In vitro biological activities of some South African Syzygium and Eugenia (Myrtaceae) species with potential as phytogenic feed additives

Antibiotic use in animals, especially as growth promoters in livestock feeds, is a risk factor for the development of antibiotic resistance in humans and contamination of the environment, constituting a one health challenge. Several alternatives including phytogenic feed additives may be potential substitutes. Plants are potential sources of compounds with antimicrobial properties.

Antiquorum sensing, anti-adherence, cytotoxicity, phytochemistry and ultrastructural effects of four South African plant crude acetone extracts; Eugenia zeyheri, E. erythrophylla, Syzygium legatii and S. gerrardii were determined. Anti-quorum sensing activity was determined by inhibition of quorum sensing (QS)-controlled violacein pigment production in Chromobacterium violaceum. Inhibition of adherence of E. coli to intestinal cells by extracts was evaluated using a Caco-2 cell enterocyte anti-adhesion model. Ultrastructural effects of E. zeyheri and S. legatii extracts on E. coli were assessed using electron microscopy. Phytochemical analysis was done using Gas Chromatography-Mass Spectrometry.

Minimum quorum sensing inhibitory concentration (MQSIC) and minimum bactericidal concentration (MBC) ranged between 0.08 and 0.16 mg/ml and 0.08 and 0.31 mg/ml respectively. Apart from S. legatii, other plant extracts and vanillin (positive control) had good antiquorum sensing activity. Eugenia zeyheri and S. gerrardii significantly reduced adhesion of E. coli to Caco-2 intestinal cells. The extracts of E. zeyheri and S. legatii damaged the cytoplasmic membrane and inner structure of E. coli. α-Amyrin, friedelan-3-one, lupeol, and β-sitosterol were abundant in the extracts.

The plant extracts have potential as phytogenic feed additives although animal trials are needed. Compounds in the extracts may be drug templates to develop novel antimicrobial agents.

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