Social Lives of Bacteria

“Support bacteria; it’s the only culture some people have.” This common sentiment is circulated throughout the internet and microbiology classes, but recent research shows that bacteria may have their own culture that mirrors aspects of ours.

When the average person hears “bacteria”, they conjure images of tiny organisms living a brutish, eat-or-be-eaten lifestyle and incapable of anything more complex than producing more of its kind. Yet research shows that various bacterial species are capable of behaviors one might expect from a sociology class –resource redistribution, cooperation, and role congruity–and not a petri dish. By exploring how various model organisms exhibit these traits, one gains not only a better appreciation of microbes but also how our society functions.

Division of Labor

Iron is a key element for all organisms, and the bacteria, Pseudomonas aeruginosa, has evolved a clever mechanism for collecting the iron a colony needs to thrive. When this pathogen takes up residence inside a host, the bacteria will secrete special iron-collecting molecules, called siderophores, to scavenge free iron from the host (Beare et al, 2002). Once the molecule has found and bonded to some iron atoms, it will return to the colony with the iron in tow (Beare et al, 2002). Although this process is an incredibly energy intensive metabolic function, the colony can work together and thrive (Beare et al, 2002). However, there is room for bacteria within the population to cheat the system.

When an individual bacterium synthesizes and secretes a siderophore compound, there is no guarantee that it will return to that specific individual; the compounds are universally accepted by any member of the species (Harrison, 2013). This odd quirk of biochemistry creates the perfect conditions for a cheating bacterium, one who does not synthesize the siderophore but takes up any iron containing siderophores, to thrive (Harrison, 2013). Although it is not completely understood, evidence shows that the bacteria who are contributing to iron collection are capable of sensing that there are cheaters among them (Harrison, 2013).

Pack Tactics

The bacteria, Myxococcus xanthus, are voracious predators in many soil ecosystems, due to their varied diet on other species of bacteria (Lloyd 2017). Like a pack of wolves, these bacteria display a unique behavior called social motility; under certain conditions, individual bacteria will swarm into massive colonies, as they hunt their prey (Astling et al, 2006).

Although an individual bacterium lacks a nose, it can pick up the scent of its prey –who secrete a molecule called acyl homoserine lactones– and start the biochemical process of forming the colony (Lloyd and Whitworth, 2017). Researchers have explored the genetic framework for this process and found that it is controlled by methylating certain genes in the Frz pathway. By regulating which genes in the Frz pathway are turned on or off, a bacterial cell can extend and retract their pili, causing them to move, and secrete special chemotaxis proteins which tell their neighbor when and where to move (Astling et al, 2006). This choreography between genes, proteins, and microscopic structures allows for hordes of bacteria to glide together in perfect harmony.

Their hunting strategy in this cooperative state is quite unique. Individuals work together to secrete bundles of secondary metabolites and digestive enzymes, such as proteases, into the environment, allowing them to feed on various other bacterial species (Lloyd 2017). However, this cooperative hunting state can’t exist permanently, so the colony performs one last act to ensure the survival of the next generation. When nutrients run low, different genes in the Frz pathway activate, telling the colony to start forming a fruiting body (Astling et al, 2006). The fruiting body is a special structure which houses thousands of endospores –tiny cells which can survive unfavorable conditions– that will awaken when better conditions arise (Astling et al, 2006; Lloyd 2017).

The Needs of the Many

Star Trek II: The Wrath of Khan ended with Mr. Spock sacrificing himself to save the entire crew from an untimely end, and many bacterial species employ similar tactics to successfully infect hosts.

When looking at various pathogenic species of bacteria, there is a great amount of phenotypic diversity within genetically identical populations; this phenomenon is called phenotypic noise (Ackermann et al, 2008). Arising from variation in cell age, metabolic activity, and epigenetics, phenotypic noise can cause microbes to change their outward appearance or alter what compounds they produce (Avery, 2006).

The most interesting of these compounds are those that are antimicrobial in nature. Various species, Escherichia coli, Salmonella typhimurium, Streptococcus pneumoniae, and Clostridium difficile, have certain individuals within a population that can produce very toxic compounds, which are lethal to all bacteria –even the bacterium who produced it (Avery, 2006; Ackermann et al, 2008). This phenotype seems to be a biological dead-end; after all, it’s not evolutionarily beneficial to produce something that will kill you before you reproduce, but when you examine this trait at the population level, its evolutionary value becomes clear.

When a pathogenic colony of bacteria invades a host, they are met with a myriad of native, commensal bacteria and several immune cells (Ackermann et al, 2008). These organisms make it difficult for the invaders to get a foothold in the host, so they send out the individuals who have self-destructing phenotypes to clear the way for others of the colony to start invading and dividing (Ackermann et al, 2008). This sacrifice of a few bacteria allows for the many bacteria of the colony to thrive in the new ecosystem, while maintaining an evolutionarily stable strategy (Ackermann et al, 2008).