Quorum sensing inhibitors increase the susceptibility of bacterial biofilms to antibiotics in vitro and in vivo

Abstract

Although the exact role of quorum sensing (QS) in various stages of biofilm formation, maturation, and dispersal and in biofilm resistance is not entirely clear, the use of QS inhibitors (QSI) has been proposed as a potential antibiofilm strategy. We have investigated whether QSI enhance the susceptibility of bacterial biofilms to treatment with conventional antimicrobial agents. The QSI used in our study target the acyl-homoserine lactone-based QS system present in Pseudomonas aeruginosa and Burkholderia cepacia complex organisms (baicalin hydrate, cinnamaldehyde) or the peptide-based system present in Staphylococcus aureus (hamamelitannin). The effect of tobramycin (P. aeruginosa, B. cepacia complex) and clindamycin or vancomycin (S. aureus), alone or in combination with QSI, was evaluated in various in vitro and in vivo biofilm model systems, including two invertebrate models and one mouse pulmonary infection model. In vitro the combined use of an antibiotic and a QSI generally resulted in increased killing compared to killing by an antibiotic alone, although reductions were strain and model dependent. A significantly higher fraction of infected Galleria mellonella larvae and Caenorhabditis elegans survived infection following combined treatment, compared to treatment with an antibiotic alone. Finally, the combined use of tobramycin and baicalin hydrate reduced the microbial load in the lungs of BALB/c mice infected with Burkholderia cenocepacia more than tobramycin treatment alone. Our data suggest that QSI may increase the success of antibiotic treatment by increasing the susceptibility of bacterial biofilms and/or by increasing host survival following infection.

Fig. 1.

Log CFU/biofilm (average ± the SD) in untreated 24-h-old B. multivorans and B. cenocepacia biofilms in MH medium and in biofilms treated with TOB or a QSI (BH or CA) alone or a combination of TOB and a QSI. Combined treatment was significantly more effective than treatment with TOB alone for both QSI and for all strains investigated (P < 0.05).

Fig. 2.

Percent survival of infected C. elegans (average ± the SD) after treatment with antibiotics (TOB for the B. cenocepacia and P. aeruginosa strains and VAN for S. aureus Mu50), QSI (BH for the B. cenocepacia and P. aeruginosa strains and HAM for S. aureus Mu50), or combinations of the respective compounds. The results are expressed as the percent survival after 48 h of infection and treatment. All combined treatments were significantly more effective than treatment with antibiotic alone (P < 0.01).

Fig. 3.

Kaplan-Meier survival-curve of infected G. mellonella receiving no treatment (CTRL) or a treatment with an antibiotic (TOB or VAN), a QSI (BH or HAM), or a combination of both. *, Significantly different compared to no treatment (P < 0.05); **, significantly increased survival compared to antibiotic treatment alone (P < 0.05).