Antimicrobial activity of plant-food by-products: A review focusing on the tropics

• The antimicrobial activity of plant food by-products of tropical origin is reviewed.
• The extracts of coconut roots and banana peels display high antibacterial activity.
• Alkaloids, phenolics and saponins from by-products show high antibacterial activity.
• By-product extracts and their isolated compounds show similar antimicrobial activity.

This review characterizes the antimicrobial potential of agricultural by-products from tropical countries, assessing their suitability as substitutes for antibiotics in animal-production farms. This study responds to an increasing trend in the use of antibiotics and other growth promoters in farm animals in tropical areas. Such use is intended to improve the daily gastrointestinal welfare and also to provide resistance or prevention against acute or chronic diseases, such as infectious diarrhoea and inflammatory bowel diseases. Such diseases pose a major challenge in all countries, but tropical conditions encourage the survival of bacteria and pathogens and commensal bacteria more than in temperate climates, and therefore tropical countries need particular attention in order to solve this dilemma. Fortunately, as a substitute to antibiotics, these countries have considerable antimicrobial potential in plants – that is, agricultural by-products contain a diverse pool of bioactive compounds with antimicrobial properties, which could be employed as feed supplements to improve animal health. By-products from tropical countries constitute rich sources of bioactive compounds, such as phenolics, carotenoids, essential oils, active peptides, saponins, and sterols. Among reviewed by-products, high activity has been detected for avocado seeds, cocoa bean shell, and banana peels, while for isolated pure compounds, high activity has been reported for: alkaloids from lupine and capsaicin; phenolics such as gallic and chlorogenic acids, naringin, exiguaflavanone D, and kenusanone A; and saponins from Capsicum seeds. Some by-product extracts have shown minimum inhibitory concentration (MIC) values very close to that of their isolated pure components. In conclusion, plant-food by-products of tropical origin contain diverse active compounds which act effectively against most pathogenic bacteria tested, avoiding well-characterized cell damage.