The quick spread of nosocomial bacterial infections and the increasing prevalence of drugresistant strains make the development of novel drugs for pathogens an urgent priority. Quorum sensing (QS) is a communication mechanism used by bacteria to recognize population density fluctuations and control gene expression, which play a critical role both in intraspecies and interspecies communications and regulates microbe-host interactions. Low-molecular-weight signal compounds, such as acyl-homoserine lactone and autoinducing peptide, are used by QS to control the expression of different pathogenic factors. Thus QS--and QS signal molecules in particular--is an attractive target for developing novel antimicrobial methods. Quorum-quenching enzymes, which hydrolyze or modify signal molecules in QS circuit systems to inhibit the expression of bacteria virulence factors, have been identified both in prokaryotes and eukaryotes. Understanding the mechanism of action of quorum-quenching enzymes also provides a promising means to control bacterial infection. This review first introduces the novel principle underling signal-based QS systems in several important pathogens and then focuses on the newly identified quorum-quenching enzymes, including lactonases, acylases, oxidoreductases, and paraoxonases; this summary introduces new concepts of antimicrobial infection.