ANTI-BACTERIAL ACTIVITY OF PLANT EXTRACTS AGAINST STAPHYLOCOCCUS.

The wide usage of antimicrobial agents for the treatment of pathogens found in mastitis in veterinary medicine is plagued with limited success due to the increasing prevalence of resistance to commonly used antimicrobial agents. Most antibiotics (particularly β-lactams) used in mastitis treatment are reported to be ineffective against Staphylococcus aureus, which is capable of releasing toxins that compromise the natural defences of the udder. Plant-derived natural products have been established as effective models for the development of new drugs since they have shown a range of useful biological activities. The use of natural plant products has increasingly become a recognized treatment for several ailments, and this is one of the rapidly growing sectors of the agribusiness industry. Given the rising prevalence of Staphylococcus species and their growing resistance to commonly used antibiotics, there is a global effort to investigate natural products as potential sources of new and safer drugs. Therefore, in the study cited the authors aimed to investigate the extracts of nine selected South African plants for their antibacterial activity against drug-resistant strains of S. aureus and non-aureus staphylococci (NAS) isolated from clinical cases of bovine mastitis. The cytotoxic effects of extracts of three plants selected based on their unique antibacterial activities were also determined to evaluate their antibacterial efficacy.

Antibacterial and cytotoxic activities of acetone and ethanol extracts of nine South African plants were determined against reference and drug-resistant staphylococci isolated from clinical cases of bovine mastitis. The nine plants, namely, Antidesma venosum, Elaeodendron croceum, Erythrina caffra, Indigofera frutescens, Pleurostylia capensis, Searsia lancea, Searsia leptodictya, Trichilia emetica, and Ziziphus mucronata, were chosen for research material. The antibacterial activity of extracts was determined using a serial micro-dilution method, while a tetrazolium-based assay was used to determine their cytotoxicity against Vero cells.

The values of antibacterial minimum inhibitory concentration (MIC) obtained were promising, with MIC ranging between 0.01 and 1.41 mg/mL. Generally, acetone extracts of most plants had better activity than their ethanol counterparts. The S. aureus strains were more susceptible to all the extracts than the NAS strains. The plant extracts’ half-maximal toxicity (LC50) was higher than 0.02 mg/mL, which is the recommended cytotoxic cut-off concentration. The ethanol extract of E. caffra, which showed better MIC values than its acetone extract, had the highest mean selectivity index (SI) of 8.30. Among the plants tested, S. lancea exhibited the most potent antibacterial activity, suggesting its potential usefulness as a broad-spectrum antibacterial agent against staphylococci associated with bovine mastitis, at relatively non-cytotoxic concentrations.