Emergence Using Reinforcing Feedback: Quorum Sensing Bacteria Quick overview: Emergent Behavior in Quorums Sensing model introduces the concept of emergence and reinforcing feedback by using as an example: how bacteria talk and work together to overwhelm something (or someone) they have infected. You’ll be able to make it difficult for these bacteria to work collectively by blocking how they communicate individually, hopefully before what they have infected gets too sick. What is emergence? “Emergence may seem mysterious, but it is actually something that we experience every day. For example, a single water molecule of two hydrogen atoms and an oxygen atom does not feel wet (assuming you could feel a single molecule). But a few billion water molecules in a cup feel wet. That is because wetness is a collective property of the slippery interactions between water molecules in a particular temperature range… Similarly, what we call a symphony is a pattern of sound that emerges out of the playing of individual instruments, and what we call a kidney is a pattern of cells working together to provide a higher level function that none of the cells could do on its own.” Eric Beinhocker- Origin of Wealth1 Take a puzzle down from a dusty shelf and shake the contents onto a table. Now take a piece of the puzzle at random and inspect it. Note the color, shape, and presence of patterns on the piece. Now take another, and do the same. Immediately you’ll be able to note certain differences that exist between two pieces and after awhile you might notice certain trends in patterns in these differences. Now consider doing this process until you’ve looked at every single piece in the puzzle. Having considered each piece, would you have a clear understanding as to what the puzzle would look like if it were completed ( without looking at the box)? Perhaps, but certainly not as clearly as you would if you were to put all the pieces on the table, assemble them together, and see what, as a whole, they revealed. Like the picture of the completed puzzle, there are certain things that only become apparent or real to us after they have been organized in a particular way. There appear to be many things that make a qualitative change or shift once a certain quantitative 1 Beinhocker, E. D. (2007). The origin of wealth: Evolution, complexity, and the radical remaking of economics. Century. ! boundary is crossed. Traffic jams2, flocking birds3, and even our own consciousness4 are examples of emergent properties that we experience in our day to day life. However, our understanding of exactly how these phenomena occur, and how to understand the significance of emergent properties is a challenging part within the study of complex systems5. What is feedback? “All dynamics arise from the interaction of just two types of feedback loops, positive (or self-reinforcing) and negative (self-correcting) loops. Positive loops tend to reinforce or amplify whatever is happening in the system: The more nuclear weapons NATO deployed during the Cold War, the more the Soviet Union built, leading NATO to build still more....Negative loops counteract and oppose change... The larger the market share of dominant firms, the more likely is government antitrust action to limit their monopoly power. These loops all all describe processes that tend to be self-limiting, processes that seek balance and equilibrium.” John D. Sterman6 Imagine two mirrors facing each other in a well lit room, and place yourself between them. It will seem that you will be represented continually and each reflection of yourself is getting smaller, stretching out into what seems like forever. This is a kind of feedback called optical feedback. A simplification of what is happening is that there is a signal that is being sent out or in this case, reflected off the surface of one mirror, toward the surface of another. The signal is received, represented as an artifact ( the reflection you see ) and rebroadcast ( reflected ) out again from the perspective of mirror that received the signal. The signal is fed back to the original mirror, and the process begins again. This process happens again and again, deteriorating the signal, certainly, but also it creates a great 2 Resnick, M. (1997). Turtles, termites, and traffic jams: Explorations in massively parallel microworlds. Mit Press. 3 Cavagna, A., Cimarelli, A., Giardina, I., Parisi, G., Santagati, R., Stefanini, F., & Viale, M. (2010). Scale-free correlations in starling flocks. Proceedings of the National Academy of Sciences, 107(26), 11865-11870. 4 Baas, N. A., & Emmeche, C. (1997, February). On emergence and explanation. Santa Fe, NM: Santa Fe Institute. 5 Anderson, P. W. (1972). More is different. Science, 177(4047), 393-396. 6 Sterman, J. D. (2000). Business dynamics: systems thinking and modeling for a complex world (Vol. 19). New York: Irwin/McGraw-Hill. ! many versions of the signal, an exciting result from a very simple feedback experiment. Feedback can be seen as the process by which an output is ‘fed’ ‘back’ into the initial input that created it. The pathway of the feedback is referred to as a loop. Feedback loops can be negative ( self-correcting ) or positive ( self-reinforcing ). Complex systems have a tendency to be driven by dynamics composed of feedback loops, making the understanding of feedback helpful in understanding the behavior of complex systems. In the instance of quorum-sensing bacteria a feedback loop drives the production of the substance ( autoinducers ) that bacteria use to communicate. When bacteria detect autoinducers, they are stimulated to create more autoinducers, as more autoinducers are made, the higher the chance there will be to make even more, creating a higher chance that even more will be made! What is quorum sensing? The word quorum, having its etymological roots in Latin ( the genitive plural of qui meaning; of whom ) means in the larger sense; a particular group’s set minimum amount of individuals needed to make a decision. In regards to this model, our quorum is a group of bacteria. This raises the question; how do bacteria know there are enough individuals in their group (a quorum) to make a decision? And, what is the decision that they are making? In the late 60’s microbiologists observing bioluminescent bacteria7 determined that individual bacteria were somehow working collectively in a manner that a great many lit up all at the same time without the presence of something controlling them to do so. The concept of a decentralized process guiding the group activity was expanded upon in the 70’s by another group of microbiologists8 showing that the bacteria were in fact inducing bioluminescence by means of an individually synthesized molecule called an autoinducer. Bacteria were sending out these autoinducers into their environment. Other bacteria of the same species were able to detect the presence of these molecules. In a sense, bacteria were communicating to each other by producing these little molecules. By being able to detect the frequency and number of autoinducers bacteria were able to answer the question; how many other bacteria are around me right now? The purpose of answering the question of how many, has to do with the reaction a bacteria has when a certain number ( threshold ) of these autoinducer molecules are 7 Kempner, E.S. and Hanson, F.E. (1968) Aspects of light production by Photobacterium fischeri. J. Bacteriol. 95, 975-979. 8 Nealson, K.H., Platt, T. and Hastings, J.W. (1970) Cellular control of the synthesis and activity of the bacterial luminescence system. J. Bacteriol. 104, 313-322. ! encountered. When a particular threshold of ‘how many are there?’ is breached, bacteria begin transcribing genes that synthesize chemicals that, depending on the bacteria transcribing them, have a wide range of outcomes, “Quorum sensing is the regulation of gene expression in response to fluctuations in cell- population density. Quorum sensing bacteria produce and release chemical signal molecules called autoinducers that increase in concentrations a function of cell density. The detection of a minimal threshold stimulatory concentration of an autoinducer leads to an alteration in gene expression....These processes include symbiosis, virulence, competence, conjugation, antibiotic production, motility, sporulation, and biofilm formation.” 9 Consider this strange situation as an analogy to quorum sensing: Imagine yourself in a perfectly dark room. You stumble around the room looking for a light switch, but find there are none. Alas, you take inventory of your situation. You hold in your hand an empty flashlight that seems to require an usual amount of batteries to turn on. You also have a bunch batteries filling all of your pockets. You try and fit each of the batteries you have in your flashlight, and find that for some reason they just don’t fit inside. Wandering around aimlessly you step on something strange and reach down to find that you’ve discovered a battery on the floor. You try this new battery and low and behold; it fits! You decide it would be a good idea to start getting rid of your other batteries, as they are of no obvious use to you, and you’re finding ones that seem to work a lot better anyway. As you walk around, each new battery you find encourages you to drop more and more of your ‘unfit’ batteries that you can’t use. Finally it seems, that every where you turn is a new battery that works with your light and you are dropping your bad batteries left and right. Finally; you find the last battery you need to turn your light on, and just before you do so you notice all around this dark room other flash lights are lighting up, you turn yours on, and before you know it, the whole room is awash in light of a great many individual flashlights.