Humboldtians in Focus
The social behaviour of bacteria
By Drandreb Earl Juanico
Antibiotics are supposed to help us defeat infectious diseases once and for all. But many pathogens have now become resistant. The social behaviour of bacteria could offer a clue to developing a new type of antibiotic.
|Different bacteria colony
patterns: (left) in an environment
hostile to growth: low nutrient con-
centration, solid culture medium;
(right) under good growth con-
ditions: high nutrient concentra-
tion, fluid culture medium. The
colour bars indicate the density
of the bacteria.
Bacteria are arguably the toughest survival artists on the planet. That these microorganisms can even be immune to antibiotics is ascribed to their ability to reorganise themselves through cooperation. Therein lies the key to bacterial survival: their exceptional adaptability to environmental conditions is manifested in colony patterns which they specifically form in hostile environments. If we can understand how they cooperate, we may just succeed in outwitting these tiny, but tough and social creatures after all. A possible application of this research would be the development of antibiotics that work by limiting the ability of bacteria to cooperate.
Numerous microorganisms use flagella to move in fluids and form specific colony patterns depending on the consistency of the culture medium. Locomotion is important to bacteria in order to absorb the nutrients they need for metabolism, growth and reproduction from different areas of their environment. If the culture medium is solidified in the laboratory by increasing its agar content, bacteria struggle to move on it using their flagella. The situation also becomes difficult for microbes when the nutrient concentration in the substrate is too low to form flagella. In short, a solid medium with a low nutrient concentration is an environment hostile to growth; nonetheless, bacteria thrive under these conditions. How do they do it, and how can we inhibit their survival mechanisms to achieve an antibiotic effect?
Evolutionary game theory could help discover how cooperation increases the ability of bacteria to survive. This theory combines principles of Darwin’s Theory of Evolution with traditional game theory: Darwin’s Theory states that in the long term it is the best adapted organisms that survive and reproduce. Traditional game theory analyses systems comprising several stakeholders and attempts to record in mathematical terms how various strategies affect each other under competitive pressure. The terms cost and benefit are of great importance here.
The game of life
|Visualisation of the cooperation
level in the bacteria colony
patterns above. The colour bars
show the degree of cooperation
from 0 (no cooperation) to 1
(extremely close cooperation). In
the environment hostile to
growth (left) the level of coopera-
tion at the edges of the colony is
In an ecological context, the microorganisms would be “players” who adopt various strategies in order to win the “survival game”. If the participants in the “survival game” are relatives who form a population, social behaviour should be an important factor and cooperation a key strategy. There are many examples of successful collaboration within a species: bacteria such as Bacillus subtilis can excrete surfactants so that fewer flagella are needed for locomotion, especially when there are insufficient nutrients to develop flagella. Other microorganisms form biofilms, jointly protecting themselves in an almost impenetrable layer of slime. They produce enzymes that bind the valuable nutrient iron and then ingest them again together with the iron, or solubilise sugar by means of yeast – all these are activities performed by individual bacteria that serve to increase the chances of survival of the entire population. However, this kind of cooperation only remains stable if the benefit to the “players” is greater than the cost.
In order to find out more about the social behaviour of bacteria, a mathematical model is being constructed that is designed to replicate the growth of a colony. The culture medium is represented as a coordinate grid across which the nutrients are evenly distributed. An initial point with bacterial cells is placed in the centre of the culture medium. These cells reproduce, and a colony grows. The system is described by two dynamic local variables: the density of the bacterial population and the nutrient concentration. Surface hardness, a parameter for the distribution of the bacteria on the surface, is also taken into account. Coupled differential equations then allow sample patterns of bacterial growth structures to be calculated. To verify the theoretical results, they can be visualised and compared with laboratory results using real bacterial cultures.
Developing a cost-benefit scheme using game theory would show how advantageous it is for bacteria to collaborate at any given point. Science does not yet know how bacteria recognise their relatives in order to cooperate with them, but there is no doubt that they are able to do so.
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