Abstract:
BACKGROUND: Antibiotics are commonly added to livestock feeds in sub-therapeutic doses as growth promoters
and for prophylaxis against pathogenic microbes, especially those implicated in diarrhoea. While this practice has
improved livestock production, it is a major cause of antimicrobial resistance in microbes affecting livestock and
humans. This has led to the banning of prophylactic antibiotic use in animals in many countries. To compensate for
this, alternatives have been sought from natural sources such as plants. While many studies have reported the
antimicrobial activity of medicinal plants with potential for use as phytogenic/botanical feed additives, little
information exists on their mode of action. This study is based on our earlier work and describes ultrastructural
damage induced by acetone crude leaf extracts of Syzygium legatii and Eugenia zeyheri (Myrtaceae) active against
diarrhoeagenic E. coli of swine origin using scanning electron microscopy (SEM), transmission electron microscopy
(TEM), and fluorescent microscopy (FM). Gas chromatography/mass spectrometry (GC-MS) was used to investigate
the chemical composition of plant extracts.
RESULTS: The extracts damaged the internal and external anatomy of the cytoplasmic membrane and inner
structure at a concentration of 0.04 mg/mL. Extracts also led to an increased influx of propidium iodide into treated
bacterial cells suggesting compromised cellular integrity and cellular damage. Non-polar compounds such as αamyrin, friedelan-3-one, lupeol, and β-sitosterol were abundant in the extracts.
CONCLUSIONS: The extracts of S. legatii and E. zeyheri caused ultrastructural damage to E. coli cells characterized by
altered external and internal morphology. These observations may assist in elucidating the mode of action of the
extracts.
