Susceptibility patterns of Escherichia coli and streptococcal isolates from bovine mastitis cases to antibiotics and selected South African plant extracts with known antibacterial activities

Bovine mastitis, an inflammatory condition affecting the mammary glands of dairy cattle, poses a significant economic burden on the dairy industry. Staphylococcus species, Streptococcus species and Escherichia coli are common pathogens. Managing mastitis typically involves antibiotics, but antimicrobial resistance (AMR) is a growing concern, promoting further research exploring alternative treatments including plant extracts. This study aimed to determine the antibiotic resistance of E. coli and streptococcal isolates from mastitis cases, and to investigate four indigenous South African plants for their selective antibacterial activities against these pathogens. The antimicrobial susceptibility of the bacterial strains was determined using a standard disc diffusion method. Minimum inhibitory concentration (MIC) values of acetone and ethanol extracts of Searsia lancea, Indigofera frutescens, Erythrina caffra and Antidesma venosum were determined against the bacterial isolates using a serial microdilution assay, and cytotoxicity was also investigated. The results showed that 82.14% of the clinical isolates tested were resistant to at least one antimicrobial agent used, and 52.17% of the antibiotic resistant isolates were multidrug resistant. All plant extracts had antibacterial activity against all the bacterial isolates, but S. lancea demonstrated higher efficacy compared to other plants. The MIC values ranged from 0.01 to 2.50 mg/mL, with the lowest range obtained with the acetone extract of S. lancea (0.01 to 0.57 mg/mL). Furthermore, the extracts were relatively non-cytotoxic to bovine dermis and Vero cells, with the highest mean selectivity index value of 25.70 recorded with S. lancea. This study highlights the growing concern of AMR in livestock management, and demonstrates the promising therapeutic potential of the selected plant species, particularly S. lancea, in treating bovine mastitis. Further exploration of S. lancea is recommended to develop novel alternative or complementary formulations for mastitis management.