Farnesol as antibiotics adjuvant in Staphylococcus epidermidis control in vitro

Farnesol is a sesquiterpenoid that has been described as impairing bacterial growth. Therefore, the goal of this study was to compare the in vitro postantimicrobial effect (PAE) of farnesol against Staphylococcus epidermidis with the corresponding values of most common practice antibiotics and also to evaluate the combined effect of farnesol with these antibiotics against planktonic and biofilm cells.

After exposure of S epidermidis cells to farnesol and antibiotics at minimum inhibitory concentration for 1 hour, the cells were regrown in medium without any antimicrobial agent. Cellular viability was assessed by colony-forming units, every hour for 12 hours, and then, the PAE was determined. The combined effect of farnesol (0, 30, 100 and 300 μM) with vancomycin, tetracycline and rifampicin was also evaluated, by using these antibiotics at peak serum concentration.

When PAE is concerned, it was found that cells grown in 100 μM of farnesol behaved similarly to cells that had never been in contact with farnesol, whereas a clear difference was obtained with cells exposed to 300 μM of farnesol, displaying a longer PAE. Farnesol showed a combined effect with the tested antibiotics against planktonic cells, although this was not so evident against biofilm cells.

Despite the reduced efficacy against biofilm cells, farnesol seems to be a potential adjuvant therapeutic agent to antibiotics against S epidermidis planktonic cells. Moreover, its long PAE makes farnesol a potential candidate in the prevention of biofilm formation because it showed to be very effective against planktonic cells alone as well.