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A Model of Social Cooperation

By Eshel Ben Jacob, Yakir Aharonov and Yoash Shapira

Bacteria, being the first form of life on Earth, had to devise ways to synthesize the complex organic molecules required for life. They are able to reverse the spontaneous course of entropy increase and convert high-entropy inorganic substances into low-entropy life-sustaining molecules. Three and a half billion years have passed, and the existence of higher organisms depends on this unique bacterial know-how. Even for us, with all our scientific knowledge and technological advances, the ways bacteria solve this fundamental requirement for life is still a mystery. We do know that this is not a solitary endeavor for the bacteria, and under natural conditions they employ chemical communication to form hierarchically structured colonies, 109-1012 bacteria each. By acting jointly, they can make use of any available source of energy and imbalances in any environments, from deep inside the Earth's crust to nuclear reactors and from freezing icebergs to sulfuric hot springs; and they can convert any available substances, from tar to metals.

The shape of the bacteria colony shown in this and the following photos  demonstrates how members of the colony cooperate to solve problems

Under unpredictable hostile environmental conditions, when the odds are against survival, the bacteria turn to a wide range of strategies for adaptable collective responses. These cooperative modes of behavior are manifested through remarkable different patterns formed during colonial self-organization. The aesthetic beauty of these geometrical patterns is striking evidence of an ongoing cooperation that enables the bacteria to achieve a proper balance of individuality and sociality as they battle for survival, while utilizing pattern-formation mechanisms that we have only recently begun to understand.

Communication

Efficient adaptation of the colony to adverse growth conditions requires self-organization on all levels--which can only be achieved via cooperative behavior of the individual cells. For that purpose, bacteria communicate by a broad repertoire of biochemical agents. Biochemical messages are also used in bacterial linguistic communication for exchange of meaningful information across colonies of different species, and even with other organisms. Collectively, bacteria can glean information from the environment and from other organisms, interpret the information (assign meaning), develop common knowledge and learn from past experience. The colony behaves much like a multicellular organism, or even a social community with elevated complexity and plasticity that afford better adaptability to whatever growth conditions might be encountered.

Social Intelligence

In multi-colonial communities (e.g., sub-gingival plaque), bacterial social intelligence is usually used for cooperation between colonies of different species. For example, each colony develops its own expertise in performing specific tasks for the benefit of the entire community, and they all coordinate the work done. Some bacteria undertake the task of keeping valuable information which is costly to maintain and may be hazardous for the bacteria to store. Frequently, such information is directly transferred by conjugation following chemical courtship played by the potential partners: bacteria resistant to antibiotics emit chemical signals to announce this fact. Some fundamental aspects of social intelligence are used to handle defectors, as is reflected by the variety of strategies Myxobacteria can use when their social intelligence is challenged by cheaters--opportunistic individuals who take advantage of the group's cooperative effort. For example, they can single out defectors by collective alteration of their own identity into a new gene expression state. By doing so, the cooperators can generate a new "dialect" which is hard for the defectors to imitate. This ever-ongoing intelligence clash with defectors is beneficial to the group as it helps the bacteria improve their social skills for better cooperation which can be utilized at other times.

Recent findings even indicate that the bacteria purposefully modify their colonial organization in the presence of antibiotics in ways which optimize bacterial survival, and that the bacteria have a special collective memory which enables them to keep track of how they handled their previous encounters with antibiotics--learning from experience. Bacteria are clearly capable of developing antibiotic resistance at a higher rate than scientists develop new drugs, and we seem to be losing a crucial battle for our health. We might even discover that the last five decades of evolution in bacterial social intelligence is largely a result of their encounter with our socially irrational massive use of antibiotic materials in agriculture and human intake.

[Bacteria colony images shown are in false color; see other examples at http://star.tau.ac.il/~eshel/]

Eshel Ben Jacob

This article is excerpted from a paper entitled "Bacteria Harnessing Complexity" published in Biofilms (2004). Eshel Ben Jacob is professor of physics at the University of Tel Aviv, Israel, and former president of the Israel Physical Society.

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