10a Ronde del Canavese Classiche

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Quorum Sensing: correlation in the bacterial world
D. Delle Side1 , V. Nassisi1 , P. Alifano2 , C. Pennetta1 , E. Giuffreda1 , A. Tal`a2 , M.S. Tredici2
1 Dipartimento
di Matematica e Fisica Ennio De Giorgi, Universit`
a del Salento and INFN Section of Lecce, Lecce Italy
2 Dipartimento
di Scienze e Tecnologie Biologiche ed Ambientali, Universi`
a del Salento, Lecce Italy
Presentation: Oral
Bacteria are known to coordinate at a colony level a diverse array of physiological activities that are
presumably productive only when groups of cells act in concert. This fascinating process, that eventually leads bacteria colonies to act as multicellular organisms, is known as Quorum Sensing (QS)[1].
From a biochemical point of view, QS is mediated by means of some small molecules called autoinducers, secreted by bacteria as a function of the colony growth. By sensing the concentration of such
autoinducers, bacteria are able to estimate their density and initiate costly processes, that would be
useless if started only by isolated individuals.
Evidently, this mechanism implicitly results in a biological example of correlation. In effect, bacteria
use the autoinducers to influence the behavior of the colony, inducing gene expression or repression as a
function of their density. In particular, bioluminescence[2], i.e. the property of some living organisms to
transform chemical energy into visible light, is a phenomenon mediated by QS, widely diffused among
bacteria.
By means of Petri dishes spotted with the bioluminescent Vibrio sp. PS1, a Vibio harveyi related
strain, we show that in conditions of high temperature (30 ± 1 ◦ C) and dry environment, bioluminescence is affected by QS-induced correlation. In particular, the distribution of the activation times of
the bioluminescent activity in bacteria, proportional to their radiant flux, shows a fat tail for large
times[3], which is well approximated by the Gumbel distribution of extreme value statistics. This happens despite no extremal process is known to take place. Furthermore, we found that this behavior
is in accordance with the rule suggested by Bramwell for the occurrence of the Gumbel distribution
in spatially averaged physical observables[4]. It is possible, indeed, to relate the total energy radiated
by the bioluminescent bacteria, which is proportional to Gumbel’s cumulative distribution function, to
the logarithm of the bacterial growth curve, described by the Gompertz model [5].
This circumstance suggests an interesting relation between bacterial growth and bioluminescent emission. In effect, it is possible to relate the position and scale parameters of the Gumbel distribution
(m and s) with the biological parameters characterizing the growth of the colony under exam, i.e. the
maximum specific growth rate µmax and the lag time λ. We present experimental evidence on this
relation, trying to elucidate the complex inter-relationships between bacterial bioluminescence, QS and
growth rates.
[l] C.M. Waters and B.L. Bassler, Annu. Rev. Cell Dev. 21, 319 (2005).
[2] K.H. Nealson and J.W. Hastings, Microbiol. Rev. 43, 496 (1979).
[3] D. Delle Side et al, Appl. Phys. Lett. 103, 253702 (2013).
[4] S.T. Bramwell, Nature Phys. 5, 443 (2009).
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[5] M.H. Zwietering, I. Jongenburger, F.M. Rombouts, and K. van ’t Riet, Appl. Env. Microbiol 56,
1875 (1990).
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