Heterogeneity and timing in the quorum sensing system of Vibrio harveyi

Bacteria produce and excrete signaling molecules, so called
autoinducers, which allow them to monitor their population density
and/or their environment in a process best known as quorum sensing.
The Gram-negative marine bacterium Vibrio harveyi regulates certain
virulence factors like type III secretion, siderophore production,
and exoproteolytic activity as well as biofilm formation and
bioluminescence using quorum sensing. The bacterium produces three
different autoinducers: HAI-1, a N-(3-hydroxybutyryl)-D-homoserine
lactone, AI-2, a furanosylborate diester, and CAI-1, a
(Z)-3-aminoundec-2-en-4-one. The autoinducers are recognized by the
hybrid sensor kinases LuxN, LuxQ and CqsS. All information is
transferred to the phosphotransfer protein LuxU and the response
regulator LuxO via phosphorelay and further transduced into the
copy number of the master regulator LuxR. LuxR induces/represses a
multitude of genes/operons (>100) including the lux-operon
responsible for the production of bioluminescence. In order to
understand how single cells behave within an autoinducer-activated
community, autoinducer-induced processes were investigated in a
homogeneous environment over time. Analysis of wild type single
cells with respect to bioluminescence revealed that even at high
cell densities only 70% of the cells of a population were bright.
Moreover, fractionation of the population was found for
autoinducer-controlled promoters (of genes coding for
bioluminescence, exoproteolytic activity, and type III secretion)
using reporter strains containing promoter::gfp fusions. These
results indicated phenotypic heterogeneity of a genetic homogeneous
population and were independent of the used cultivation medium,
temperature or strain. An artificial increase of the autoinducer
concentrations resulted in an all-bright cell population similar as
observed for a luxO deletion mutant. Both, wild type and deletion
mutant switched to biofilm formation at high cell density. However,
the capability of the mutant to produce biofilm was significantly
reduced. These data suggest that a population of the
non-differentiating bacterium Vibrio harveyi takes advantages of
division of labor. In addition, a temporal variation of the
autoinducer concentrations over time was found. The extracellular
concentrations of the three autoinducers and quorum
sensing-regulated functions of Vibrio harveyi were monitored in a
growing culture. In the early and mid-exponential growth phase only
AI-2 was detectable and bioluminescence was induced. In the late
exponential growth phase both, HAI-1 and AI-2 reached their maximum
values, bioluminescence stayed high and exoproteolytic activity was
induced. The stationary phase was characterized by equal
concentrations of HAI-1 and AI-2, exoproteolytic activity reached
its maximum, and CAI-1 activity was detectable in the culture
fluids. Furthermore, only a stable and mature biofilm was formed,
when HAI-1 and AI-2 were present in the above described ratios over
time. CAI-1 had no influence on the biofilm formation in Vibrio
harveyi. These results demonstrate that not the cell density per se
is important, but that autoinducers rather control the development
of a Vibrio harveyi population.