ORIGINAL ARTICLE Presence of Quorum-sensing Systems Associated with Multidrug Resistance and Biofilm Formation in Bacteroides fragilis Lilian Pumbwe & Christopher A. Skilbeck & Hannah M. Wexler Received: 15 August 2007 / Accepted: 6 December 2007 / Published online: 11 January 2008 # Springer Science + Business Media, LLC 2008 Abstract Bacteroides fragilis constitutes 1–2% of the natural microbiota of the human digestive tract and is the predominant anaerobic opportunistic pathogen in gastroin- testinal infections. Most bacteria use quorum sensing (QS) to monitor cell density in relation to other cells and their environment. In Gram-negative bacteria, the LuxRI system is common. The luxR gene encodes a transcriptional activator inducible by type I acyl-homoserine lactone autoinducers (e.g., N-[3-oxohexanoyl] homoserine lactone and hexanoyl homoserine lactone [C6-HSL]). This study investigated the presence of QS system(s) in B. fragilis. The genome of American-type culture collection strain no. ATCC25285 was searched for QS genes. The strain was grown to late exponential phase in the presence or absence of synthetic C6-HSL and C8-HSL or natural homoserine lactones from cell-free supernatants from spent growth cultures of other bacteria. Growth, susceptibility to antimicrobial agents, efflux pump gene (bmeB) expression, and biofilm formation were measured. Nine luxR and no luxI orthologues were found. C6-HSL and supernatants from Yersinia enterocoli- tica, Vibrio cholerae, and Pseudomonas aeruginosa caused a significant (1) reduction in cellular density and (2) increases in expression of four putative luxR genes, bmeB3, bmeB6, bmeB7, and bmeB10, resistance to various antibiotics, which was reduced by carbonyl cyanide-m-chlorophenyl hydrazone (CCCP, an uncoupler that dissipates the transmembrane proton gradient, which is also the driving force of resistance nodulation division efflux pumps) and (3) increase in biofilm formation. Susceptibility of ATCC25285 to C6-HSL was also reduced by CCCP. These data suggest that (1) B. fragilis contains putative luxR orthologues, which could respond to exogenous homoserine lactones and modulate biofilm formation, bmeB efflux pump expression, and susceptibility to antibiotics, and (2) BmeB efflux pumps could transport homoserine lactones. Introduction Bacteroides fragilis constitutes 1–2% of the natural micro- flora of the human digestive tract and is the predominant anaerobic opportunistic pathogen in gastrointestinal infec- tions. Various factors contribute to its ability to persist in high numbers in the gut. A critical factor is that bacterial cells communicate with one another and synchronize behavior requiring cooperation between cells. In most bacteria, cell–cell communication has been demonstrated to greatly depend on quorum sensing (QS) by which bacteria respond to chemical hormone-like molecules called auto- inducers. As the population density increases, so does the concentration of autoinducer that they secrete. Once the autoinducer reaches a critical threshold representing a “quorum” of bacterial cells, the bacterium responds by altering the expression of several genes (e.g., genes involved with antibiotic, exoenzyme, capsular exopolysaccharide synthesis, and biofilm formation) and cell growth [7, 11]. Depending on the bacterial species, the physiological processes regulated by QS are extremely diverse. Three major QS circuits have been described: (1) communication by modified peptides (Gram-positive bacteria), (2) The Microb Ecol (2008) 56:412–419 DOI 10.1007/s00248-007-9358-3 L. Pumbwe (*) : C. A. Skilbeck : H. M. Wexler Greater Los Angeles Veterans Administration Healthcare Systems, Wadsworth Anaerobe Laboratory, Bldg. 304, Room E3-224, 691/151J, Los Angeles, CA 90073, USA e-mail: lilskil@hotmail.com L. Pumbwe : H. M. Wexler Department of Medicine, University of California, Los Angeles, CA, USA