Bacteria in the natural ecosystem frequently live as adherent communities called biofilms. Some chemical compounds are known to affect biofilm formation. We investigated the effect of exogenous small molecules, N-acylhomoserine lactones (AHLs), beta-lactam antibiotics, and adenosine, on biofilm formation in the beta-lactam antibiotic-resistant bacterium Acidovorax sp. strain MR-S7. Biofilm formation was induced by the addition of various types of AHL isomers and beta-lactam antibiotics, whereas the addition of adenosine strongly interfered with the biofilm formation. A gene (macP) encoding adenosine deaminase (that converts adenosine to inosine controlling intracellular adenosine concentration) was successfully cloned from MR-S7 genome and heterologously expressed in Escherichia coli. The purified MacP protein clearly catalyzed the deamination of adenosine to produce inosine. A transcriptional analysis revealed that biofilm-inducing molecules, an AHL and a beta-lactam antibiotic, strongly induced not only biofilm formation but also adenosine deaminase gene expression, suggesting that an elaborate gene regulation network for biofilm formation is present in the beta-lactam antibiotic-resistant bacterium studied here. (C) 2013, The Society for Biotechnology, Japan. All rights reserved.