Pathogenic fungi pose an enormous global threat to humanity, leading to millions of deaths and substantial financial losses annually (Fisher et al., 2012; Rhodes, 2019). Morbidity and mortality rates from opportunistic fungal pathogens, such as Candida albicans, Aspergillus fumigatus, and Cryptococcus neoformans, have been increasing for some years, especially in immunocompromised patients (Pal, 2017; Linder et al., 2019; de Sousa-Neto et al., 2020). Addressing the pathogenesis of these fungal pathogens and finding controllable strategies are crucial and urgent. To tackle this threat, model organisms are required to conduct research focusing on the identification of virulence factors, screening of effective antifungal agents, and exploring appropriate treatment approaches.

Several model organisms have been adopted for studying of dimorphic and filamentous pathogenic fungi, including invertebrate models such as Drosophila melanogaster (Lamaris et al., 2008; Regulin and Kempken, 2018; Sampaio et al., 2018; Wurster et al., 2019), Galleria mellonella (Gomez-Lopez et al., 2014; Long et al., 2018; Silva et al., 2018; Staniszewska et al., 2020), Bombyx mori (Matsumoto et al., 2013; Uchida et al., 2016; Nakamura et al., 2017; Matsumoto and Sekimizu, 2019), Caenorhabditis elegans (Okoli and Bignell, 2015; Song et al., 2019; Wong et al., 2019; Ahamefule et al., 2020a), and vertebrate models such as mice (Fakhim et al., 2018; Skalski et al., 2018; Wang et al., 2018; Mueller et al., 2019), guinea pigs (Vallor et al., 2008; Nadăş et al., 2013; Garvey et al., 2015), and zebrafish (Chen et al., 2015; Knox et al., 2017; Koch et al., 2019; Kulatunga et al., 2019).

C. elegans is a microscopic multicellular nematode that lives freely in soil (Muhammed et al., 2012; Kim et al., 2017). Advantages, such as short life cycle, physiological simplicity, transparent body, complete sequenced genome, mature genetic manipulation system, and no requirement for ethical license, have greatly encouraged the wide adoption of this nematode as a model organism in scientific research with assorted applications across several research fields (Okoli et al., 2009; Ballestriero et al., 2010; Huang et al., 2014; Jiang and Wang, 2018). Some of these applications have been established for decades now whereas others are still in their nascent stages undergoing several studies. Nematode infection by the natural nematophagous obligate filamentous fungus Drechmeria coniospora is a common incidence in nature. C. elegans is usually applied for studying the innate immunity of nematodes to this fungus (Engelmann et al., 2011; Couillault et al., 2012; Zugasti et al., 2016). This nematode model has also been explored as an in vivo model for studying infections of human pathogenic filamentous fungi (Okoli and Bignell, 2015; Ahamefule et al., 2020a).

Application of the nematode model for dimorphic pathogenic fungi studies has resulted in numerous publications whereas only a few publications thus far have been recorded for human filamentous pathogenic fungi studies, such as A. fumigatus (Okoli and Bignell, 2015; Ahamefule et al., 2020a; Eldesouky et al., 2020a). Here, we have extensively portrayed C. elegans-dimorphic fungi (in particular Candida spp.) infection models for determining virulence factors (reported within the last decade) and evaluated the effectiveness of anticandidal agents, including drugs, bioactive compounds, and live biotherapeutic products (reported within the last 5 years). The practical challenges constraining the applications of the C. elegans model for filamentous fungi are elaborated, and possible solutions are raised for future improvement.

C. elegans has been extensively used for studying several dimorphic fungi of clinical relevance. The most devastating and pathogenic dimorphic fungus that has been adequately explored with this nematode model is Candida albicans (Hans et al., 2019a; Hans et al., 2019b; Song et al., 2019; Venkata et al., 2020) and a few other non-albicans species such as C. tropicalis (Brilhante et al., 2016; Feistel et al., 2019; Pedroso et al., 2019), C. krusei (De Aguiar Cordeiro et al., 2018; Kunyeit et al., 2019), and C. auris (Eldesouky et al., 2018a; Mohammad et al., 2019). Another important clinical dimorphic fungus, Taloromyces (Penicillium) marneffei, has also been studied in a C. elegans model for both virulence tests and antifungal agent efficacy evaluations (Huang et al., 2014; Sangkanu et al., 2021).

Virulence factors of C. albicans such as genes involved in hyphal filamentation and biofilm formation (Romanowski et al., 2012; Sun et al., 2015; Holt et al., 2017), intestinal adhesion and colonization (Rane et al., 2014a; Muthamil et al., 2018; Priya and Pandian, 2020), important virulence enzymes (Ortega-Riveros et al., 2017; Song et al., 2019), transcription factors (Jain et al., 2013; Hans et al., 2019a), and environmental and nutrient factors (Hammond et al., 2013; Lopes et al., 2018; Hans et al., 2019b; Wong et al., 2019) have been identified in a C. elegans model to strengthen our understanding of the in vivo pathogenesis of this important fungal pathogen ( Table 1 ). The virulence traits of some other non-albicans species (both dimorphic and nondimorphic) have also been investigated with this nematode model ( Table 1 ). Similarly, virulence factors such as pigmentation and hyphal filamentation have been demonstrated to be critical pathogenic features of T. marneffei in a C. elegans infection model (Huang et al., 2014; Sangkanu et al., 2021). C. elegans glp-4; sek-1 worms have mostly been used in these studies (aside from the wild-type strain, N2) because of their inability to produce progeny at 25°C due to the glp-4 mutation and their susceptibility to pathogens due to sek-1 mutation, thus making the worms immunocompromised for infection by opportunistic human fungi (Huang et al., 2014; Okoli and Bignell, 2015; Ahamefule et al., 2020a)

Moreover, the adoption of a C. elegans model for searching and screening of effective bioactive compounds against several species of Candida has also received much attention. Effective bioactive compounds from marine habitats (Subramenium et al., 2017; Ganesh Kumar et al., 2019), plant parts (Shu et al., 2016; Pedroso et al., 2019), and other sources ( Table 2 ) have been discovered because of their in vivo efficacies against several Candida species and were simultaneously evaluated for their cytotoxicity in a C. elegans model. Compounds such as alizarin, chrysazin, sesquiterpene, and purpurin were discovered to be quite effective in in vivo assays with effective doses ranging from 1 to 10 µg/ml ( Table 2 ), indicating potential future prospects for antifungal drug research and discovery. Other compounds such as thymol (Shu et al., 2016), coumarin (Xu et al., 2019), and theophylline (Singh et al., 2020), were only effective at high concentrations of 64, 2, and 1.6 mg/ml, respectively ( Table 2 ). Most of these compounds were certified as nontoxic at such effective concentrations as they were able to rescue infected nematodes and significantly elongated their lifespan ( Table 2 ).

The drug resistance threat of Candida species, similar to most other pathogens, is constantly increasing, leading to increased incidences of mortality and morbidity (Sanguinetti et al., 2015; Popp et al., 2017; Popp et al., 2019; Prasad et al., 2019). C. elegans has also proven to be an effective in vivo model for studying the infection of several azole-resistant C. albicans (Chang et al., 2015; Sun et al., 2018) and C. auris (Eldesouky et al., 2018a; Eldesouky et al., 2018b) strains. Studies have demonstrated the in vivo efficacy of some bioactive compounds applied singly or in combination with initially resistant antifungal drugs in the treatment of infected nematodes ( Table 3 ).

Compounds such as 2-(5,7-dibromoquinolin-8-yl)oxy)-N′-(4-nitrobenzylidene) acetohydrazide (Elghazawy et al., 2017; Mohammad et al., 2018) and phenylthiazole small molecules (Mohammad et al., 2019) are among the recently demonstrated effective compounds with good outcomes in nematode candidiasis (with effective dose concentrations of ≥4 and ≥5 µg/ml, respectively) against fluZ-resistant C. albicans and/or C. auris ( Table 3 ). The combination of caffeic acid phenethyl ester (CAPE) and fluZ (Sun et al., 2018) as well as the sulfamethoxazole and voriconazole (voZ) combination (Eldesouky et al., 2018a) effectively rescued C. elegans worms infected by azole-resistant C. albicans and C. auris, respectively ( Table 3 ).

The search for alternative treatment drugs with new inhibition mechanisms against pathogenic fungi such as C. albicans is a pressing need. Obtaining effective compounds that may not necessarily have a direct effect on Candida planktonic cells but affect critical virulence factors has recently been made possible by evaluating the efficacy of the compounds in a C. elegans infection model (Graham et al., 2017; Subramenium et al., 2017; Manoharan et al., 2018) ( Table 4 ).

Remarkably, some compounds such as loureirin A (Lin et al., 2019), camphor, and fenchyl alcohol (Manoharan et al., 2017b) are effective compounds protecting infected worms at concentration doses less than the in vitro MICs ( Table 4 ). Cascarilla bark oil, α-longipinene, linalool (Manoharan et al., 2018), and Enterococcus faecalis bacteriocin (EntV) (Graham et al., 2017) were reported to be quite potent in rescuing infected worms at low effective concentration doses, such as ≥0.001% for cascarilla bark oil, α-longipinene and linalool and 0.1 nM for EntV ( Table 4 ).

These compounds usually rescue infected nematodes through other pathways such as direct effects on cardinal virulence factors and/or by stimulating/enhancing the immune responses of the host against pathogens (Okoli et al., 2009; Peterson and Pukkila-Worley, 2018; Ahamefule et al., 2020b). Such compounds may only be screened and identified through in vivo assays since they usually show little or no antimicrobial activities in in vitro assays. The adoption of simple in vivo models such as C. elegans significantly supports the screening and identification of more such compounds, which may expand the narrative of the usual antifungal therapies that primarily address direct effects on causative pathogens.

The application of live biotherapeutic products (LBPs) consisting mainly of probiotics is another alternative approach for the treatment of nematode candidiasis. Such alternative therapy is an interesting and promising option since pathogenic fungi are currently developing resistance to the few clinically available antifungal drugs (Sanguinetti et al., 2015; Prasad et al., 2019). Several species of Lactobacillus such as L. rhamnosus (Poupet et al., 2019a; Poupet et al., 2019b) and L. paracasei (de Barros et al., 2018) as well as probiotic yeasts—Saccharomyces cerevisiae and Issatchenkia occidentalis (Kunyeit et al., 2019)—have demonstrated efficient rescue of worms infected with a number of Candida species. These therapeutic microorganisms drastically reduced the burden of the pathogens in the C. elegans intestine approximately 2 to 4 h postinfection treatment ( Table 5 ).

The efficacy of these LBPs in reducing and/or eliminating fungal burden implies the future potential of LBPs in addressing the fungal menace. The demonstrated significant increase (p < 2 × 10⁻¹⁶) in worm mean lifespan (Poupet et al., 2019a; Poupet et al., 2019b) is so high that it has not been reported in any potent bioactive compounds or even established antifungal drugs. The fact that most of these LBPs are already established probiotics is yet another important parameter that would advance future research beyond nematode models.

The in vivo efficacy of known antifungal drugs and a number of repurposed drugs have also been applied in the treatment of nematode candidiasis. Several azoles (Souza et al., 2018; Hernando-Ortiz et al., 2020), echinocandins (Souza et al., 2018), polyenes—particularly amphotericin B (Hernando-Ortiz et al., 2020), and β-lactam antibiotics (in combination with vancomycin) (De Aguiar Cordeiro et al., 2018) have been evaluated for their in vivo efficacy at varying effective concentrations in rescuing worms infected with Candida species ( Table 6 ). Synthesized azole drugs, such as 1-(4-cyclopropyl-1H-1,2,3-triazol-1-yl)-2-(2,4-difluorophenyl)-3-(1H-1,2,4-triazol-1-yl) propan-2-ol, have also been evaluated for both efficacy and cytotoxicity in a C. elegans model (Chen et al., 2017).

Given that decades of searching for new antifungal agents have not truly resulted in new antifungal drugs, drug repurposing is a less expensive and welcome research prospect. The C. elegans infection model for evaluating the efficacy of repurposed drugs on candidiasis has attracted attention (Eldesouky et al., 2020b; Singh et al., 2020) ( Table 6 ) due to the advantages of saving extensive time, cumbersome labor, and enormous cost of searching and obtaining new antifungal drugs.

One of the biggest challenges usually encountered in the applications of the C. elegans model for filamentous fungal infection is the difficulty in infecting the worms through conidia. Worms usually avoid eating conidia unless they starve with no other option (Okoli and Bignell, 2015). This avoidance is unlike the case of dimorphic fungal and bacterial pathogens, where infection is never much of a problem as worms easily feed on the cells of these pathogens when they replace or are mixed up with nematode choice food (E. coli OP50 or HB101) (Breger et al., 2007; Johnson et al., 2009; Kirienko et al., 2013; Okoli and Bignell, 2015).

Giving the worms more time to starve and more access to the conidia (placed at four cardinal points) for ingestion is very important for establishing mold preinfection assays. Okoli and Bignell (2015) adopted a 12-h preinfection technique, while we modified to 16 h (Ahamefule et al., 2020a). The fact is that worms must be given such ample time to “force” them to ingest the mold conidia in a preinfection system since coinfection approach (which is usually adopted for most dimorphic fungi modeling) cannot work well for mold pathogens (Okoli et al., 2009; Okoli and Bignell, 2015; Ahamefule et al., 2020a). As conidia germinate very fast even before the worms have ingested enough spores in killing assay medium, a relatively less nutritious medium was adopted for pre-infection assay to avoid the quick growth and flooding of hyphal filaments in the rich killing assay medium (brain heart infusion medium); otherwise later experimental procedures will be severely limited (Okoli and Bignell, 2015; Ahamefule et al., 2020a).

Another challenging aspect in setting up the C. elegans-mold model is the separation of noningested conidia from worms after pre-infection stage. Failure at this stage leads to the germination of unseparated spores in killing or antifungal screening media thus obstructing experimental progress. Although our designed membrane-attached-on-tube filter (with a 35-µm pore diameter) was able to remove a great deal of noningested conidia, the separation was not 100% efficient. Modifying the membrane pore size to an appropriate diameter should help improve the filtration efficiency by allowing faster and better removal of conidia while keeping the preinfected L4/young adult worms ( Figure 1 ). Even though the separation efficiency of noningested spores becomes 100% or close to it, hyphae growth in killing medium would still not be completely eliminated, particularly if the experiment is scheduled to go beyond 72 h postinfection. This is because we have discovered that some conidia could be egested out of the nematode intestine into the killing medium and still retain their viability of germinating to hyphae, which is a big challenge to tackle and severely affect the experiment.

Hyphal filamentation usually occurs in infected worms. Unlike most studied dimorphic fungi whose external hyphae protrude when worms were already dead (and could therefore be easily transferred), numerous worms infected with filamentous fungi such as A. fumigatus (Ahamefule et al., 2020a), A. flavus, and some strains of Penicillium (that we have studied in our laboratory), were discovered to still be alive with protruded hyphae. This makes these worms stuck to the killing assay plates and therefore difficult to remove (Ahamefule et al., 2020a). Such filamentation usually becomes profuse, growing and spreading very fast and may eventually obstruct visibility and affect the experimental results. Regulating the number of immunocompromised worms in killing assays, especially for highly virulent pathogenic molds, is an option to ameliorate this menace ( Figure 1 ).

The tremendous health hazards of pathogenic fungi cannot be overemphasized. Better understanding of in vivo pathogeneses and identification of virulence factors are urgent and imperative to fight against these fungi. Screening, identifying and repurposing effective compounds/drugs against them as well as obtaining and optimizing effective treatment alternatives are desirable at this time. Therefore, developing, optimizing and applying better modelling organisms such as C. elegans is meaningful not only for dimorphic fungi but also for mold pathogens. Our review of the breakthrough applications of C. elegans for dimorphic fungi studies and progress/modifications of the C. elegans-mold infection model will provide a reference for studying fungal infections and developing antifungal agents.

CA and BE wrote the initial manuscript. JO, AM, AI, BW, CJ, and WF revised the manuscript. WF supervised the manuscript. All authors have read and agreed to the published version of the manuscript.

This work was supported by National Natural Science Foundation of China (31960032, 32071279), Guangxi Natural Science Foundation (2020GXNSFDA238008) to WF, Research Start-up Funding of Guangxi Academy of Sciences (2017YJJ026) to BW, and Bagui Scholar Program Fund (2016A24) of Guangxi Zhuang Autonomous Region to CJ.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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