The individual behavior of prokaryotic organisms such as bacteria often gives rise to complexity that is commonly associated with collective behavior. The transition from unicellular to collective behavior occurs in response to chemical signals called autoinducers, which bacteria generate and receive internally within a given population. These autoinducers control the gene expression necessary for the emergence of group-behavior-phenotype. This phenomenon is called quorum sensing. An example of the quorum sensing control of gene regulation is the luminescence (lux) operon in Vibrio fischeri. V. fischeri is a marine bacterium that lives in symbiosis with squids and fish. The light produced by the bacteria is used by the squid to mask its shadow from being formed on the sea-floor, thereby fooling its predators. The squid, on the other hand, provides nutrients to the bacteria.

It was discovered that luminescence in V. fischeri is regulated by two main signalling mechanisms - the luxI and ainS mechanisms. These two mechanisms control luminescence through the autoinducers C6-HSL and C8-HSL. These autoinducers bind to LuxR, a transcriptional activator of lux operon, which activates expression of lux genes causing an increase in luminescence. There exists a third system called luxS, which was also shown to regulate luminescence through AI-2, in conjunction with C6-HSL and C8-HSL. Studies have shown that quorum signaling also affects motility and biofilm formation in bacteria. However, the evidence with respect to these phenotypes is conflicting and inconclusive, the reason being the state of quorum is ambiguously defined. It is not properly known whether the observed collective behavior is purely a result of physical crowding of bacteria or that both chemical signaling, and crowding contribute to this phenomenon. My work attempts to address these issues by studying luminescence, motility, and biofilm, a diverse set of behaviors, yet closely linked to each other in V. fischeri-squid symbiosis.

Bright Field Image of a 24 hour biofilm of ES114, a wild-type strain of V. fischeri

Signalling mechanisms in V. fischeri