Reversal of Polymicrobial Biofilm Tolerance to Ciprofloxacin by Blue Light plus Carvacrol

Chronic wound infections are often caused by multi-species biofilms and these biofilm-embedded bacteria exhibit remarkable tolerance to existing antibiotics, which presents huge challenges to control such infections in the wounds. In this investigation, we established a polymicrobial biofilm composed of P. aeruginosa, S. aureus, K. pneumoniae, and A. baumannii. We tested a cocktail therapy comprising 405-nm blue light (BL), carvacrol (Ca), and antibiotics on the multispecies biofilm. Despite the fact that all strains used to form the biofilm were susceptible to ciprofloxacin (CIP) in planktonic cultures, the biofilm was found to withstand ciprofloxacin as well as BL-Ca dual treatment, mainly because K. pneumoniae outgrew and became dominant in the biofilm after each treatment. Strikingly, when ciprofloxacin was combined with BL-Ca, the multispecies biofilms succumbed substantially and were eradicated at an efficacy of 99.9%. Mechanistically, BL-Ca treatment increased membrane permeability and potentiated the anti-biofilm activity of ciprofloxacin, probably by facilitating ciprofloxacin's entrance of the bacteria, which is particularly significant for K. pneumoniae, a species that is refractory to either ciprofloxacin or BL-Ca dual treatment. The results suggest that bacterial membrane damage can be one of the pivotal strategies to subvert biofilm tolerance and combat the recalcitrant multispecies biofilms.