By fair means or foul
Posted on April 15, 2014 by Jon Fuhrmann
Bacteria can be altruistic in their behaviour or downright mean. Today at the SGM’s Annual Conference, Professor Kevin Foster will tell us about his work studying cooperation and competition in bacterial colonies.
When thinking about microbes, one may imagine primitive and ultimately solitary creatures concerned only with their own reproduction. In fact, scientists usually study micro-organisms in isolation from other cells, without possibility for interaction. The underlying assumption is that wherever they are found – in a primordial sludge, a petri dish in a lab, maybe even inside another living being – they are solely concerned with their own ability to reproduce. As a result, they will battle any other organism they encounter for space, nutrients and the resulting ability to divide faster.
Professor Foster from the Department of Zoology at the University of Oxford is working to change this rather narrow view of microbial interaction. He studies the concepts of co-operation and social behaviour in the world of micro-organisms. After all, he says, a lot of the effects microbes have on us – whether good or bad – are associated with how they interact among themselves. The notion that single-celled organisms are too primitive to do anything other than multiply as quickly as possible and directly compete against all other organisms they encounter is incorrect: microbes certainly interact and co-operate with each other in a variety of ways.
At first glance, the concept of “social” interaction as we know it in humans cannot easily be transcribed to the world of microbes. However, Professor Foster notes that it is important to remember that micro-organisms don’t “think”; the conscious reasoning and awareness of future benefits that characterise human relationships simply don’t apply. Microbes interact on a more primordial, purely molecular level – but upon closer observation the social aspect becomes apparent.
There are four types of social interaction in microbes, characterised by their impact on both the actor and the affected cell. Any action undertaken by an organism can be either beneficial or harmful to itself and to the recipient cell as well. An organism putting the benefit of others before itself is known as “altruistic”, such as worker bees who never reproduce and exist purely for the benefit of the Queen bee.
By contrast, “mutually beneficial” co-operation occurs if an action benefits both organisms involved. Consider the bacterium Rhizobia, which lives on plant roots and converts nitrogen into a form that the plants can use. Too much oxygen prevents this process from working, however, so it is crucial for Rhizobia that oxygen concentrations are low in its surroundings. In order to aid Rhizobia, plant roots produce cells that are extremely effective at taking up oxygen (but not nitrogen). This allows Rhizobia to produce nitrogen, which it will then supply to the plant through its roots. Both Rhizobia and the plant, therefore, benefit from each other’s actions.
A more unusual microbial scenario is the “spiteful” behaviour observed in Escherichia colibacteria. E. coli often grows in large accumulations of microbes called biofilms, in which they coexist with many other species. In such an environment competition is intense – and E. coli have an innovative, and somewhat gruesome, way to gain more space and nutrients for themselves. Since neighbouring E. coli tend to be genetically identical, it is an evolutionary success for one individual bacterium if as many around it as possible survive to divide.
While cell division is the ultimate goal, then, damage to the DNA of a bacterium lowers its chances of successful division. Therefore, E. coli with higher levels of DNA damage may release a toxin called colicin which kills nearby non-E. coli organisms. However, this process causes the bacterium to “burst” and die. By sacrificing itself in this manner, the bacterium clears more space and makes resources available for other E. coli, killing itself but increasing the likelihood that its genetic material is passed on by nearby identical bacteria. In E. coli’s spiteful behaviour towards other bacteria, then, we find altruistic behaviour towards other members of its own species which allows them to be more successful.
It is almost impossible to simulate a truly lifelike natural environment in a lab – there are simply too many factors to control. For example, we can study exactly how a bird’s wing allows it to fly in a lab – but this may not help us understand the bird’s flight patterns in the sky. Professor Foster says that this remains something of an obstacle as laboratory techniques underlie any microbiologist’s work. However, he thinks that social interactions must be considered when putting findings from the laboratory into a real-world context.
