Preliminary Study to Compare the Antimicrobial Efficacy of Aqueous and Ethanolic Ultrasound-Assisted Aloe vera Extracts

The growing threat of antimicrobial resistance has driven the search for natural alternatives, such as Aloe vera extracts, whose phytochemical composition and biological activity depend on the extraction solvent. This preliminary study compared the antimicrobial efficacy of aqueous and ethanolic (99%) A. vera extracts obtained by ultrasound-assisted extraction (UAE) under identical conditions, and explored their FTIR spectroscopic profiles. The extracts were characterized by FTIR and evaluated against Staphylococcus aureus ATCC 12493 and Escherichia coli ATCC 33876 using the agar well diffusion method and broth macrodilution to determine the Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC). In the well diffusion assay, the ethanolic extract produced larger inhibition halos than the aqueous extract (22.3 and 20.0 mm vs. 20.6 and 17.4 mm against S. aureus and E. coli, respectively, at 200 mg/mL), and two-way ANOVA showed significant effects of extract type and microorganism (p < 0.001). The ethanolic extract also showed a trend toward lower mean MIC values (62.0 and 85.6 mg/mL) than the aqueous extract (90.7 and 145.0 mg/mL); however, given the high standard deviations and the limited number of replicates (n = 3), the MIC/MBC results are inconclusive, as no specific statistical analysis was applied to them. Likewise, the FTIR spectra showed a relatively higher intensity of bands consistent with polysaccharides (O–H and C–O–C) in the aqueous extract and of the 1600–1650 cm−1 band (C=O/C=C), a spectral feature potentially associated with compounds such as anthraquinones and flavonoids, in the ethanolic extract; nevertheless, owing to band overlap, the semi-quantitative nature of the comparison and the absence of complementary analyses (e.g., HPLC-MS or NMR), these results are also inconclusive regarding the specific compound classes involved. Overall, the findings indicate a trend toward higher antimicrobial activity of the ethanolic extract against the two reference strains evaluated, which should be confirmed in future studies using broth microdilution with a larger number of replicates, identification of bioactive compounds, and a broader panel of microorganisms, including resistant clinical isolates, together with cytotoxicity assays.