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 acylhomoserine 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.Biofilm-associated infections are often very difficult to treat with conventional antibiotics (7,17,28,38). Hence, novel targets are needed to combat biofilm infections. One of them could be the bacterial communication system (quorum sensing [QS]). Bacteria coordinate their cell-density-dependent gene expression by excreting small signaling molecules (26). When a certain extracellular threshold concentration is reached, these molecules bind to a receptor, thereby activating the QS system. The typical QS system in Gram-negative bacteria consists of three components: a LuxI synthase homolog, acyl-homoserinelactone (AHL) signaling molecules, and a LuxR receptor homolog (10). Gram-positive bacteria generally use small peptide signaling molecules, which are transported out of the cell and bind to a membrane-associated two-component receptor (42). Binding to the receptor activates a signal transduction system leading to the transcription of QS-regulated genes. A third QS system using autoinducer 2 (AI-2) as signaling molecule is widespread in both Gram-positive and Gram-negative bacteria and is considered to be responsible for interspecies communication (43). QS has been shown to regulate biofilm formation in several bacterial species (15,20). AHL QS mutants of Burkholderia cenocepacia and Pseudomonas aeruginosa form biofilms with a drastically altered str...

Exploiting Quorum Sensing To Confuse Bacterial Pathogens

“…In tests where treatments were initiated 11 days postimplant, they were not beneficial, indicating the need to intervene before bacteria enter a chronic infection state and/or before the host immune system walls off the infection. In separate studies, combinatorial application of the previously identified QSI compounds baicalin hydrate, hamamelitannin, and cinnamaldehyde with antibiotics was tested on Burkholderia spp., S. aureus, and P. aeruginosa to measure viable counts of bacteria in biofilms and in animal models of infection (283,284). Synergistic effects were most pronounced in Burkholderia cepacia biofilms treated with baicalin hydrate combined with tobramycin, which resulted in an additional 2-log decrease in viable cell counts compared to tobramycin treatment alone.…”

Biofilms: Microbial Cities of Scientific Significance

“…One of the major reasons for the increase in resistance to antibiotics is the expression of different genes that encodes a set of protein that confers the microbial community with this character. In addition to genetic factors which involve the expression of vital genes in favor of the microorganisms, the extra polymeric matrix prevents the penetration of the antimicrobial agents in to biofilms and quorum sensing is another vital factor that favors the microbial communities within biofilms [92]. The biofilm structure enables the pathogens to tolerate the antimicrobial agent which is an in built character of the bacterial biofilms [93].…”

Garlic ointment inhibits biofilm formation by bacterial pathogens from burn wounds

“…Garlic extract has also been shown to increase the susceptibility of P. aeruginosa to tobramycin and to phagocytosis Rasmussen et al, 2005), and, in the murine model of lung infection, it has been shown to significantly reduce mortality rate and promote rapid clearing of P. aeruginosa from the lung . Furthermore, garlic extract blocks P. aeruginosa and Acinetobacter baumannii quorum sensing systems Harjai et al, 2010;Rasmussen et al, 2005;Sarkar & Chakraborty, 2008), which have been shown to control biofilm development (Brackman et al, 2011;Davies et al, 1998).…”