Phenolic Plant Extracts Versus Penicillin G: In Vitro Susceptibility of Staphylococcus aureus Isolated from Bovine Mastitis

Antibiotics are the elective drugs in bovine mastitis (BM) treatment, despite their low rates of efficiency and effectiveness and increasing risk of pathogen resistance. In this sense, it is urgent to discover new and effective antimicrobial agents to apply in BM control and even treatment. Plant extracts have been widely recognized as a rich source of phytochemicals with antimicrobial potential. Thus, the present work aims to compare the bioactivity of <i>Eucalyptus globulus</i> and <i>Juglans regia</i> extracts against <i>Staphylococcus aureus</i> bovine mastitis strains with penicillin G. At non-toxic concentrations, <i>E. globulus</i> exerted a bacteriostatic effect in planktonic cells and <i>J. regia</i> had no antimicrobial activity. Penicillin G, at minimum inhibitory concentration (MIC), demonstrated bactericidal activity, but just for <i>S. aureus</i> 3, 5, 6 and ATCC 25923, while the other strains seem to have acquired resistance. On the other hand, <i>E. globulus</i> and penicillin G in combination demonstrated synergy, being the most effective approach against <i>S. aureus</i> 1, 2 and 4. Thus, penicillin alone and in combination with <i>E. globulus</i> or <i>J. regia</i> seems to be promissory strategies to control bovine mastitis infections.