Propolis, a resin collected by bees, exhibits a considerable variety of well-established pharmacological activities, and its antimicrobial potential has been widely studied, especially against oral pathogens [1
]. Most of these studies describe the activity of propolis collected by A. mellifera
, which increased the market price of this product. Geopropolis is a type of propolis collected by stingless bee species native to tropical countries, which, in addition to resin and wax, has soil in its composition, leading to low yield extracts, a fact that can partly justify its low economic value and the lack of studies on its biological activity [3
In this study, EEGP showed interesting antimicrobial activity, especially against S. aureus
, S. mutans
, and MRSA strains, with MIC values below 50 μg/mL, but it presented weak inhibition of P. aeruginosa
growth, a Gram-negative bacillus. Our findings are corroborated by Velikova et al. [4
], who reported that Brazilian geopropolis samples showed significant activity against S. aureus
but presented weak activity against E
, as well as by Duarte et al. [20
], who affirmed that crude extracts from natural products are considered promising when the MIC value is below 500 μg/mL.
Several types of A. mellifera
propolis extracts have their activity against S. mutans
well described in the literature. Duarte et al. [21
] showed that the ethanolic extract of Brazilian propolis type 6 inhibited S. mutans
UA 159 growth at concentrations between 25 and 100 μg/mL and Hayacibara et al. [19
] reported that Brazilian propolis types 3 and 12 were able to inhibit bacterial growth at 25–50 μg/mL and 200–400 μg/mL, respectively. EEGP inhibited the growth of S. mutans
UA 159 between 25 and 50 μg/mL, also demonstrating strong inhibitory activity with bacteriostatic character, suggesting its ability to act on the virulence factors of the microorganisms involved in the etiology of dental caries. In case of an infection in the oral cavity, actions that have impact on the virulence factor of the microorganisms seem to be the best way to control their development and pathogenesis, since total and permanent elimination of bacteria from the oral environment is not viable [22
]. Such effect of geopropolis, provided that it is confirmed by specific studies, indicates the presence of compounds that can be effective in controlling and preventing caries. S. aureus
and MRSA infections have acquired great clinical importance, because these organisms appear to be resistant to β-lactam, aminoglycoside, and macrolide antibiotics as well as to certain antiseptic substances [23
]. In the present study, EEGP demonstrated to be a promising source of bioactive compounds against this pathogen, showing the lowest MIC and MBC values against both S. aureus
strains tested. Furthermore, when compared to other strains, MRSA was the most sensitive microorganism, with all fractions tested showing low MIC and MBC values.
In order to verify whether the chemical separation was efficient, HF, CF, and EAF were tested against the same microorganisms and their MIC values were compared to the values obtained for EEGP. HF proved to be the most potent fraction, reducing MIC and MBC values (between two to four times) for S. mutans, E. faecalis, and A. naeslundii, and maintaining these values against S. aureus 25923 compared to EEGP. Against MRSA, HF was less active than EEGP and CF. In general, all the other fractions showed low bacterial growth inhibition compared to HF and EEGP. Such effect suggests that nonpolar compounds present in geopropolis should be the main substances responsible for its biological activity.
EEGP and HF (defined as the most active fraction) were also able to inhibit the adherence of S
growing cells at sub-MIC concentrations. The inhibition rates observed indicate that HF probably presents higher 2activity than EEGP because of a concentration of bioactive compounds in the nonpolar fraction, which suggests that this fraction might contain promising anti-caries agents. Other types of Brazilian propolis also showed this antibiofilm activity with a similar mechanism of action, especially Brazilian propolis type 6, which presented activity against adherence of S
growing cells due to its activity on glucan synthesis by inhibiting glucosyltransferases [14
]. Furthermore, other kinds of geopropolis collected by other bee species and from different regions demonstrated similar mechanisms of action against S
, reducing the cell viability of the biofilm formed by this microorganism [5
According to Fouche et al. [24
], extracts of natural products with antiproliferative activity can be classified as inactive (TGI > 50 μg/mL), presenting weak activity (15 μg/mL < TGI < 50 μg/mL), moderate activity (6.25 μg/mL < TGI < 15 μg/mL), and potent activity (TGI < 6.25 μg/mL). EEGP was inactive against normal murine fibroblast cells and a weak inhibitor of human keratinocytes. Among the human cancer cell lines tested, EEGP showed moderate inhibition against melanoma and ovarian cancer lines. These results indicate that EEGP exhibited a nontoxic profile against normal cell lines and toxicity against cancer cell lines, i.e., a selective antiproliferative activity. Additionally, HF maintained the weak activity against HaCaT cells, promoted a six-fold reduction in TGI value against the melanoma line compared to EEGP, and presented potent activity against prostate and ovarian tumors.
Umthong et al. [10
] described the selective antiproliferative activity of propolis collected by Trigona laeviceps
, a stingless bee species, against some cancer cell lines and low cytotoxic activity against normal cell lines. Comparatively, M. scutellaris
geopropolis seems to be a promising source of anti-tumor bioactive compounds, showing moderate or strong inhibition of a wide range of cell lines. Although these are preliminary results obtained from in vitro
evaluations, they indicate that the compounds present in EEGP and HF could be used to treat some types of infections and tumors without causing significant damage to the normal cells tested here. In fact, the concentrations of EEGP and HF that can affect the normal cell lines were higher than those effective against some bacteria or tumor cell lines.
RP-HPLC analyses confirmed the presence of low polarity compounds in M. scutellaris
geopropolis, evidenced by high elution times (RT between 80 and 120 min), corresponding to less polar compounds, and also by the concentration of substances in HF. Other types of propolis and Brazilian geopropolis are essentially nonpolar due to the presence of terpenes and benzophenones [4
]. The UV spectra of the major compounds (4, 5, and 7, Figure ) ranged from λmax
279 to 282 nm, suggesting a possible chromophore with characteristics of polyprenylated benzophenones [25
]. Our findings also indicate the absence of flavonoids, usually reported as responsible for the pharmacological activities attributed to some types of A. mellifera
propolis, as well as of markers of some types of propolis [3
CG-MS data showed the presence of compounds belonging to similar chemical classes, indicated by the fragmentation pattern of the mass spectra. The fragment at m/z 105 [(C6
, observed in the fragmentation pattern of compounds 6, 8, and 9, suggests that they have characteristics of the class of benzophenones, and the presence of fragments at m/z 55, 69, and 77 indicates that they present prenylations [27
]. These findings about the chemical profile of M. scutellaris
geopropolis corroborate the differentiated profile and the not yet entirely elucidated nature of its bioactive compounds. This stimulates the search for a detailed description of its chemical composition and the potential use of its bioactive compounds as complementary food or medicine, thus increasing the economic and social value of a natural product not fully recognized.
The presence of benzophenones, especially polyprenylated ones, has been described in some types of propolis. Ishida et al. [25
] attributed the antimicrobial activity of propolis samples from the Brazilian Amazon region to benzophenones, such as epi
-nemorosone and 7-epi
-clusianone, which are also described as typical metabolites produced by Clusiaceae (Guttifarae), a family of plants widely distributed in Brazil. Studies on the chemical composition and biological activity of Brazilian propolis type 6, collected by A. mellifera
in the state of Bahia, showed certain similarities to the geopropolis studied herein, from the same state, although they were collected by bees with completely different biological characteristics [21
]. The studies reported that Brazilian propolis type 6 has an essentially nonpolar composition, showing the possible presence of benzophenones and the absence of phenolic acids and flavonoids [21
]. Similarly to the geopropolis of the present study, HF was the fraction responsible for the best activity of Brazilian propolis type 6, and its biological activity was attributed to hyperibone A, which also acts against the adherence of S. mutans
]. The similarities between the chemical and biological profiles of geopropolis collected by M. scutellaris
and Brazilian propolis type 6 suggest that the possible activity of the former is due to the presence of a benzophenone.