer beetles' progeny has not been fully investigated. The aim of the present study was to determine the insecticidal activity of B. bassiana on X. affinis adult females and their progeny in an artificial sawdust diet bioassay model. The B. bassiana strains CHE-CNRCB 44, 171, 431, and 485 were individually tested on females at concentrations ranging from 2 × 10⁶ to 1 × 10⁹ conidia mL^-1. After 10 d of incubation, diet was evaluated to count laid eggs, larvae, and adults. Insect conidia loss after exposure was determined by attached conidia to each insect after 12 h of exposure. The results showed that females' mortality ranged between 3.4% and 50.3% in a concentration-response manner. Furthermore, we did not observe statistical differences among strains at the highest concentration. CHE-CNRCB 44 showed the highest mortality at the lowest concentration and reduced larvae and laid eggs at the highest concentration (p < 0.01). Strains CHE-CNRCB 44, 431, and 485 significantly decreased larvae, as compared with the untreated control. After 12 h, up to 70% of conidia was removed by the effect of the artificial diet. In conclusion, B. bassiana has the potential to control X. affinis adult females and progeny.

Improvement of beauvericin production by Fusarium oxysporum AB2 under solid-state fermentation using an optimised liquid medium and co-cultures

The production of beauvericin (BEA) by Fusarium oxysporum AB2 in liquid medium (SmF) was compared to that on solid medium (SSF) on inert support (polyurethane foam or PUF), using a previously optimised medium. The analysis included two different concentrations of the medium (1 × and 3 ×). Under SSF, the production of BEA (22.8 mg·L^-1) was higher relative to SmF (0.8 mg·L^-1). The production increased proportionally in the concentrated medium (3 ×) (65.3 mg·L^-1); using the concentrated medium in SmF, the production of BEA was completely inhibited, although more biomass was produced. The peak of BEA production was reached on day 7 and remained stable until day 11; sustained production after several days has not been achieved in similar reports. The presence of BEA was corroborated by high-performance liquid chromatography (HPLC) and mass spectrometry. The BEA production profile is shown performing mixed cultures of Fusarium oxysporum AB2 and Epicoccum nigrum TORT using the same system, increasing the production of BEA up to 84.6 mg·L^-1. We propose SSF using polyurethane foam (PUF) as a solid support as a new culture system for obtaining secondary metabolites such as BEA.

Formalin-casein enhances water absorbency of calcium alginate beads and activity of encapsulated Metarhizium brunneum and Saccharomyces cerevisiae

The control of root-feeding wireworms has become more challenging as synthetic soil insecticides have been progressively phased out due to environmental risk concerns. Innovative microbial control alternatives such as the so-called attract-and-kill strategy depend on the rapid and successful development of dried encapsulated microorganisms, which is initiated by rehydration. Casein is a functional additive that is already used in food or pharmaceutical industry due to its water binding capacity. Cross-linked forms such as formalin-casein (FC), exhibit altered network structures. To determine whether FC influences the rehydration of alginate beads in order to increase the efficacy of an attract-and-kill formulation for wireworm pest control, we incorporated either casein or FC in different alginate/starch formulations. We investigated the porous properties of alginate/starch beads and subsequently evaluated the activities of the encapsulated entomopathogenic fungus Metarhizium brunneum and the CO₂ producing yeast Saccharomyces cerevisiae. Adding caseins altered the porous structure of beads. FC decreased the bead density from (1.0197 ± 0.0008) g/mL to (1.0144 ± 0.0008) g/mL and the pore diameter by 31%. In contrast to casein, FC enhanced the water absorbency of alginate/starch beads by 40%. Furthermore, incorporating FC quadrupled the spore density on beads containing M. brunneum and S. cerevisiae, and simultaneous venting increased the spore density even by a factor of 18. Moreover, FC increased the total CO₂ produced by M. brunneum and S. cerevisiae by 29%. Thus, our findings suggest that rehydration is enhanced by larger capillaries, resulting in an increased water absorption capacity. Our data further suggest that gas exchange is improved by FC. Therefore, our results indicate that FC enhances the fungal activity of both fungi M. brunneum and S. cerevisiae, presumably leading to an enhanced attract-and-kill efficacy for pest control.

Improved conidiation from entomopathogenic fungi through 26% oxygen pulses in solid-state culture depends on a balance between headspace volume and substrate amounts

Oxygen-enriched atmospheres applied as periodic pulses increased conidia production from entomopathogenic fungi in agar surface cultures. However, this advantage has not been obtained in solid-state cultures (SSC), probably as a result of different biomass production between both culture systems. In this work, the biomass formation from two Isaria strains was limited in SSC using 5, 2·5 and 1 initial grams of substrate (gds). In the system with 5 gds, conidia production decreased in 26% oxygen-enriched pulses compared to the normal atmosphere. Conversely, 26% oxygen pulses increased conidiation up to one order magnitude in systems with 2·5 and 1 gds, respective to the normal atmosphere. These results were explained by oxygen depletion and high CO₂ accumulation in the 5 gds system. Whereas in systems with 2·5 or 1 gds, oxygen levels remained high enough to stimulate conidiation. These results were attributed to the headspace volume:gds ratio, which is suggested to be ≥48 ml per gds. This ratio is proposed as a scaling-up criterion for bioreactor design when oxygen-enriched pulses are used in SSC for improvement of conidia production. SIGNIFICANCE AND IMPACT OF THE STUDY: Oxygen-enriched atmospheres applied as periodic pulses increase conidiation in entomopathogenic fungi (EF). However, this remained restricted to agar surface cultures, since conidiation decreased when carried out in solid-state culture (SSC) which is used as large-scale production system. We identified that in SSC the ratio between the headspace volume containing 26% oxygen-enriched pulses and the grams of substrate determines the conidiation response to oxygen-enriched pulses. For the first time, oxygen-enriched pulses increased conidiation in SSC respective to the normal atmosphere in four EF. This ratio is proposed as a bioreactor criterion design for large-scale conidia production of EF using oxygen-enriched pulses.

Transient anoxia during Metarhizium robertsii growth increases conidial virulence to Tenebrio molitor

Little is known about the phenotypic effects of hypoxia and transient anoxia on the virulence of an entomopathogenic fungus. Conidia of Metarhizium robertsii were produced on: (1) potato dextrose agar medium (PDA) under normoxia; (2) PDA medium under continuous hypoxia; (3) PDA medium under transient anoxia; and (4) minimal medium with lactose (MML) under normoxia. Conidia produced under transient anoxia and produced on MML were the most virulent to Tenebrio molitor. Conidia produced under normoxia and hypoxia were the least virulent. Conidial production and germination speed of conidia produced under normoxia, hypoxia, and transient anoxia were similar; however, MML produced less conidia, but germinated faster than any other treatments.

Comparison between superficial and solid-state cultures of Isaria fumosorosea: conidial yields, quality and sensitivity to oxidant conditions

Conidia production and quality from mycoinsecticides in solid-state cultures (SSC) are frequently inferred from superficial culture (SC) results. Both parameters were evaluated for two Isaria fumosorosea strains (ARSEF 3302 and CNRCB1), in SC and SSC, using culture media with the same chemical composition. For both strains, conidia production was higher in SC than SSC in terms of conidia per gram of dry substrate. Germination in both strains did not show significant differences between SC and SSC (>90 %). Similarly, conidia viability in ARSEF 3302 strain did not show differences at early stages between SC and SSC, but was higher in SC compared to SSC in the late stage of culture; in contrast, conidia from CNRCB1 strain did not differ between both culture systems. Some infectivity parameters improved in conidia from SSC, compared to SC at the early stages, but these differences disappeared at the final stage, independently of the strain. Both strains showed decreased conidia production when 26 % O2 pulses were applied; nevertheless, conidiation in SSC was two orders of magnitude more sensitive to oxidant pulses. In SC with 26 % O2 pulses, conidia viability for both strains at early stages, was higher than in normal atmospheric conditions. Infectivity towards Galleria mellonella larvae was similar between conidia from normal atmosphere and oxidant conditions; notably, for the strain ARSEF 3302 infectivity decreased at the final stage. This study shows the intrinsic differences between SC and SSC, which should be considered when using SC as a model to design production processes in SSC.

Reactive oxygen species production, induced by atmospheric modification, alter conidial quality of Beauveria bassiana

AIM

The aim of this study was to determine the relationship between reactive oxygen species (ROS) production and conidial infectivity in Beauveria bassiana.

METHODS AND RESULTS

Beauveria bassiana Bb 882.5 was cultured in solid-state culture (SSC) using rice under three oxygen conditions (21%, or pulses at 16 and 26%). Hydrophobicity was determined using exclusion phase assay. Bioassays with larvae or adults of Tenebrio molitor allowed the measurements of infectivity parameters. A fluorometric method was used for ROS quantification (superoxide and total peroxides). NADPH oxidase (NOX) activity was determined by specific inhibition. Conidial hydrophobicity decreased by O2 pulses. Mortality of larvae was only achieved with conidia harvested from cultures under 21% O2 ; whereas for adult insects, the infectivity parameters deteriorated in conidia obtained after pulses at 16 and 26% O2 . At day 7, ROS production increased after 16 and 26% O2 treatments. NOX activity induced ROS production at early stages of the culture.

CONCLUSION

Modification of atmospheric oxygen increases ROS production, reducing conidial quality and infectivity.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first study in which conidial infectivity and ROS production in B. bassiana has been related, enhancing the knowledge of the effect of O2 pulses in B. bassiana.

Conidiation of Penicillium camemberti in submerged liquid cultures is dependent on the nitrogen source

OBJECTIVE

To study the ability of a commercial Penicillium camemberti strain, used for Camembert type cheese ripening, to produce conidia during growth in liquid culture (LC), in media containing different sources of nitrogen as, industrially, conidia are produced by growth at the surface of a solid state culture because conidiation in stirred submerged aerobic LC is not known.

RESULTS

In complex media containing peptic digest of meat, hyphae ends did not differentiate into phialides and conidia. Contrarily, in a synthetic media containing KNO3 as sole nitrogen source, hyphae ends differentiated into phialides producing 0.5 × 10(7) conidia/ml. Conidia produced in LC were 25 % less hydrophobic than conidia produced in solid culture, and this correlates with a seven-times-lower expression of the gene rodA encoding hydrophobin RodA in the mycelium grown in LC.

CONCLUSION

Conidiation of P. camembertii is stimulated in iquid medium containing KNO3 as sole source of nitrogen and therefore opens up opportunities for using liquid medium in commercial productions.