Modes‐of‐action of antifungal compounds: Stressors and (target‐site‐specific) toxins, toxicants, or toxin–stressors

Abstract

Fungi and antifungal compounds are relevant to the United Nation's Sustainable Development Goals. However, the modes‐of‐action of antifungals—whether they are naturally occurring substances or anthropogenic fungicides—are often unknown or are misallocated in terms of their mechanistic category. Here, we consider the most effective approaches to identifying whether antifungal substances are cellular stressors, toxins/toxicants (that are target‐site‐specific), or have a hybrid mode‐of‐action as toxin–stressors (that induce cellular stress yet are target‐site‐specific). This newly described ‘toxin–stressor’ category includes some photosensitisers that target the cell membrane and, once activated by light or ultraviolet radiation, cause oxidative damage. We provide a glossary of terms and a diagrammatic representation of diverse types of stressors, toxic substances, and toxin–stressors, a classification that is pertinent to inhibitory substances not only for fungi but for all types of cellular life. A decision‐tree approach can also be used to help differentiate toxic substances from cellular stressors (Curr Opin Biotechnol 2015 33: 228–259). For compounds that target specific sites in the cell, we evaluate the relative merits of using metabolite analyses, chemical genetics, chemoproteomics, transcriptomics, and the target‐based drug‐discovery approach (based on that used in pharmaceutical research), focusing on both ascomycete models and the less‐studied basidiomycete fungi. Chemical genetic methods to elucidate modes‐of‐action currently have limited application for fungi where molecular tools are not yet available; we discuss ways to circumvent this bottleneck. We also discuss ecologically commonplace scenarios in which multiple substances act to limit the functionality of the fungal cell and a number of as‐yet‐unresolved questions about the modes‐of‐action of antifungal compounds pertaining to the Sustainable Development Goals.

Scientific novelty beyond the experiment

“…This classification gave rise to further work relating to cellular stresses imposed by the metabolic products of biofuel-producing microbes, including a decision tree to identify modes-of-action of inhibitory substances (Figure 1 of Cray, Stevenson, et al, 2015). This classification of antimicrobials has also formed the basis of other subsequent studies (e.g., Alves et al, 2015;Heinz et al, 2021;Noel et al, 2023;Yakimov et al, 2015).…”

“…This 1998 study re‐assessed terminology by re‐assigning ethanol from presumed toxin to being, in reality, a stressor (an issue explored in more detail in later studies; Cray, Stevenson, et al, 2015; Hallsworth, 2018; Hallsworth et al, 2003; Noel et al, 2023); and by challenging the assumption that water‐mediated cellular stresses are necessarily caused by osmotically active substances. J.E.H.…”

“…A more‐recent explanation of ethanol‐induced stress by the J.E.H. group was published by Cray et al (2015b) and the distinctions between toxicity and stress are examined in more detail in Noel et al (2023).…”

“…However, research activity in this field is now rapidly increasing, and the works based on that study (Supplementary Text 'Work that followed from a theorybased study of ethanol stress') have now been cited by ~3000 other papers (Google Scholar; August, 2022), used as the basis of further experimental studies (e.g.,Belilla et al, 2019;Cubillos et al, 2019;Heinz et al, 2022;Martínez et al, 2021;Rubin et al, 2017;Zajc et al, 2014), and feature in more than 50 textbooks.A more-recent explanation of ethanol-induced stress by the J.E.H. group was published byCray et al (2015b) and the distinctions between toxicity and stress are examined in more detail inNoel et al (2023).5 The term situational-functional describes functionality within any prevailing or given context, albeit this term is rarely used(McCammick et al, 2010; Tsukuba Language Group, 1995). 6 Artificial intelligence methodologies have also been successfully employed to predict the physical properties of proteins from their gene sequences.…”

The Pareto principle: To what extent does it apply to resource acquisition in stable microbial communities and thereby steer their geno−/ecotype compositions and interactions between their members?

“…Once this stable community was reached, daughter chemostats were created and subjected to a variety of intentional temporary perturbations, during which the composition of the community was observed to change substantially, but after which it always returned to that of the parent chemostat. The most instructive perturbation was an increase in the dilution rate beyond the metabolic capacity of the community, which resulted in the accumulation of toxic/stressful metabolites that included protoanemonin (Blasco et al, 1995(Blasco et al, , 1997, 4-chlorocatechol and 3-chloro-cis,cis-muconate, which poisoned the culture or induced stress through exerting chaotropicitymediated damage (Bhaganna et al, 2010;Noel et al, 2023).…”