Evaluation of leaf extracts of several tree species for activity against Salmonella

Abstract

Handling and consumption of contaminated foods (meat, milk and eggs) are considered a major source of infection in humans. This public health risk reinforces the need for close monitoring and collaboration between public health and veterinary authorities to mitigate the risk at the humananimal interface necessary for health and food security. Gastroenteritis that is not self-limiting is mostly treated with a wide range of antibiotics. The indiscriminate use of these antibiotics has resulted in the upsurge of resistant and multiresistant strains of bacteria. This complicates treatment, especially in patients with human immunodeficiency virus (HIV) infection, necessitating the search for novel, cheaper, safer and efficacious antibacterial products. Recent in vitro studies have revealed that indigenous South African plants possess antimicrobial properties against gastrointestinal disorders and diarrhoea-causing organisms. In a preliminary screening, the antibacterial activities of acetone, ethanol, methanol and water extracts of the leaves were determined using a two-fold serial microdilution method against a range of pathogenic bacteria (Staphylococcus aureus, Enterococcus faecalis, Bacillus cereus, Escherichia coli, Salmonella Typhimurium and Pseudomonas aeruginosa). The plant species investigated were Protorhus longifolia, Searsia leptodictya, Carissa macrocarpa, Combretum bracteosum, Kirkia wilmsii, Loxostylis alata, Brachychiton acerifolium, Brachychiton bidwillii, Noltea africana and Blighia unijugata. All the extracts had activity against at least one of the test organisms over an incubation period of 24 hours. The average MIC values of the plant extracts against the different bacteria ranged from 0.2 mg/ml to 1.4 mg/ml. The Gram-positive bacteria (S. aureus, B. cereus and E. faecalis) were more susceptible to the plant extracts than the Gramnegative bacteria (E. coli, S. Typhimurium and P. aeruginosa). P. longifolia and L. alata extracts were the most active against nearly all the bacteria tested with MIC values as low as 0.02 mg/ml. L. alata was selected for further work to isolate compounds active against Salmonella species.