BACKGROUND : Diarrhoea, a global economically important disease burden affecting swine and, especially piglets, is
commonly caused by infection with entero-toxigenic E. coli (ETEC). Adherence of ETEC to porcine intestinal
epithelial cells following infection, is necessary for its pathogenesis. While antimicrobials are commonly given as
therapy or as feed additives for prophylaxis against microbial infections, the concern over increased levels of
antimicrobial resistance necessitate the search for safe and effective alternatives in livestock feed. Attention is
shifting to natural products including plants as suitable alternatives to antimicrobials.
The activity of acetone crude leaf extracts of nine under-explored South African endemic plants from the Myrtaceae
family with good antimicrobial activity were tested against pathogenic E. coli of porcine origin using a microplate
serial dilution method. Bioautography, also with p-iodonitrotetrazolium violet as growth indicator was used to view
the number of bioactive compounds in each extract. In vitro toxicity of extracts was determined against Caco-2
cells using the 3-(4,5-dimethythiazolyl-2)-2,5-diphenyltetrazolium bromide reduction assay. The antimicrobial
susceptibility of E. coli isolates was tested on a panel of antimicrobials using the Kirby-Bauer agar diffusion method
while the anti-adherence mechanism was evaluated using a Caco-2 cell enterocyte anti-adhesion model.
RESULTS : The MIC of the extracts ranged from 0.07–0.14 mg/mL with S. legatii having the best mean MIC (0.05 mg/
mL). Bioautography revealed at least two active bands in each plant extract. The 50% lethal concentration (LC50)
values ranged between 0.03–0.66 mg/mL. Eugenia zeyheri least cytotoxic (LC50 = 0.66 mg/ml) while E. natalitia had
the highest cytotoxicity (LC50 = 0.03 mg/mL). All the bacteria were completely resistant to doxycycline and colistin
sulphate and many of the plant extracts significantly reduced adhesion of E. coli to Caco-2 cells.
CONCLUSIONS : The extracts of the plants had good antibacterial activity as well as a protective role on intestinal
epithelial cells against enterotoxigenic E. coli bacterial adhesion. This supports the potential use of these species in
limiting infection causes by E. coli. Some of these plants or extracts may be useful as phytogenic feed additives but
it has to be investigated by animal feed trials.