The end of egoism
The bacterial societies found in natural environments differ greatly from the pure cultures most microbiologists work with in the laboratory. – And the neutral term ‘biofilm’ scarcely describes the diverse and complex interactions that go on in these communities. Biofilms are the slimy overlays one comes across on all sorts of damp substrates, or on the surfaces of puddles and ponds. Such habitats are all around us, and they provide models of primitive biological communities.
Indeed, 95% of all bacterial species follow a communal lifestyle, for it offers single-celled organisms substantial advantages and improves their chances of survival. For instance, multispecies communities provide opportunities for genetic exchange, which helps their members to cope with varying environmental conditions.
Heterogeneous communities are also better equipped to exploit essential nutrients that can be obtained by the enzymatic degradation of complex natural products such as the lignin in dead wood or the chitin of insect exoskeletons. Biofilms, which consist of mixtures of sugars, lipids, proteins and water, also offer protection against antibiotics, noxious chemicals and UV radiation. They can form in industrial installations and in air conditioning systems – and when you and I get sick, there are bacteria waiting for the chance to take up residence in medical devices and implants, and in our vulnerable tissues.
Communicating via “quorum sensing”
Like other forms of communal life, social interactions between microorganisms depend on the ability to communicate, at least at a rudimentary level. Kirsten Jung, Professor of Microbiology at LMU’s BioCenter, decodes the grammar of these systems – interprets the language of bacteria, so to speak – hoping to unearth the rules that enable communal modes of life to emerge. She also acts as Coordinator of a Priority Program devoted to the topic, which is funded by the German Research Foundation (DFG).
It has been known for about 20 years that microbial cells communicate via “quorum sensing,” a biochemical mechanism that serves to monitor cell numbers. Cells secrete signaling molecules that are recognized and bound by surface receptors on nearby cells. When the concentration of the signal (and thus the population density) exceeds a certain threshold, a signal transduction process is triggered, which leads to activation of particular genes via the receptors. As a result, cells may begin to generate the matrix for a biofilm, produce bioluminescence or synthesize toxins that harm other species. Eventually, all individuals in a population behave in synchrony. The architecture of the signaling system ensures that coordinated behaviors are implemented only when a large number of individuals is present to efficiently exploit available resources. At low cell densities, the cost/benefit ratio would be too small. The study of such systems is now termed sociomicrobiology. Jung’s goal is to understand how the underlying communication processes work, how microorganisms acquire the ability to react to environmental conditions. What kind of stimuli can they perceive, and how does the cell process information on variables like temperature or pH?
Kirsten Jung studied Biochemistry in Leipzig before German reunification, and has since dissected whole reaction cascades and identified the receptors that recognize signal substances. “For instance, we now know that bacteria possess a kind of memory. They respond more rapidly to a stimulus they have previously encountered than to a factor that is new to them,” she says. For her, this property represents a primitive form of intelligence.
Kathrin Burger, ph
The complete article appeared in the latest issue of insightLMU , LMU’s international newsletter.
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