INTRODUCTION

Many species of bacterium exchange chemical signals to help them monitor their population densities, a phenomenon referred to as quorum sensing. Quorum sensing was first described over two decades ago in two luminescent marine bacterial species, Vibrio fischeri and V. harveyi (40), which have served as model species for studies of cell-density-dependent gene expression. In both species, the enzymes responsible for light production are encoded by the luciferase structural operon luxCDABE (13, 39) and light emission occurs only at high cell density in response to the accumulation of secreted autoinducer signaling molecules (40). In the 1980s, Eberhard et al. (11) purified the first homoserine lactone autoinducer from V. fischeri and showed that it was indeed a specific inducer of luminescence. In 1983, the basic features of the autoinduction system were revealed at the molecular level when the lux genes of V. fischeri were successfully cloned and expressed in Escherichia coli (12).

Although quorum sensing regulation has been analyzed in great detail in V. harveyi and V. fischeri, the study of quorum sensing phenotypes in the clinically important Vibrio species V. cholerae was virtually non-existent until quite recently. This was partly because, unlike V. fischeri and V. harveyi, V. cholerae does not possess luciferase genes and it was therefore unclear whether it possessed any genes that were regulated by quorum sensing. However, when the V. cholerae genome sequence was completed (22) it was revealed that V. cholerae contains several quorum-sensing genes similar to those of V. harveyi.