1. Introduction
Propolis is a natural resinous substance produced by bees from substances collected from the vegetation in which antifungal, antibacterial, antiviral and antiparasitic activities have been recognized, showing variation in their biological activity depending on their geographical origin [
1,
2,
3].
In Latin America, there is a great variety of ecosystems each with a very diverse vegetation from which native bees’ extract propolis, which in turn results in great medicinal richness. In general, to be able to identify the origin from which bees extracted the material with which they elaborate their propolis, is not as simple as it is to do so with honey or pollen, and for this reason it has been proposed that chemotaxonomic studies should be undertaken or the behavior of each bee species in each region to be observed [
4,
5,
6,
7]. Since ancient times, products elaborated by stingless bees from the species
Scaptotrigona Mexicana and
Tetragonisca angustula have been used, both species being highly employed in central American beekeeping, however, very little information exists in the currently available literature concerning the scientific evidence demonstrating the medicinal efficacy of their products. Also, propolis from the
Apis mellifera species presents a fungicidal effect against several fungi species [
8,
9,
10]. Moreover, its applications can prove beneficial in veterinary medicine, as in the case of canines. Propolis coming from
Apis mellifera has been used as a prophylactic agent against gastrointestinal and respiratory diseases, mycoses, as well as a wound healing agent, and its therapeutic use has spread to many areas [
11,
12], one example being that of canine external otitis, which is the inflammation of the external auditory canal and represents between 5 to 20 % of consultation reasons. The main causative agent of canine external otitis is the yeast
Malasezzia pachydermatis, which is part of the normal microbiota of the external auditory canal in dogs [
13,
14,
15,
16]. Propolis possess an alternative for the treatment of canine otitis, instead of common antifungals, for patients with a high incidence of relapse, because of its antifungal, anti-inflammatory and wound healing properties; however, up to this time the only propolis that has been evaluated is that obtained from the bee
Apis mellifera [
17]. To undertake studies regarding the potential antimycotic properties of propolis from native bees can provide the scientific ground for its use as an alternative treatment against canine otitis.
4. Discussion
Many of the identified metabolites of propolis stingless bees have been reported to exhibit a myriad of biological activities, including antimicrobial, anti-inflammatory, cytotoxic, antioxidant, hepatoprotective, antiulcer, among others [
39,
40,
41,
42]. Analyzed propolis extracts exhibit chemical compositions with significant differences among them, this being consistent with other studies which have found that the quantity and diversity of compounds found in propolis is high [
43,
44], in comparison with the obtained results from the present study for
Scaptotrigona mexicana and
Tetragonisca angustula as well as other species such as
Melipona beecheii and others worldwide [
2].It is important to consider that floral diversity, collection time of the year and bee species are all determinant factors for the final composition of each propolis. Compounds belonging to the sesquiterpenes were primarily detected, these have been attributed with antimicrobial activity [
34,
45,
46,
47]
Regarding
Scaptotrigona mexicana, it is important to mention that a bibliographic research of furan-2,5-dicarbaldehyde, which is a heterocyclic compound with aldehyde groups, was unsuccessful using that exact denomination, however we did find mention of a similar compound, 2-acetil-5-metilfurane, reported to exhibit antimicrobial activity against
Escherichia coli, Candida albicans and
Staphylococcus aureus [
33].
For
Tetragonisca angustula, the antibacterial and antifungal activity of solavetivone has been reported [
36,
37]; on the other hand, concerning l-(1S,6R,9S)-5,5,9,10-tetramethyltricyclo [7.3.0.0(1,6)]dodec-10(11)-ene, there in no specific information regarding its activity; nonetheless, antifungal and antibacterial activities have been reported for a similar compound, namely 3,3,7,7-tetramethyl-5-(2-methyl-1-propenyl)-tricyclo [4.1.0.0(2,4)]heptane [
37].
Antimycotic potential of propolis extracts, mainly from
Apis mellifera, has been demonstrated against
Candida albicans. Also, fungicidal and fungistatic properties of green and red propolis extracts from Brazil against other fungi genera such as
Saccharomyces [
48] have been observed; inhibition and morphologic alterations of
Cyptococcus neoformans [
49].
None the less, information regarding the evaluation of the antimycotic activity of propolis coming from stingless bees mainly against
Candida albicans has been studied for the following species:
Lestrimellata spp., Melipona favora orlinge, Melipona marginata, Melipona quadrifasciata, Melipona scutellaris, Nannotrigona testaceicornis, Plebeia droriana, Plebeia remota, Scaptotrigona bipunctata, Tetragona clavipes [
7] and
Tetragonisca angustula and that of propolis from
Tetragonisca fiebrigi against
Candida glabrata [
50] .In the case of propolis coming from the Malaysia-stingless bee
Trigona thoracica, it has been demonstrated that it acts against
Cryptococcus neoformans [
51]. Furthermore, an Indonesian propolis from the species
Tetragonula sp. has been evaluated as a possible therapeutic agent for the treatment of vaginal candidiasis [
52].
Regarding
Malassezia pachydermatis, there are studies evaluating the activity of propolis from
Apis mellifera, but not from native bees, against this yeast. In a recent study a correlation was established between the antifungal activity of EPP from Brazilian green and red propolis against
M. pachydermatis, with a MIC between 4 y 8 mg/mL and a MFC of 8 and 16 mg/mL. It was determined that as the total content of phenols and flavonoids increases, propolis exhibits a better biological activity, proposing that the action mechanism of EEP is due to the rupture of the cellular wall, given the fact that during the investigation it was observed that some azole- resistant
Malassezia pachydermatis strains were inhibited by the EEP. However, they also extern concern because it is still unclear if high EPP concentrations could induce cytotoxicity. Therefore, there remains work to be done in order to accurately identify the active principles of propolis, as well as their action mechanisms. Currently the are two theories aiming to explain the antifungal activity of propolis, the first one proposing that it elicits cellular wall lysis and the other proposing that it damages the cellular membrane by inhibiting ergosterol synthesis [
53]. Likewise, the efficacy of an Argentinian propolis against
M. pachydermatis was evaluated by different in vitro techniques, which concluded that the yeast was vulnerable to all employed propolis concentrations, with a MIC of 0.30 mg/mL, however, they were not able to determine the MFC [
17]. The efficacy of a 2.5 % EEP against 48 clinical strains of
Malazessia pachydermatis isolated from dogs diagnosed with external otitis was also proven, as it was found that all of them were susceptible to it [
54]. Antimycotic activity against clinical isolates from dogs with external otitis has also been observed in a EEP from Rio Grande do Sul, Brazil, with a MIC of 2.6 mg/mL and a MFC of 5.3 mg/mL [
55]. In contrast, our results for both propolis of the evaluated bee species regarding MIC and MFC was of 32 mg/mL and 64 mg/ml respectively, both concentrations being significantly higher than those reported in studies of propolis coming from
Apis mellifera.
According to the images obtained, the EEP proved to be able to penetrate the membrane cellular, causing severe damage and eventually death of yeasts with structural and functional damage by membrane disruption. Calcofluor white has a high affinity for fungal wall components [
56,
57]. The alteration observed with this stain was mainly deformity of the morphology and in some cases, we have the hypothesis that the complete destruction of the cell wall caused the yeasts not to be observed, which would be a possible effect by the sesquipertenes present in the EEP as previously described [
58]. It would be advisable to perform computational chemistry studies to specifically establish the damage of these compounds in the yeast wall.
On the other hand, propidium iodide binds to nucleic acids and increases its red color when there is damage to the cell membranes. which indicates severe cell damage and death, which was seen in the reference strains and the clinic treated with both EEP, which demonstrates the effectiveness of this type of propolis. This effect with propidium iodide has been observed in other fungi such as
Fusarium species and
Colletotrichum fructicola, [
59] when naturally occurring compounds have been evaluated against their growth [
60].
These stains have been used to detect cellular damage using
Apis mellifera propolis against other yeast of medical importance such as
Candida albicans [
61,
62,
63] as well as the bacterium
Staphylococcus aureus [
64].
Therefore, it is shown that propolis of the Mexican stinglees bee Scaptrigona mexicana and Tetragonisca angustula have antimycotic effect and cause structural damage on Malazessia pachidermatis, which supports its possible use for therapeutic purposes in the treatment.. The findings and their implications should be discussed in the broadest context possible. Future research directions may also be highlighted.