Dermatophytes develop resistance to the monoterpenes geraniol and citronellol

Productos naturales


Dermatophytes develop resistance to the monoterpenes geraniol and citronellol

Desarrollo de resistencia de los dermatofitos a los monoterpenos geraniol y citronelol


Gustavo Nunes Cardoso1
Kaltz Victor Souza Silva1
Maria Islaine de Oliveira Lima1
Juliana Moura Mendes Arrua2
Fillipe de Oliveira Pereira1

1Universidade Federal de Campina Grande, Laboratório de Bioquímica, Unidade Acadêmica de Saúde, Centro de Educação e Saúde. Cuité, Brasil.
2Universidad Nacional de Asunción Laboratorio de Biotecnología, Centro Multidisciplinario de Investigaciones Tecnológicas, Dirección General de Investigación Científica y Tecnológica. San Lorenzo, Paraguay.


Introduction: Dermatophytoses are fungal infections whose treatment has been the subject of much concern throughout the world. The emergence of antifungal resistance (azoles) in clinical therapeutics is well known. However, few studies demonstrate the capacity of dermatophytes to develop resistance against natural products such as terpenes. The monoterpenes geraniol and citronellol have recognized antifungal potential and are found in various essential oils.
Objectives: To investigate the capacity of Trichophyton rubrum, Microsporum canis, and Microsporum gypseum to acquire resistance against citronellol and geraniol.
Methods: The minimum inhibitory concentration of the tested drugs was determined by microdilution. The fungal strains were subjected to eight successive subcultures in Sabouraud dextrose agar containing the monoterpenes in sub-inhibitory concentrations. After this period of adaptation, the susceptibility profile to drugs was assessed by microdilution. Finally, after eight passages in culture medium without the drugs, resistance stability was again evaluated by microdilution test.
Results: From the initial citronellol and geraniol minimum inhibitory concentration values we observed an increase. In addition, a cross over effect was observed against the azole compounds. Finally, the fungi reversed their profiles of resistance against the natural drugs and showed no reversal in resistance to azoles. Although in dermatophytes the development of resistance to monoterpenes was observed, the phenomenon was not stable, as was observed against the azole drugs.
Conclusions: Citronellol and geraniol were active against resistant isolates even after dis-habituation. Thus these monoterpenes present themselves as potential therapeutic alternatives with fewer complications in the emergence of resistance.

Keywords: resistance; antifungal; habituation; terpenes; azoles.


Introducción: Las dermatofitosis son infecciones fúngicas cuyo tratamiento es motivo de preocupación en todo el mundo. En la terapéutica clínica, la aparición de la resistencia antifúngica (azoles) es muy conocida. Sin embargo, pocos estudios han demostrado la capacidad de los dermatofitos de desarrollar resistencia a productos naturales como terpenos. Los monoterpenos geraniol y citronelol tienen un reconocido potencial antifúngico y se encuentran en diversos aceites esenciales.
Objetivo: Investigar la capacidad de Trichophyton rubrum, Microsporum canis y Microsporum gypseum para desarrollar resistencia a los monoterpenos geraniol y citronelol.
Métodos: Se determinó, mediante microdilución, la concentración inhibitoria mínima de las drogas sometidas a ensayo. Las cepas fúngicas fueron sometidas a ocho subcultivos en agar a base de dextrosa Sabouraud que contenía concentraciones subinhibitorias de los monoterpenos ensayados. Pasado este periodo de adaptación, se evaluó el perfil de susceptibilidad por microdilución. Finalmente, después de ocho pases en medio de cultivo sin las drogas, se evaluó la estabilidad de la resistencia nuevamente mediante la prueba de microdilución.
Resultados: Se observó un incremento a partir de la concentración inhibitoria mínima inicial del citronelol y geraniol, además del efecto cruzado frente a los compuestos azólicos. Finalmente, los hongos revertieron su perfil de resistencia frente a las drogas naturales, sin mostrar resistencia a los azólicos. Aunque se observó el desarrollo de resistencia a los monoterpenos en los dermatofitos, el fenómeno no fue estable como el demostrado frente los fármacos azólicos.
Conclusiones: El citronelol y el geraniol fueron activos contra asilados resistentes incluso con posterioridad a la deshabituación. De este modo, estos monoterpenos se presentan como posibles alternativas terapéuticas y con pocas complicaciones relacionadas con la aparición de la resistencia.

Palabras clave: resistencia; antifúngicos; habituación; terpenos; azoles.


Recibido: 01/06/2019
Aceptado: 25/01/2020




Dermatophytoses are fungal infections in keratinized tissues such as nails, hair and the stratum corneum of the skin, caused by fungi called dermatophytes which, comprise the genera Microsporum, Trichophyton and Epidermophyton.(1) Dermatophytes infect both males and females of all ages and have a worldwide distribution. However, hot and humid climates as found in tropical and subtropical areas contribute to the large numbers of cases in Latin America, Africa, and Asia.(2,3)

Dermatophytoses can be treated topically or systemically or using drug associations usually reserved for more complicated or chronic cases; or for patients with Tinea unguium or Tinea capitis.(4) Certain agents are used clinically to treat dermatophytosis, among which are ketoconazole imidazole and triazole compounds i.e. itraconazole and fluconazole.(5) Although it seems that the number of antifungal drugs available in the market is large, they group simply into a few chemical classes, and thus often present a restricted spectrum of action. Antifungal treatments often fail because dermatophytosis is frequently caused by T. rubrum, associated with high relapse and drug resistance, and like other micro-organisms; fungal cells have great capacity to develop resistance to antifungal drugs.(1)

Natural products are emerging as a both great alternative therapies, and as one of the most successful strategies for discovering new molecular candidates for future drugs.(6) Essential oils are complexes of volatile compounds found in various parts of plants. They are widely used and constantly sought due to the strong demand for natural constituents to treat diseases.(7) Monoterpenes deserve to be highlighted. Geraniol and citronellol are alcoholic monoterpenes found in the essential oils of aromatic and medicinal plants such as Cymbopogon winterianus (poaceae),(8) and others.(9,10)

The repellant(11) and antimicrobial(12) activity of monoterpenes has already been highlighted. We therefore investigated their effect on the resistance profiles of T. rubrum, M. canis, and M. gypseum against azole drugs such as ketoconazole, fluconazole, and itraconazole using sub-inhibitory concentrations of citronellol and geraniol (to induce antifungal resistance).





Citronellol, geraniol, ketoconazole, fluconazole and itraconazole were purchased from Sigma-Aldrich® (Brazil). Emulsions were freshly prepared for the tests by dissolving first in dimethylsulfoxide (DMSO), and sterilized distilled water to obtain a concentration of 1024 µg/mL. From this concentration, dilutions were performed to achieve a concentration of 1 µg/mL using RPMI 1640 medium


The fungal strains T. rubrum LM 305 (skin), M. canis LM 216 (scalp) e M. gypseum LM 305 (skin) were taken from the culture collections of the (Federal University of Paraíba) Laboratory of Mycology. The fungi were grown in potato dextrose agar (Difco ®) at 28° C for 7 days to obtain the fungi inocula in sterile saline (0.9 % NaCl). Turbidity of the final inocula was adjusted to 106 conidia/mL, at a wavelength of 520 nm, and transmission adjusted to 70 % in a UV-5100 Spectrophotometer.(13)


Minimum inhibitory concentration (MIC)

MIC values of the drugs-test were determined against dermatophyte fungi by microdilution technique using 96 well flat bottom micro-titer plates. (13,14) To each row of the plate was added 100 µL of the diluted test drugs in RPMI 1640. To each well of the plate was added 100 µL of a previously prepared inoculum diluted in RPMI 1640 at a ratio of 1:50. A fungal control was performed by replacing the test drug using sterile saline (growth control). A sterility control and DMSO were also performed. The plates were sealed and incubated at 28° C for 7 days. MIC was the lowest concentration of drugs capable of inhibiting observed fungal growth in the wells by 100%. The assays were performed in triplicate and the MIC modal values expressed as geometric means of the results.

Induction of antifungal resistance

Test tubes containing Sabouraud dextrose agar with citronellol and geraniol at 1/2MIC and (controls) free of drugs, were inoculated with a mycelial fragment of 4 mm2 from each fungal strain, newly-grown on potato dextrose agar. The tubes were incubated at 28°C for 5 days. This procedure was repeated 8 times under the same conditions. The final cultures were used for the preparation of the respective inoculates with saline solution. Afterwards, the new sensitivity profile of these strains (adapted) was examined; determining the MICs for geraniol and citronellol by microdilution.(15) Development of resistance was observed as an increase in the MIC value in relation to the baseline value. (16) The assays were performed in triplicate and the MIC values expressed as geometric means of the results.

Cross-resistance assessment

To verify sensitivity profiles against ketoconazole, fluconazole and itraconazole, the dermatophyte strains were first treated with sub-inhibitory concentrations of monoterpenes.(13,14) A micro-organism control was also performed in parallel in RPMI 1640, where no azole drugs were added. Thus, we would be able to compare the new fungal sensitivity profiles, and also confirm if cross-resistance to azole drugs had occurred as a result of adaptation to monoterpenes in these dermatophyte strains.

Evaluation of the resistance stability

To evaluate the stability of both direct resistance and cross resistance to the antifungals, a mycelial fragment of 4 mm2 of the resistant strains was placed on Sabouraud dextrose agar surfaces in tubes free of drugs. The tubes were incubated at 28 °C for 5 days. This procedure was repeated 8 times. The final cultures were used for preparation of the respective inoculant in saline solution. Finally, we analyzed the new sensitivity profile of these adapted strains using microdilution. Thus, it was possible to confirm whether or not there had been any reversal in dermatophyte resistance to the test drugs.(17)




Initially, the MICs of ketoconazole, fluconazole, itraconazole and the citronellol and geraniol monoterpenes against the strains (T. rubrum LM 305, M. canis LM 216 and M. gypseum LM 305) were determined. The results are demonstrated in table 1. The dermatophytes were most resistant to fluconazole, since growth was inhibited only by a greater concentration of the drug (2 µg/mL). Citronellol and geraniol presented their best activity against T. rubrum LM 305 at MIC = 64 µg/mL.

The fungal strains (in the crop series) were thus subjected to sub-inhibitory treatment concentrations of citronellol and geraniol (antifungal pressure); cultivated in both the presence and absence of drugs, and to respective determinations of their MIC values (Table 2). In mediums free of drugs (controls), for all strains tested, there were no changes in the MIC values in relation to the initial states (data not shown).

The T. rubrum strain LM 305, after citronellol antifungal pressure, showed an increase in its MIC values (resistance) against citronellol. There was also a change in its MIC values against fluconazole and geraniol, (cross-resistance). For geraniol, the MIC also changed from 64 µg/mL to 256 µg/mL; however, this geraniol adapted strain showed no cross-resistance against the azoles, only against citronellol.

After antifungal pressure processing, M. canis strain LM 216 developed resistance to both monoterpenes, and, the MIC values for ketoconazole, fluconazole and itraconazole increased, presenting cross-resistance, this after treatment with either geraniol or citronellol. While treatment of this strain with citronellol induced cross-resistance to geraniol, interestingly, when treated with geraniol, this same strain became more sensitive to citronellol.

M. gypseum LM 305 also developed resistance to geraniol and citronellol after application of antifungal pressure. In relation to the emergence of cross-resistance to azoles, the strain showed behavior similar to M. canis LM 216, and, the treatment of this strain with citronellol did not induce cross-resistance to geraniol. However, when treated with geraniol, the strain became more sensitive to citronellol.

After the tested strains acquired resistance, it was necessary to verify the influence of subsequent dis-habituation cycles (for monoterpene action) and the reversibility of direct and cross fungal resistance to antifungals. The results are expressed in table 2. One can see that the resistance developed against citronellol was not permanent; with the exception of the MIC for geraniol against T. rubrum LM 305 (already citronellol resistant), indicating cross-resistance stability. Similarly, this occurred with the MICs of the natural monoterpene products in the strains that were resistant to geraniol; the majority also reversed their MICs. T. rubrum alone displayed stabilization of cross-resistance to citronellol. However, the MICs of the azole drugs when retested, (both when treated with citronellol or geraniol), did not indicate resistance reversal, except for M. canis LM 216 resistant to geraniol; (against ketoconazole).




Mechanisms of antifungal activity of monoterpenes have been reported in the literature. Against strains of T. rubrum, previous studies have suggested that inhibition of ergosterol biosynthesis is the likely mechanism of antifungal activity for citronellol and geraniol.(12) However, the literature lacks reports on the resistance of dermatophytes against natural products, including monoterpenes. The present study sought to know if in vitro resistance development is possible, and also if monoterpenes can yield cross resistance to azole compounds since they both share the same mechanisms of antifungal activity.

Reports in the literature concerning acquired resistance in dermatophyte fungi against essential oils and their phytochemicals are scarce. Osborne et al.(18) investigated acquired resistance to terbinafine in T. rubrum strains when cultivated with sub-inhibitory concentrations of the drug. More recently, a study by Hryncewicz-Gwo'z'dz' et al.(15) investigated T. rubrum resistance development when subjected to antifungal pressure using fluconazole and itraconazole. In this study, the authors report that sequential passages fluconazole or itraconazole against clinical isolates of T. rubrum resulted (in the majority of isolates) in MIC increases for both drugs. This clearly indicates the ability of T. rubrum to acquire resistance through exposure to sub-inhibitory drug concentrations.

The mechanisms of resistance to azole derivatives seen in T. rubrum and other dermatophyte species are very little understood, yet certain biochemical mechanisms have been reported. One proposed mechanism to explain T. rubrum resistance to azole drugs is effluxing, and an increase in the expression of efflux pumps. TheTrichophyton interdigitale pleiotropic drug resistance gene pdr1 (previously named mdr1, and mdr2), encodes different ABC transporters that are over-expressed when in the presence of the various antifungal drugs.(19) Another mechanism that may be involved in resistance to azole compounds is increasing modulation of lanosterol 14-alpha-demethylase, the molecular target for azole compounds. After exposure to sub-inhibitory concentrations of ketoconazole, overexpression of the erg11 gene (which encodes the enzyme) was evidenced in T. rubrum.(3)

It was observed in this study that sequential passages of clinical T. rubrum, M. canis and M. gypseum isolates in presence of citronellol and geraniol resulted in MIC increases for citronellol, geraniol, and the azole drugs tested. This is the first report of resistance to natural products acquired by dermatophytes. This finding is of the utmost importance, since resistance modulation in vivo is also possible and may cause therapeutic failures.

According to Santos et al,(20) this in vitro observation has its reflections in vivo; and is supported by the increase in MIC values for azole agents, including itraconazole and ketoconazole in strains of T. rubrum obtained from patients with onychomycosis, after treatment with ketoconazole or a combination of itraconazole and terbinafine. The results also point to the development of cross-resistance, both to the azole test drugs and to the monoterpenes. This is probably because both present similar toxic mechanisms to the fungal cell.

The stability and reversibility of direct and cross fungal resistance to antifungals are parameters that must also be analyzed due to their clinical implications. Borst et al.(21) cultivated isolates of C. glabrata with acquired resistance to fluconazole in a drug-free medium for 122 days. The resistance phenotype was lost was when subcultures in liquid medium were changed to solid. This is an important correlation for our results, since the subcultures were performed directly on solid mediums and resistance phenotype loss was also observed.

Pippi et al(22) investigating resistance stability showed that for isolates of resistant C. parapsilosis, the resistance phenotype was preserved even after subculture in the fluconazole free medium. By this, it can be inferred in relation to natural products that biochemical changes in the fungal cells occurred only in the presence of citronellol and geraniol stressor agents. This highlights an advantage for these natural products, since azole resistance plays an important role in therapeutic failures and consequently contributes to the persistence and chronicity of infections.

Several theories try to explain how genetic changes causing resistance to drugs emerge, and become established in fungal populations. Biochemical mechanisms involved in the acquisition of resistance by dermatophyte fungi, may explain both irreversible as well as reversible mutations. (23) An organism with the ability to modulate its rates of spontaneous mutation and recombination, keeping them low during conditions of low stress and increasing them in conditions of high stress, has a selective advantage over organisms with constitutively constant (high or low) rates of mutation and recombination.(24)

From the results presented in this study, one may conclude that dermatophytes develop resistance when submitted to successive sub-inhibitory concentration treatments of citronellol and geraniol. This resistance may also lead to the emergence of resistance to other drugs such as ketoconazole, fluconazole and itraconazole. These results are new, and place natural drugs, including monoterpenes as therapeutic agents susceptible to the mechanisms of resistance developed by pathogenic fungi. It was observed that resistance to these natural products is reversible.

In conclusion, our results reveal that citronellol and geraniol as having great potential as future therapeutic agents; yielding fewer complications in the emergence of resistance. Although controlled clinical studies are needed to define their true effectiveness, these monoterpenes were active against resistant isolates even after dis-habituation, which did not occur with the majority of the azoles.




The authors thank the Mycology Laboratory, Federal University of Paraíba for the supply of fungal strains.




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Conflict of Interest

The authors declare that they have no conflict of interest.


Author contributions

Gustavo Nunes Cardoso: Conceptualization; Data Curation; Writing - Original Draft.

Kaltz Victor Souza Silva: Conceptualization; Data Curation; Formal analysis.

Maria Islaine de Oliveira Lima: Conceptualization; Data Curation; Formal analysis.

Juliana Moura Mendes Arrua: Supervision; Writing - Review & Editing.

Fillipe de Oliveira Pereira: Conceptualization; Project administration; Writing - Review & Editing.

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Copyright (c) 2020 Gustavo Nunes Cardoso, Kaltz Victor Souza Silva, Maria Islaine de Oliveira Lima, Juliana Moura Mendes Arrua, Fillipe de Oliveira Pereira

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