Evaluation of antimicrobial activity and phenolic and flavonoid quantification of Caesalpinia ferrea stem bark

Caesalpinia ferrea Mart. (Libidibia ferrea), Leguminosae, has been used in traditional medicine because of its anti-inflammatory, analgesic, antimicrobial and antipyretic properties¹. Some of these effects are related to phenolic compounds such as tannins and flavonoids produced in this species2. Seasonal variability may modify the secondary metabolism of plants, causing variations in the class and quantity of these compounds produced throughout the year. In this sense, this work aimed to correlate the antimicrobial activity with the total phenolic and flavonoid contents in ethanolic extracts of the stem bark of C. ferrea. This plant was obtained seasonally in the caatinga and Atlantic forest biomes of Bahia. The extracts were tested against the microorganisms Staphylococcus aureus, Bacillus subtilis, B. cereus, Escherichia coli, Salmonella choleraesius, Candida albicans and Fusarium oxysporum by the broth microdilution method3. Flavonoid and total phenolic contents were quantified by the spectrophotometric methods with AlCl3 (standard: quercetin) and Folin-Ciocalteau (standard: gallic acid). The coefficients of determination for the flavonoid and total phenolic determinations were 0.9994 and 0.9977, respectively. Among the extracts tested, the ethanolic and aqueous extracts of the fruit were active for S. aureus, with minimum inhibitory concentration (MIC) of 250 and 1000 μg mL-1, respectively. The aqueous extract was also active for C. albicans (MIC = 500 μg mL-1). Ethanolic extract of the fruit presented the highest contents of phenolics and total flavonoids in relation to the aqueous extract. Stem bark extracts showed activity for C. albicans fungus with MIC values ≤ 250 μg mL-1. Among the tested extracts, the most active was the stem extract collected from the Atlantic Forest during spring (MIC = 125 μg mL-1). The phenolic and flavonoid contents for this extract were 304.88 µg mg-1 EGA (Gallic Acid Equivalent) and 5.11 µg mg-1 EQ (Quercetin Equivalent), respectively. Stem extracts showed activity for S. aureus, B. subtilis and B. cereus. We highlight the extracts collected in the Atlantic Forest during winter and spring with MIC = 250 μg mL-1 for the microorganisms S. aureus and B. cereus. These extracts also presented higher phenolic contents among those of the Atlantic Forest, with values of 296.58 and 296.58 µg mg-1 EGA, in the order. The antibacterial effect of the extracts may be associated with flavonoids. Antifungal activity against C. albicans can be inferred from the presence of phenolics such as tannins. All extracts had much lower flavonoid content than phenolic content, so it can be inferred that phenolic and flavonoid contents are not directly related. This statement can be explained by the presence of other classes of phenolic compounds, such as tannins in these plant organs.