Antimicrobial potential of Pleurotus ostreatus against bovine mastitis pathogens: an in vitro screening study

Abstract Mastitis is a common multifactorial disease in dairy herds, usually associated with infections caused by bacteria, fungi or pathogenic microalgae and commonly treated with antibiotics. Widespread antibiotic resistance has prompted researchers around the world to search for new alternatives to combat infections. For centuries, plants, herbs and other natural-origin antimicrobial agents have been used in medicine worldwide. Therefore, the aim of this preliminary in vitro screening study was to evaluate the antibacterial effects of 10 Pleurotus ostreatus preparations against Staphylococcus aureus and Escherichia coli isolated from bovine mastitis cases. The preparations obtained using different extraction and preparation methods were evaluated at concentrations of 10%, 5%, 2.5%, 1.25%, and 0.625% using an XTT-based bacterial viability assay. The effects varied substantially among bacterial strains, preparations and concentrations. B9 (derived from oil-press pomace after hot-water treatment and subsequent methanol maceration) and B10 (derived from slow-juicer pomace by methanol maceration) showed the most consistent overall reductions in bacterial viability across the tested conditions, whereas other preparations produced pronounced reductions only in selected pathogen–concentration combinations. The observed responses were preparation-, pathogen-, and concentration-dependent and did not follow a uniform concentration-response pattern. In contrast, some preparations and concentrations were associated with bacterial viability values similar to or higher than those of the control, indicating a non-linear concentration-response pattern. These preliminary findings identify promising P. ostreatus preparations for further chemical characterization and safety assessment and support their continued investigation as potential sources of naturally derived antimicrobial compounds. However, the present results do not demonstrate bactericidal activity or therapeutic efficacy in cattle, and further studies, including evaluation in bovine mammary cells and subsequent in vivo validation, are required before any potential veterinary application in bovine mastitis can be considered.