## 4 Discussion  
_Beaurevia bassiana_ is a dimorphic and entomogenic fungus widely used for pest biocontrol [18]. Morphological transition in dimorphic fungi is crucial for pathogenicity but simultaneously defines its biology and prevalence in different environments [5]. In the present study, we examined the dimorphism of two _B. bassiana_ strains previously isolated from different environmental niches. Strain AS5 was isolated from soil and AI2 from a mycosed insect cadaver. According to [PERSON] et al. [28], AI2 was more lethal than AS5 when inoculated with conidia on larvae in stage L2 from the insect pest _Spodoptera frugiperda_. Notably, both strains killed the larvae, but the median lethal time was different. The results of the toxicity assays in _C. elegans_ showed that the cell-free medium obtained from the aerobic growth of AI2 in PDB increased nematode killing compared with that observed with AS5, suggesting an important difference between the fungal secretomes that may induce mortality in _C. elegans_ and _S. frugiperda_. The nematicidal effects of _B. bassiana_ are widely known and different cyclic depsieptides and proteins are directly related to this activity [33; 34; 35]. Hence, it would be interesting to perform a further analysis to reveal and compare the secreted metabolites of both strains to provide more information on the molecular players that participate in fungal colonization and virulence. It is noted that any change in the insect host, carbon/nitrogen sources, inoculum size, or other types of stimuli could accelerate or slow down the life cycle of _B. bassiana_ with direct repercussions on virulence [36; 37]. In the case of AS5, both the culture media YPG and YNB, along with oxygen availability and pH, modified conidial germination and mycelium morphology, which are important morphological attributes for viability, colonization, and persistence in different environments [4; 36; 38; 39; 40; 41; 42]. _B. bassiana_ thrives on a wide variety of substrates and pH ranges. AS5 showed low conidial germination at pH 3 and optimum mycelial growth at pH 6 on PDB medium under aerobic conditions. This is consistent with the results of previous studies [36; 38; 39]. Furthermore, pH 9 allowed for more branched mycelial growth. This modification in mycelial morphology may be the result of an adaptation to alkaline conditions [41]. This could represent an advantage for the saprophytic colonization of alkaline soils. The relationship between morphological characteristics such as conidial germination speed, growth rate, and virulence capacity is controversial in _B. bassiana_[39; 40; 41; 42; 43; 44]. Our observations using the MacConkey agar culture medium showed that the more virulent strain AI2 developed faster than AS5. Notably, AI2 switched faster between the two morphologic forms than AS5 did. Using an inoculum size of \(10^{6}\) spores, a higher germination response and earlier dimorphism were observed in AI2 than in AS5, thus showing varying development with consequential differences in the timing of the dimorphic change. The speed of the dimorphic transition was dependent on the inoculum size. The highest inoculum size of \(10^{7}\) enabled the transition of yeast cells to hyphal growth as early as 2 d post inoculation for both strains, whereas with a lower inoculum size of 1000 or less, only AI2 formed mycelia on day 6. These results suggest that the dimorphic transition speed is dependent on the threshold cell population density and the production of chemical signals that trigger yeast-to-mycelium conversion, as seen in other dimorphic fungi [2; 3; 5; 45; 46]. Furthermore, the speed in the transition of AI2 is a feature that could be associated with its increased virulence against _S. frugiperda_. The QS system in entomopathogenic fungi has previously been reported in _Metarhizium rileyi_ and _Ophicocordyceps sinensis_[46; 47]. According to these studies, the production of QS signals occurs in the hemolymph in response to a quorum of hyphal bodies during the advanced stages of insect infection.