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.

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.