Testing the hypothesis of quorum sensing in vibrio fischeri : luminescence, motility, and biofilm

Abstract

The individual behavior of prokaryotic organisms such as bacteria often gives rise to complexity typically associated with multicellular systems. The transition from unicellular to multicellular behavior occurs in response to chemical signals called autoinducers, which bacteria produce and detect within a population. These autoinducers regulate gene expression necessary for group-behavior phenotypes, a phenomenon known as quorum sensing (QS).

A classic example is the luminescence operon in Vibrio fischeri. The quorum signaling systems luxI and ainS work together to regulate luminescence. LuxI and AinS are acyl-synthases that produce autoinducers C6-HSL and C8-HSL, respectively. These bind to LuxR, a transcriptional activator of the lux operon, which induces expression of lux genes and increases luminescence. QS has also been implicated in motility and biofilm formation, though evidence remains conflicting, partly due to ambiguity in defining quorum.

Luminescence Studies Luminescence induction in cultures is not always proportional to cell density and shows growth-phase-dependent behavior.

Luminescence per cell is a more accurate measure than bulk luminescence.

Exogenous addition of C6-HSL and C8-HSL increased luminescence per cell, with C6-HSL being more effective.

Luminescence peaked during the mid-exponential phase and declined thereafter.

qRT-PCR analysis showed maximum expression of luxI and luxR at mid-exponential phase, correlating with luminescence.

Kinetic modeling confirmed that luxI mRNA expression follows the luminescence trend.

Motility Studies Motility is critical for V. fischeri colonization of squid light organs.

Previous studies suggested ainS signaling represses flagellar activity, but these were based on soft agar assays.

Real-time planktonic motility assays with ES114 and mutant strains showed that autoinducers do not affect motility.

Reduction in motility was attributed to physical crowding, not chemical signaling.

Biofilm Studies Crystal violet assays revealed that V. fischeri produces exopolysaccharides in both planktonic and biofilm states.

qRT-PCR analysis showed genes associated with exopolysaccharide production are expressed under both conditions.

Results were inconclusive regarding the role of signaling in biofilm formation.

Biofilm formation may result primarily from physical accumulation of bacteria.