Susannah Kate Devitt (mnemosynosis) wrote,
Susannah Kate Devitt
mnemosynosis

Bacterial Cognition

This post was written using notes taken during Pamela Lyon's talk at ASCS '07.

INTRODUCTION

Cognitive science must have an answer for two key questions: what is cognition, and what kinds of creatures or systems are cognitive? Descartes argued that only humans were cognitive agents due to their rationality, reflectiveness and perhaps creativity. In the 20th century, behaviourism showed that much of animal behaviour--including human behaviour--could be explained without recourse to mentalizing of any kind, simply by examining stimulus and response. The cognitive revolution in the 1960s showed that even animals such as rats must use mental maps and representations of the external world to make decisions. At the same time computers were being programmed to undertake a multitude of tasks often considered cognitive (e.g. play chess, interpret sentences (e.g. Eliza). These two research paths suggested that perhaps cognition was not the unique faculty of humans at all?

Nevertheless, solving the puzzle of exactly what is cognition has remained. It is within this context that the study of bacterial cognition is interesting. Bacteria--and by extension unicellular eukarotes--have long been considered to simple, too reactive and too determined to be a member of the cognitive gang. However, Pamela Lyon argues that this exclusion is unwarranted. She suggests that bacteria are sensitive, communicative and decisive organisms and bacterial responses are more flexible, complex and adaptable than generally believed. In terms of re-defining cognition, Lyon argues that behaviour at the microbial level is precisely what must be understood in order to comprehend how more complex and specialized forms evolved and now function. Lyon claims that cognition is part of basic biological function, like respiration.[1] A good way to examine this is through a case study of myxococus xanthus (MX).

Case study - myxococcus xanthus

M.xanthus is a gram negative rod-shaped proteobacteria (picks up a stain on a test). M.xanthus dwells in soil and organic waste. M.xanthus is predatory, territorial and highly social. Almost no stage of its life cycle is solitary. The demands of cooperative living and navigating an epistemically polluted environment significantly shape its behaviour. M.xanthus dwells in a challenging, constantly fluctuating, informationally rich environment that is teeming with predators as well as prey; e.g. one gram of soil is estimated to habour up to 10 billion microbes of possibly thousands of species, which live alongside eukaryotic organisms (plant and animal). M.xanthus is well-equipped to cope with this environment.

There are three arguments against bacterial cognition, 1) Flexibility 2) Complexity and 3) Distilty

FLEXIBILITY

Lay-people generally assume that bacterial responses are inflexible and unmodifiable. That is, bacterial behaviour presumed to be rigidly determined by the organism’s genetic endowment and the nature of the stimulus. It is also thought that Bacterial behaviour does not require memory, is not sensitive to context or does not involve selection among alternative actions.

However, decision-making in bacteria has been known for over 30 years. Responses integrating multiple or conflicting stimuli are not always additive or linear. Bacteria have a huge repertoire of responses including: Swarming motility, ‘Wolf pack’ hunting and deceptive signaling, e.g. M.xanthus lures E. coli within killing range.

Bacteria sense features of their environment via two types of signal transduction system: 1-component and 2-component systems. 2CSTs have separate protein sensor and response regulators. The total number of ST systems has been likened to ‘bacterial IQ’ and the predominant type (1CST or 2CST) as reflecting ‘introversion’ or ‘extroversion’. To give an idea of how complicated bacteria are, E.coli shown to have 30 different sensors and 34 response regulators of differing activities and reaction speed—and that’s just its 2CSTs. The number of STs in M.xantus is unknown. However, due to complex lifestyle it is estimated to have 5 times as many as E. coli.

These findings suggest that bacterial behaviour is highly flexible and involves complicated decision-making.

COMPLEXITY

Bacteria are thought to be very simple creatures, without a sufficiently complex nervous system to constitute a cognitive agent. They are not supposed to have a life-cycle or development. However, not only are bacteria sufficently complex as individuals, they also have communal living arrangements with incredible emergent complexity (e.g.colonies, swarms, multispecies biofilms). Not only this, but the information processing systems that generate bacterial behaviour share mechanistic similiarities (not simply conceptual similiarities) with those that generate human behaviour.

Biofilms

A biofilm is a highly structured living arrangement with 100s of species of bacteria. An analogy can be made between the biofilm and a city because within the biofilm there is division of labour and mutual living (i.e. bacteria live off each others waste-products and productions.) The biofilm allows access to resources that individuals cannot obtain or use. The biofilm provides protection against predators, optimized survival and reproduction.




Catheter biofilm

Bacterial development

Development in bacteria refers to changes to cell morphology or function. M.xanthus forms fruiting bodies similar to slime mold. This aggregation is like ‘great herd migrations’. In the formation of the fruiting bodies, 10-20% will form spores and survive and 80% commit suicide (autolysis) to provide nutrients for the group. A small percentage become ‘sentries’ on the perimeter. Incompetent cells are provided with proteins by conspecifics to make them competent. Cheater mutants never prosper in these ecosystems/set-up. Occasionally there is the arrival of very efficient cooperators who are very good at providing proteins (like a messiah bacteria)


Under starvation conditions, m.xanthus undergoes a magnificent developmental process in which roughly 100,000 individual cells aggregate to form a structure called the fruiting body over the course of several hours.


M.xanthus fruiting body

These examples show that bacteria are complex, social creatures.

DISTALITY

'Distality' is a concept most easily understood by contrast to proximity. The term is used to investigate the distance between objects, events or properties that are significant for the perceiver and the behavioural response. Bacterial responses are generally thought to be proximal in three ways. a) they are within direct contact of stimulus physically b) they have a small amount of mechanistic complexity involved in processing the stimulus information (aka Dennett's unity position on intentionality) and c) temporally continuous (i.e. no memory), lack of decoupling mechanisms.

Although I don't have detailed notes for all aspects of the distality argument, an amazing example of a distal capacity of bacteria is their ability to communicate. Bacteria have a chemical messengering system called 'Quorum Sensing' (QS) that they use in the biofilm. There are intraspecies as well as interspecies quorum sensing (called bacterial ‘Esperanto’). There are at least five different quorum sensing systems known so far. Some bacterial strains are known to use three systems. Quorum sensing molecules are proximal indicators of distal states of affairs (e.g. the presence of (non) conspecifics).

Another distal example involves swarms of M.xanthus have been found to migrate non-randomly toward glass beads from a distance of 10 body lengths.[2]

It is also unclear exactly why distality is a necessary feature of cognition, rather than one of the properties that certain forms of cognition feature.

CONCLUSION

This post has a small fraction of information about bacterial cognition. Even though bacteria may not be aware, they certainly have complex behaviour and decision-making worth examining. Bacteria perceive, remember, problem-solve, learn and communicate. Understanding how they make group and individual decisions may contribute importantly to our understanding of cognition across many species including humans.

[1] This reminds me of Searle's argument that consciousness must be understood as a fundamentally biological output, just as lactose cannot come from equations mimicking function, thought cannot come from computation alone.

[2] An incredible article on bacterial directed movement towards objects can be found HERE.
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