Purpose: To evaluate the antimicrobial activity of methanolic extracts of varieties of nopal cactus and correlate this activity with the content of antioxidant compounds.
Methods: Five cultivars of nopal (four of Opuntia and one of Nopalea) were used in this study. Plant material was washed and dried and macerated in methanol. Preliminary antimicrobial assays were performed using the agar well diffusion technique, using LB agar, PCA agar and MH agar supplemented with 5% sheep blood for Vibrio cholerae, Campylobacter perfringens and C. jejuni/coli,respectively. Minimum bactericidal concentrations (MBCs) of nopal varieties were evaluated by a microdilution method. Assays for bioactive compounds and antioxidant activity included total phenols, total flavonoids and DPPH radical scavenging activity (%RSA).
Results: All extracts showed antimicrobial and antioxidant activities. The MBC ranged from 0.5-4.0 mg/ml, 0.4-2.5 mg/ml and 0.5-7.0 mg/ml for C. perfringens, C. jejuni/coli and V. cholera, respectively. High content of total phenols and flavonoids was found in Opuntia ficus-indica var. Villanueva, (193.02 mg GAE/g, and 139.7 mg CAE/g, respectively), while the extracts of var. Copena showed the lowest phenols and flavonoids content (43.53mg GAE/g and 29.31 mg CAE/g, respectively). DPPH radical scavenging activity was related with content of bioactive compounds. The Villanueva variety has the highest % RSA at 94.7, while the lowest activity was recorded for Copena at 42.7%.
Significance : Our results demonstrate the antimicrobial activity of all varieties of nopal that parallel the antioxidant activity of the extracts. These characteristics can be exploited in this abundant vegetable for its use in the food industry.