1. Introduction
Pathogenic microorganisms (e.g., bacteria, fungi, and viruses) that cause infections are a serious public health problem that is increasing considerably, mainly by the high rate of genetic changes, resistance mechanisms, or wrong and excessive use of antimicrobials [
1,
2,
3]. In addition, bacterial resistance to antibiotics increases infection rates (i.e., Gram-negative bacteria, 61.3%; Gram-positive bacteria, 34.8%; yeasts, 2%; and other pathogens, 1.9%) mainly in developing countries [
2,
3,
4,
5].
The Enterobacteriaceae family is the largest and most heterogeneous group of Gram-negative bacteria of clinical importance [
6]. Within this group, genera such as
Citrobacter,
Enterobacter,
Escherichia,
Klebsiella,
Proteus,
Serratia,
Shigella, and
Salmonella are the most frequent causes of human infections [
7,
8,
9]. About 80% of infections, including urinary tract infections, pneumonia, diarrhea, meningitis, sepsis, and endotoxic shock, among others, are caused by Enterobacteriaceae [
5,
9,
10,
11,
12].
On the other hand, according to the reviewed literature, an alarming decrease in the discovery of antibiotics has been observed during recent decades [
13,
14], which probably involves factors such as the lack of interest from the pharmaceutical industry [
15], as well as the lack of resources intended for research and bioprospecting of organisms [
16]. In this sense, the need arises to look for new molecules or proteins with antibacterial properties, such as synthetic or natural compounds, investigated mainly in plants and very poorly in fungi [
17].
Fungi constitute a promising group of interest for the search for bioactive compounds [
18], in addition to being a highly diverse group of organisms, with an estimated 1.5 to 5 million species in the world [
19], of which only a small proportion of <100,000 species have been described, according to Baldrian et al. [
20]. This group of organisms is able to adapt to and survive extreme conditions in several ecosystems [
21]. This characteristic may be due to the production of a wide range of bioactive compounds [
22]. For example, filamentous fungi, mainly Ascomycetes (e.g.,
Aspergillus,
Cladosporium, Fusarium, Penicillium notatum), are able to produce enzymes, microbial biomass, and secondary metabolites, including antibiotics (e.g., fusidic acid, cephalosporin, and penicillin), that are applied for the treatment of various infectious diseases [
18,
23,
24]. Likewise, Basidiomycete fungi generate a large number of metabolites that have demonstrated antibacterial, antifungal, antiviral, cytotoxic, and hallucinogenic capacities [
25,
26].
Among the fungi, the macro Basidiomycetes
Lentinus edodes, followed by species within the genera
Boletus,
Ganoderma, and
Lepista are promising candidates for the search for compounds with antibiotic activity against Gram-positive and Gram-negative bacteria [
25]. Likewise, other fungi with resupinate and corticioid characteristics, such as
Perenniporia spp. and
Antrodia spp. in the order Polyporales, have been evaluated for antimicrobial activity [
26]. However, there is still a low number of prospective mushroom studies, especially in neotropical areas such as Ecuador, where these organisms are still poorly cataloged [
27,
28,
29,
30], characterized morphologically and molecularly [
31,
32,
33,
34], or even more chemically characterized by evaluating the metabolites that they generate [
35]. However, mushrooms with medicinal properties, from an ethnomycological point of view, have been reported in Ecuador [
36,
37,
38,
39].
Most research focused on fungal antibacterial compounds has been conducted mainly from macrofungi due to their ease of characterization and cultivation [
25]. However, the study of fungi that are almost imperceptible to the naked eye, such as “resupinates”, which are distributed in several taxonomic groups within Homobasidomycetes and Heterobasidomycetes, has been left aside [
40]. Resupinated fungi exhibit diverse basidiomata forms (e.g., corticoid, trechisporoid, jaapiaid, poliporoid, russuloid, Heimenoquetide, and Cantharelloides) [
40] and fulfill ecological roles, such as saprotrophs (decomposing organic matter) [
41,
42], symbionts (forming mycorrhizae) [
31,
43], or parasites of insects or plants [
44]. The integration of molecular, morphological [
32], and biochemical [
35] characterization has allowed the discovery of new species with different biotechnological potentials [
45].
Therefore, this research aims to evaluate the antibacterial potential of seven fungal strains isolated from resupinate basidiomata from a tropical montane forest (Podocarpus National Park (PNP)) in southern Ecuador, against bacteria of clinical interest, such as Escherichia coli, Serratia sp., and Klebsiella sp. We describe Gloeocystidiellum lojanense as a new species to science, integrating a morphological and molecular (ITS1-5.8S-ITS2; partial LSU D1/D2 nrDNA) characterization, and we report for the first time the antibacterial activity of G. lojanense against the Gram-negative bacteria Escherichia coli.
3. Results
Six of seven strains identify as five Basidiomycetes, and one ascomycete under six orders (
Table 2) was not characterized taxonomically because they did not present antibacterial activity. On the other hand,
Gloeocystidiellum sp., which presents positive antibacterial activity against
E.
coli, is taxonomically described as a new species for science. The BLAST identity percent found between 87 and 97 suggests a possibility to find some new species for the science in this tropical forest.
3.1. Taxonomy
Gloeocystidiellum lojanense A. Jaramillo, D. Cruz & C. Decock., sp. nov. (
Figure 2 and
Figure 3).
ETYMOLOGY: The specific epithet refers to Loja province (Ecuador) where the species was found for the first time.
BASIDIOMA—resupinate, ceraceous, or subceraceous. Hymenial surface is bright grayish white (#ecf0d4) and slightly light yellow (#b0ac6e), extending smoothly and slightly tuberculate over the substrate up to about 15 cm2; bounded margins.
HOLOTYPE—South America, Ecuador, Loja Province, and Canton, Podocarpus National Park, Cajanuma sector, alt. c. 2900 m, on fallen decomposing branch of unknown tree, 23 February 2021. A. Jaramillo [HUTPL(F)2181].
Microscopic Structure. Hyphal system monomitic: generative hyphae 3–4 µm diameter, septate, thin- to thick-walled hyaline with clamp connection with calcium oxalate crystals, gloeocystidia abundant, tubular or cylindrical, slightly tapering towards the apices, thick-walled throughout, up to approximately 80–90 μm long, 7–8 μm wide, the protoplasmic content granular and yellowish in KOH, blackish in SA (positive reaction), basidia clavate 25–35 long × 5–6 μm wide, transversely septate at the basal zone, with four sterigmata, basidiospores hyaline ellipsoid and slightly verrucose, thin-walled, 6.5–8 μm long × 3.4–4.5 μm wide, and slightly amyloid under Meltzer’s reagent (
Figure 2 and
Figure 3).
Gloeocystidiellum lojanense is morphologically related to
Gloeocystidiellum formosanum,
G. compactum,
G. aspellum, and
G. rajchenbergii [
50,
57]. These species are closely similar as far as their morphology is concerned, especially the basidiospores and gloeocystidia features (
Table 3); additionally, all of them present an amyloid reaction in Meltzer’s reagent. Microscopically, the shape and size (
Table 3) of basidiospores and gloeocystidia in
G. lojanense are indistinguishable from those of its allied species listed above. Although the size of the basidiospores has been considered as the main characteristic to define species in this group [
50,
57,
63,
64,
65], it is evident that they can overlap each other, causing taxonomic confusion, as has already been indicated in other groups of fungi (e.g.,
Tulasnella spp.), where molecular data allowed revealing some cryptic morphological species [
32].
Another reason to propose
G. lojanense as a new neotropical species is their biogeographic and ecological location, as has been described in other
Gloeocystidiellum species [
64,
65].
G. lojanense was found in a tropical montane rainforest in southern Ecuador at 2900 m asl; different to the places in eastern Asia (Taiwan, China) or Patagonia Andean of Chile, about 600 to 1250 m asl, reported to the closest morphological species [
50,
57,
66].
3.2. Phylogenetic Hypothesis for Gloeocystidiellum lojanense
The phylogenetic results, based on the ITS or the concatenated data sets (
Figure 4 and
Figure 5;
Figures S2 and S3) showed that
G. lojanense form an independent well-supported clade (respectively, 96 and 100% bootstrap) that is here interpreted as a new phylogenetic species.
Gloeocystidiellum lojanense forms a sister clade (
Figure 5, BS 98/99% for ML/NJ) to the clade gathering
G. aspellum,
G. compactum, and G.
formosatum from several countries (
Table 1). A similar topology if observed is when the ITS1 region (63/56% BS for ML/NJ), including the sequence from the species
G. rajchenbergii, is analyzed.
The phylogenetic tree obtained from the concatenated dataset (ITS1-5.8S-ITS2 and partial LSU D1/D2 regions) (
Figure 4) is consistent with the phylogenies from independent analyses for the ITS1 (
Figure 5) or ITS1-5.8S-ITS2 (341 bp) regions (
Figure S1) and LSU D1/D2 partial (471 bp) (
Figure S3), where
G. lojanense is grouped as a sister group with the species
G. aspellum,
G. compactum,
G. formosatum, and
G. rajchenbergii but remains as an independent clade with BS values higher than 95/97% for ML/NJ.
The resulting interspecific groups are totally supported when the sequences were analyzed by complete deletion and partial deletion; for example, the range for the ITS1-5.8-ITS2 region is between 6.45% and 6.90% (
Table 4), ITS1 is between 2.94 and 3.69%, and partial LSU D1/D2 is 0.81% (
Table 4).
3.3. Antibacterial Activity
Gloeocystidiellum lojanense HUTPL(F)550 was the unique strains tested by inhibiting the bacterial strains of
Escherichia coli and
Serratia sp. (
Table 5). The other six strains (
Table 2) were not further charactrized because they were totally inactive against the bacterias tested.
Gloeocystidiellum lojanense was observed to be more effective at inhibiting mainly two pathotypes of
E. coli (i.e.,
Escherichia coli ATCC 25922 and
Escherichia coli O157:H7). The inhibition halos generated for these strains were larger in size (between 22.78 and 24.33 mm in diameter (
Table 5) and statistically significant (<0.001) versus the other tests (
Figure 6,
Table 6). However, the inhibition halos generated by
G. lojanense did not exceed the halos generated from the positive controls (30.33–34.33 mm in diameter) for the strains
Escherichia coli ATCC 25922 and
Escherichia coli O157: H7. Inhibition halos varied according to the incubation time (
Table 5).
The bacteria Enterobacter aerogenes, Enterobacter cloacae, Klebsiella pneumoniae, Klebsiella pneumoniae ATCC BAA-1706, and Pseudomonas aeruginosa did not exhibit inhibitory activity by G. lojanense.
The box plot clearly indicates that
Gloeocystidiellum lojanense presents greater inhibition in the bacteria
Escherichia coli ATCC 25922 (22.44/22.78 mm) and
Escherichia coli O157:H7 (24.33 mm), increasing during 48 and 72 hours, compared to
Escherichia coli. UPEC FG: A,
Escherichia coli UPEC FG: B2 and Eb, EGA, and
Serratia sp., which were more stable over time (
Figure 6).
The analysis of variance indicated that the fungal species, time, as well as their interaction between the two factors, significantly affect the inhibition of the bacteria, showing significant values of each factor (<0.001) (
Table 6).
4. Discussion
Gloeocystidiellum lojanense represents a species new to science from a tropical montane rainforest in southern Ecuador. This species is phylogenetically supported, forming an independent clade with BS values greater than 96% for the ITS1-5.8S-ITS2 and partial LSU regions, analyzed independently or concatenated. This species presents an interspecific genetic divergence for the ITS1-5.8S-ITS2 region greater than 6.45% with respect to the other species. This interspecific divergence of 6.45% would generate a barcode gap with respect to the 3% or 4% thresholds of intraspecific variability studied for fungi [
32,
67].
Morphologically and molecularly,
G. lojanense is suggested as a sister group to the clade comprising
G. formosanum,
G. compactum,
G. aspellum, and
G. rajchenbergii [
57]. All these species are practically indistinguishable morphologically, mainly due to the overlap of shapes and sizes of basidiospores and gloeocystidia. This problem, discussed as “cryptic species”, is already well known to other groups such as
Tulasnella spp. [
32], where several morphologically cryptic species were delimited genetically. Other species (i.e.,
Gloeocystidiellum clavuligerum and
G porosum) are morphologically [
63,
64,
65] closely related to
G. lojanense but genetically distant (>6.45% ITS1-5.8S-ITS2), forming a different clade (
Figure 4). Additionally, the interspecific genetic difference for the ITS1-5.8S-ITS2 region is greater than 6.45%, exceeding the so-called barcode gap discussed by [
32].
Gloeocystidiellum lojanense exhibited antibacterial activity against the four strains of the
E. coli species, with
E. coli ATCC 25922 and
E. coli O157:H7 (
Figure 6) showing greater inhibition than the other
E. coli strains. This inhibitory action may be due to the fact that these
E. coli pathotypes are less virulent and resistant [
68]. Other pathotypes (e.g.,
Escherichia coli UPEC FG: A,
Escherichia coli UPEC FG: B2) are reported to be resistant to antibiotics [
69], observed in slight size changes in the inhibition halos. Likewise,
G. lojanense did not significantly inhibit the growth of
Serratia sp. This species is also known to have a high bacterial resistance to antibiotics [
70], as evidenced in the positive control with a smaller size of the inhibition halo. Suay et al. [
71] present a study indicating that the species
Gloeocystidiellum porosum has antibacterial activity against
Pseudomonas aeruginosa,
Serratia marcescens, and
Staphylococcus aureus, which is why a further exploration of this group of fungi is required.
Antibacterial inhibition increased between 48 and 72 h, probably because the fungal metabolites exhibiting these antibacterial properties were generated secondarily by nutrient depletion, or simply by recognition of the bacterial foreign agent [
72]. Six different strains of resupinated fungi within (Amylocorticiales, Cantharellales, Hymenochaetales, Polyporales, Russulales) and one
Hypocrea sp. (Hypocreales) did not exhibit inhibition activity against any bacterial strain tested. The
Gloeocystidiellum sp. (active against bacteria) and
Xenasmatella sp. (inactive against bacteria) share the same order Russulales but differ in antibacterial activity. Many studies report different fungi species in the same order such as this study (e.g., Polyporales, Russulales) but positive for bacteria inhibitions [
25,
26]. Probably, the antibacterial activity of fungi is restricted at the species level by specific genetic adaptations [
73] requiring further analysis of the gene expression of these fungi.
Statistically, the antibacterial activity of
G. lojanense is significant mainly for two pathotypes of
E. coli (i.e.,
E. coli ATCC 25922 and
E. coli O157:H7). However, our analysis requires an evaluation of the minimum inhibitory concentration by obtaining its extracts, as has already been evaluated for other fungi [
74].
Ecuador, considered within the megadiverse countries worldwide [
75,
76,
77], has as consequence a great diversity of fungi [
27,
29,
33,
34] that require integrative taxonomic research to discover new species such as
G. lojanense with bioactive potential and applications in several sectors such as human health.