Antimicrobial activity of phytochemical-antibiotic combinations against pathogenic bacteria

Biofilms underpin major health and economic burdens, accounting for up to 80% of all microbial infections in humans. Moreover, the treatment of biofilms has been increasingly troubled due to the rising of antibiotic resistance, which makes the use of antibiotics alone ineffective for treating biofilm-associated infections. In this way, novel antimicrobial strategies should be designed outside the constricted antibiotics box. Nowadays, natural products, in particular phytochemicals, have been explored as a promising antimicrobial approach due to their attractive source of eco-friendly, relatively inexpensive and broad-spectrum antimicrobials with low levels of cytotoxicity, coupled with their multiple action mechanisms that can prevent the emergence of resistant bacteria. However, though hopeful, the antimicrobial potential of phytochemicals is often weaker than that of common antibiotics. To address these issues, the present study investigated the antimicrobial potential of selected phytochemicals alone and in combination with standard antibiotics against Staphylococcus aureus, including a drug-resistant strain, and Escherichia coli. The antimicrobial activity of the phytochemicals alone and their combinations with gentamicin, mupirocin, and fusidic acid were determined through the minimum bactericidal concentration (MBC). Among the selected phytochemicals, citronellol presented the highest antimicrobial activity against S. aureus (MBC of 512 µg/mL) and cis-6-nonen-1-ol displayed the highest antimicrobial activity against both drug-resistant S. aureus and E. coli (MBC ranging from 1024 to 2048 µg/mL). In addition, bacterial cells were found to be eradicated at lower doses of selected phytochemicals and antibiotics when combined. This study highlights the promising phytochemical-antibiotic combinatorial approach for dealing with biofilm-associated infections.