The social lives of bacteria

“Man is by nature a social animal; an individual who is unsocial naturally and not accidentally is either beneath our notice or more than human.” —- Aristotle

This famous quote by Aristotle gets at the heart of what is considered the most human quality – being social. However, biologists have long known that ‘social behaviors’ are not the sole domain of human beings. Examples of cooperation, conflict, altruism and even spite have been known to exist across the animal kingdom. Popular examples include social hierarchies in groups of non-human primates, pheromone-based signaling for attracting mates, male-male competition for access to mates and many more. Are these exceptions? It turns out that social interactions are pervasive across the living world, and even the simplest living organisms – single-celled, microscopic bacteria – engage in a wide range of social behaviors!

Over the last three decades, a large body of scientific research has provided examples of a fascinating range of social behaviors that bacteria engage in. In the language of social behavior, social acts are classified according to their effects on the individual performing the act (actor) and the individual receiving it (recipient). As shown in Figure 1, these acts include mutual benefit, altruism, selfishness, and spite.

Mutual Benefit: Mutually beneficial acts benefit both the actor and recipient. A popular example of mutual benefit in bacteria is siderophore production. Siderophores are iron-scavenging molecules. Iron commonly exists in an insoluble form in the environment making it difficult for bacterial cells to acquire it. Siderophore molecules released into the environment can help bind iron molecules for uptake by all bacterial cells in the environment. Thus, siderophore-producing cells as well as other neighbouring cells benefit, making the act mutually beneficial.

Selfishness: Selfish acts benefit the actor but harm the recipient. A common example of selfish behavior among bacteria is resource competition whereby bacterial cells consume resources in the environment rapidly to promote their own growth while depleting resources for other cells in the same environment.

Behaviors that are COSTLY to the actors: Social behaviors that incur costs to the actors themselves can be classified as altruistic if the recipients benefit and spiteful if the recipients also suffer a cost. How do bacteria engage in these behaviors? Almost all known bacteria produce costly toxins called ‘bacteriocins’ that can kill closely related cells but not identical cells. Producing these toxins is ‘costly’ due to energetic investment into the act. In fact, releasing these toxins may often result in lysis and death for the producing cells. Interestingly, bacteriocin production is a premier example of an act that can be considered both altruistic as well as spiteful. How is that possible? The answer lies in who we consider to be the recipient of the act.

Bacteriocins produced by a given population can target and kill cells from a different population. Thus, if we consider a recipient cell that belongs to a different population which is sensitive to the bacteriocin, then the act is spiteful. However, killing competitors cells from a different population also benefits other cells of the producer’s own population by eliminating competition. From this perspective, this act of bacteriocin production which is costly to the producing cell is also beneficial to other cells in its own population, making this an altruistic sacrifice.

 

Examples of spiteful behaviors are rare in nature. In fact, for a long time researchers believed that it may just be a theoretical construct. However, that was only until bacteriocin production was found to be rampant in bacterial populations. Here at Indiana University, Dr. Farrah Bashey’s lab in the Department of Biology studies bacteriocin-mediated interactions among bacteria that are native to the woodlands in Indiana. In collaboration with Prof Curt Lively, Dr. Bashey’s work has previously demonstrated that bacteriocin production can benefit ‘kin’ (related individuals) which is consistent with theoretical work that strives to explain why behaviors that are costly to the actors can persist. Current research in the lab is further investigating how costly bacteriocin production is maintained among these bacteria. My dissertation research is addressing whether the costs of bacteriocin production are minimised by being expressed only in the presence of competitors. Stay tuned to ScIU for more updates about this work.

Edited by Victoria Kohout and Karna Desai