Quorum sensing: a quantum perspective

Abstract

Quorum sensing is the efficient mode of communication in the bacterial world. After a lot of advancements in the classical theory of quorum sensing few basic questions of quorum sensing still remain unanswered. The sufficient progresses in quantum biology demands to explain these questions from the quantum perspective as non trivial quantum effects already have manifested in various biological processes like photosynthesis, magneto-reception etc. Therefore, it’s the time to review the bacterial communications from the quantum view point. In this article we carefully accumulate the latest results and arguments to strengthen quantum biology through the addition of quorum sensing mechanism in the light of quantum mechanics.

Introduction

Bacterial communication to each other is very efficiently described by the process of Quorum Sensing (QS) which enables bacteria to coordinate their behavior with environment. QS bacteria produce and release chemical signal molecules called autoinducers (AIs) that increase in concentration as a function of cell density. The detection of a minimal threshold stimulatory concentration of an AI leads to an alteration in gene expression (Miller & Bassler, 2001). QS controls genes that direct activities that are beneficial when performed by groups (a multiple of bacteria always being stronger than a few and thus by union are able to overcome obstacles too large for the few) of bacteria acting in co-operation and synchrony. However, there are many situations when the bacterial population behave co-operatively and recognize self and nonself which can be highly advantageous, particularly in the contexts of sex, symbiosis, niche adaptation, production of secondary metabolites combined with the defense mechanisms of higher organisms and for facilitating population migration where the prevailing conditions in a specific environmental niche have become unfavorable (Majumdar, 2016). QS is not limited to bacteria but it is observed in fungi and social insect also controlling various biological processes such as bioluminescence, sporulation, competence, antibiotic production, biofilm formation, virulence factor secretion etc. In the year 2014, research revealed traveling waves in response to a diffusing QS signal in spatially extended bacterial colonies (Langebrake et al., 2014). The results predict the direction and activating or deactivating nature of a wave of gene expression in experimentally controlled bacterial populations subject to a diffusing AI signal. Although QS has been extensively studied in well-mixed systems, the ability of diffusing QS signals to synchronize gene expression in spatially extended colonies is not well understood. A recent discovery developed a new QS enabled system with enhanced feedback to reduce cell heterogeneity. This narrows the population distribution of protein expression, leading to significant per cell and overall increases in productivity (Zargar et al., 2016).

Classical non-local theory

The fluidity of the water is modified in the presence of bacteria. Under such conditions swimming bacterial suspensions impose a coordinated water movement on a length scale of the order (10–100) micrometers compared with a bacterial size of the order of 3 μm (Roy & Llinas, 2016). The fundamental question arising about the study of quorum state and its non-local hydrodynamics. Classical mathematical model of quorum sensing fails to explain this phenomenon. Moreover, the origin and the effect of nonlocal noise in quorum state are not investigated. In this framework of nonlocal hydrodynamics, viscosity is generated by self-induced noise. This viscosity leads the actively moving bacteria into the meta-stable states required to support quorum, given the non- local nature of stresses mediated by pheromones. The shear stress created non-locally within this framework depending on the non-local noise of granularity and the viscosity associated to this noise can be tested experimentally. To treat this non-local problem mathematically, one needs some modification over the homogeneous relaxation equation of internal variables and to fulfill this purpose the famous Ginzburg-Landau equation (Ginzburg et al., 1950) has to come into the picture to handle the non-locality of the model. In nonlocal fluid mechanics, the weakly nonlocal field can be introduced to determine the origin of viscosity, or adhesive force, in the physically more realistic setting of large-scale structure formation at the cosmological scale. This was first introduced by Zeldovich (1970) with soft initial assumptions to understand the large scale structure formation. In this approach viscosity is driven by stochastic force and the dynamics is governed by the famous Burger’s equation (Burgers, 1974) which is the simplest equation taking care of both the non-linearity and the diffusion in the system. Whereas, noisy Burger’s equation in one dimension provides the simplest possible continuum description of an open driven system to handle the non-equilibrium phenomena in chemical reactions, biological systems, turbulence in fluids etc. So for the purpose of dealing with the non-local theory of classical quorum sensing in presence of driving force one needs to incorporate these equations carefully.

Quantum approach

Quantum biology is a concept that has been discussed for many years. But it can still be argued that there is no unequivocal example of a quantum process which play an important role in a complex biological phenomenon. The quantum effects at room temperature in biological systems is a remarkable observation. Recent research shows the important role of quantum mechanics in photosynthetic proteins, vision, electron- and proton-tunneling, olfactory sensing, and magneto-reception. Erwin Schrödinger noted in his famous book “What is Life?” (Schrödinger, 1944) that quantum mechanics accounts for the stability of living things and their cellular processes through our understanding via quantum mechanics of the stability of molecules, and the fact that quantum effects create, sometimes large, energy gaps between different states of chemical systems.

The quantum effect in the biological science (Mohseni et al., 2014) in a new age of research. In quantum theory, coherence indicates the wave patterns to be persisting for a very long time. But in the noisy and uncertain realm of cell communication, coherence fails to retain a microsecond and that’s why it needs review and more efforts to understand the bacterial world from quantum perspectives. Researchers have already started to develop the bacterial quantum biology using quantum theory. Researchers (like Seth Lloyd, physicist at the Massachusetts Institute of Technology in Cambridge) are already talking about “quantum hanky-panky”. “We hope to be able to learn from the quantum proficiency of these biological systems,” says Lloyd (published in Nature (Ball, 2011)). Quantum coherence has already satisfactorily explained the photosysthesis (Engel et al., 2007). So, we can hope that the very tiny (micro) length scale and uncertain nature of bacterial world will enhance the scope of quantum mechanism in quorum sensing and consequently it can have more powerful explanation on the basis of quantum coherence which needs redefinition in more intelligent way.

In several cases of QS system, it has been found that bacteria use more than one AIs and integrate the information conveyed by them. It has not yet been satisfactorily explained why bacteria evolve such signal integration circuits and what is the advantage of using more than one AIs since all signaling pathways merge in one. With the aim to answer this question, Quantum Gate Circuit Model of Signal Integration (Karafyllidis, 2012) has been proposed which incorporates quantum information processing as a theoretical framework for the study of signal processing in biological system. This model reproduces recent static experimental results accurately and explains the dynamic response of the quorum sensing circuit quite reasonably. A simulation algorithm based on this model has been developed and numerical experiments that analyze the dynamical operation of the quorum sensing circuit were performed for various cases of AI variations (Karafyllidis, 2012). Simulation showed that it is possible that bacteria can distinguish between different states of their environment and produce different responses. This in turn strongly suggests that the variations in AIs contain significant information about the environment in which bacteria exist though a better understanding of how non trivial quantum effects are still maintained in living organisms ( in hot and wet environments) is still to meet. Once we get the answer, we certainly would expect that quantum computation in biological systems will be understood and may also serve as an example for artificial quantum computing systems.

On the other hand, the analysis of noise in quorum sensing circuit plays a pivotal role in gene functionality. But it still remains unclear how crosstalk between C8HSL and 3OC6HSL affects the information that the bacterium obtains through quorum sensing. Apart from that, origin of noise in QS system is still undiscovered. The detail understanding of the dynamics at biomolecular length and timescales in noisy biological systems can uncover novel phenomena and concepts and hence present a fertile ground for truly multidisciplinary research. Quantum effect must play a crutial role to find the origin of noise and hidden signature in the wide rage of quorum sensing molecules. So it is the dawn of quantum quorum sensing.