Abstract:
BACKGROUND : Antimicrobial resistance (AMR) remains an important global health issue but the gap between AMR and
development of new antimicrobials is increasing. Plant extracts may have good activity per se or may be sources of
effective antimicrobial compounds which can act against planktonic and/or biofilms of pathogens. We determined the
antimicrobial efficacy and cytotoxicity of some under-investigated plants from the Myrtaceae family endemic to South
Africa. The ability of the plant extracts to inhibit or destroy pre-formed bacterial biofilms was also determined.
METHODS : Based on previous preliminary in vitro screening and on chemotaxonomy, nine species from the Myrtaceae
family were selected. The antimicrobial activity of the crude acetone leaf extracts was determined against six common
nosocomial pathogens, namely: Gram-positive bacteria (Bacillus cereus, Enterococcus faecalis, Staphylococcus aureus), Gramnegative
bacteria (Escherichia coli, Pseudomonas aeruginosa, Salmonella Typhimurium) using a two-fold serial microdilution
assay with p-iodonitrotetrazolium violet as growth indicator. The number of antimicrobial compounds present in extracts
was determined by bioautography. Cytotoxicity of extracts was determined against Vero kidney cells using a colorimetric
tetrazolium-based assay. The total antibacterial activity (TAA) in ml/g and selectivity index (LC50/MIC) of the plant extracts
were calculated. A modified crystal violet assay was used to determine the antibiofilm activity of the extracts.
RESULTS : Syzygium legatii, Syzygium masukuense, and Syzygium species A had the best activities against Gram-negative and
Gram-positive bacteria (MIC) values ranging from 0.04–0.08 mg/ml. Eugenia erythrophylla had the best MIC (0.02 mg/ml)
against Bacillus cereus. Many extracts had relatively low cytotoxicity (LC50>20 μg/ml) leading to reasonable selectivity indices.
Three leaf extracts (Syzygium masukuense, Syzygium species A, and Eugenia natalitia) were moderately cytotoxic (20 μg/ml <
LC50<100 μg/ml). The plant extracts had a good capacity to reduce biofilm formation and good to poor potential to
destroy pre-formed biofilms.
CONCLUSIONS : The plant species examined in this study had varying degrees of antibacterial activity against bacterial planktonic and biofilm forms with some having good activity against both forms. Several of these selected species may be potential
candidates for further investigation to isolate antimicrobial compounds and to determine the mechanism of activity.
