In vitro biological activities of Combretum molle R.Br. ex G. Don (Combretaceae) against mastitis-causing organisms

Abstract

Bovine mastitis, caused mostly by Staphylococcus species, has gained global importance owing to the increasing prevalence of multi-drug resistant bacteria, which comprise a risk to food security and public health. In this study, the antibacterial, anti-biofilm and quorum quenching activities of six leaf extracts of Combretum molle R.Br. ex G. Don prepared using different solvents were assessed against clinical isolates of Staphylococcus aureus and two Staphylococcus ATCC strains (S. aureus ATCC 29213 and S. epidermidis ATCC 35984). The antioxidant and anti-inflammatory activity and cytotoxicity of the extracts was also evaluated. The antibacterial and anti-biofilm potential of the extracts was determined via serial microdilution and crystal violet and INT assays. Quorum quenching activity was ascertained via inhibition of violacein production in Chromobacterium violaceum ATCC 12472. The antioxidant activity was determined using in vitro chemical assays. The 15-lipoxygenase enzyme inhibition and the nitric oxide inhibition assays were utilized to ascertain the anti-inflammatory activity of the extracts. The tetrazolium-based colorimetric (MTT) reduction assay was used to determine the cytotoxicity of the extracts against Vero African green monkey kidney cells. The antibacterial activity of the extracts was moderate to good, with minimum inhibitory concentration (MIC) values of 0.02–0.63 mg/mL. The extracts had promising biofilm inhibition (≥50 %) against four of the test strains. The effects of the extracts on the metabolic activity of the test strains showed ≥ 50 % inhibition in most of the test strains at the different test times. All extracts had moderate to excellent quorum quenching activity at different concentrations with an inhibition ranging between 32.81- 96.32 %. The methanol and cold water extracts had the best antioxidant activity against the 2, 2-diphenyl-1-picrylhydrazyl (DPPH) and 2, 2′-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radicals with IC50 of 7.79 and 2.34 µg/mL, respectively. The hot water extract (AQH) had the best anti-inflammatory activity in terms of 15-lipoxygenase enzyme inhibition with 85.26 % inhibition, while the methanol extract had the best nitric oxide inhibition in LPS-induced RAW 264.7 macrophages with inhibition of 96.75 % at 100 µg/mL. Selectivity index values of the extracts were as high as 50. Combretum molle, therefore, offers promising potential as a source of antibiofilm and anti-quorum sensing agents with the ability to disrupt microbial virulence factors. Additionally, the extract had good antioxidant and anti-inflammatory properties, supporting further research into its use in preventing and treating bovine mastitis.