## 3 Results and Discussion
### Determination of the Antimicrobial Activity  
Substantial advances have been made in the antimicrobial study of red propolisils extract, making it an interesting alternative to be potentially applied against a range of microorganisms [97; 98]. This is implied in the growing quantity of publications on the extracts of red propolisis obtained by different extraction methods and their bioactive compounds isolated and differentiated in recent years [99]. Table 3 presents the MIC and MBC values found for the ten Brazilian red propolisils extracts obtained in this study. All extracts showed activity against _S. aureus_ (representative Gram-positive) and _E. coli_ (representative Gram-negative) bacteria; however, this parameter depended on the extraction patterns applied. The bacterial growth of the controls chosen in the study was not affected. Because the sample that was not treated with ultrasound (E:UT) presented lower MIC and MBCvalues compared with some of the treated extracts (including under the same temperature conditions for MBC), we cannot claim that the ultrasound treatment was efficient at increasing the antimicrobial potential of the extracts against the studied bacteria for some of the tested temperatures. The MIC for _S. aureus_ ranged from 6 to 23 \(\mathrm{\SIUnitSymbolMicro mL^{-1}}\), whereas the variation for _E. coli_ ranged from 23 to 93 \(\mathrm{\SIUnitSymbolMicro mL^{-1}}\) (Table 3). To obtain whole antimicrobial properties, investigations to determine the MBC were conducted for all the studied extracts. The MBCs for _S. aureus_ and for _E. coli_ varied between 11 and \(>\)1000 \(\mathrm{\SIUnitSymbolMicro mL^{-1}}\) and between 93 and \(>\)1000 \(\mathrm{\SIUnitSymbolMicro mL^{-1}}\), respectively (Table 3). The literature suggests a promising MIC of a raw extract of a natural product is less than 500 \(\mathrm{\SIUnitSymbolMicro mL^{-1}}\), thus causing robust investigations to be conducted to clarify its mechanism of action [100, 101]. Hence, regardless of the given parameters, the low MIC values observed for the ten red propolisis extracts show that the study subjects are promising antimicrobial compounds. In general, when the extraction conditions in use are compared, the extracts presenting higher antimicrobial activities were those that exhibited the highest antioxidant activities, with high total contents of phenolic compounds and flavonoids (Figure 1). The extracts with higher active concentrations against _S. aureus_ and _E. coli_ were E:50\(-\)20 and E:25\(-\)30, respectively, as represented by the lower MIC and MBC values. Among all the conditions tested here, four extracts did not show active bactericidal action against _S. aureus_, and three did not show activity against _E. coli_ (as represented by MBC \(>\) 1000 \(\mathrm{\SIUnitSymbolMicro mL^{-1}}\)). As observed in other studies and confirmed in this experiment, the extracts tested here showed significant activity against the Gram-positive strain (_S. aureus_), in contrast to the Gram-negative strain (_E. coli_). This behavior can be explained by the bacterial cell wall, which, due to its structural differences between Gram-positive and Gram-negative bacteria, may have facilitated the action of propolis in the case of Gram-negative bacteria [102, 103]. Similar results were obtained in other studies investigating red propolisis extracts with other origins [24, 104]. The antimicrobial activity mechanism of propolis is multifactorial and may be credited to the presence of some bioactive compounds, in particular flavonoids [105]. The antimicrobial action of propolis has been associated with different mechanisms that are essential for survival and proliferation, such as inhibition of nucleic acid synthesis, impacts on energy metabolism, biofilm formation and cytoplasmic membrane fluidity [106, 107].