Bacteria: Social critters!?

Bacteria enlist “soldiers” to fight for group!

People communicate by sight and sound. Dogs rely on smell. And bacteria are the chemists of the natural world, using chemistry for offense, defense and communication.

In a study¹ published today, scientists looked at offense and defense among Vibrionaceae, a family of oceanic bacteria that contains the bug that causes cholera.

What they found suggests that some bacteria live in genetically similar clusters that cooperate among themselves while battling against outsiders.

They even found specialists among those clusters: A few warriors, called “super-killers,” make antibiotics that kill outsiders while sparing members of the in group, which carry genes to defeat these antibiotics.

Previously, microbiologists thought that antibiotics would be deployed shotgun style, but this dog-eat-dog, go-it-alone view of the microbial world is undermined by the new study.

Instead, the situation is starting to look more like Montagues and Capulets, or Jets and Sharks, or Hatfields and McCoys.

Motto: We are the in group, and we take care of our own.

A watery world

Some oceanic bacteria live free-floating, solitary lives, but many colonize organic particles, or the skin and gut of ocean animals. These clusters form “ecological populations that share common resources and common habitats, much like what we see in plants and animals,” says Martin Polz, the study’s senior author and a professor of civil and environmental engineering at MIT.

Bacteria can compete by producing antibiotics to kill other microbes.

To explore the relationship among related strains of bacteria that are genetically different, Polz and colleagues grew pairs of Vibrio side by side. Fully 44 percent of the 185 strains could inhibit or kill at least one other strain.

And these patterns of assault and resistance exactly followed the genetics, Polz says. Bacteria attacked strangers, but not close relatives, which carried genetic protection from the antibiotic.

Beware the gangsta bugs!

Furthermore, up to 5 percent the 185 strains were super-killers — bacterial thugs that could stifle at least 25 percent of other strains.

Call them what you like — soldiers, gangsters or enforcers — but Polz sees the situation as “a social structure, with some organisms taking on functions that benefit the group.”

This contradicts the standard view of antibiotic-making genes “as the ultimate selfish genes,” Polz says. Once a microbe has an antibiotic gene, he says, “We thought its ecological effects would be most pronounced if it killed its closest relatives, because they share resources. If they die, then you have access to more resources.”

Instead, these genes kill bugs that are, in genetic terms, more distant — the strangers in their midst. “This indicates that there is a social structure, and these antibiotic-makers act as a public good,” says Polz.

The battlin’ bacteria remind us of soldier ants, which protect the colony in return for housing, food and the hope that their queen will pass along their genes.

But for Vibrio, it’s not clear how the soldier microbes benefit. “We don’t have a good explanation,” Polz admits, “because the guys who produce antibiotics are incurring a cost that the others who are resistant are not incurring. We suspect there are many such functions in the ecological cluster.”

In other words, some bacteria do certain jobs while others benefit; eventually, both jobs and benefits balance out and are maintained over the course of evolution.

Proceed with caution!

It’s hard to say from one study whether many types of bacteria are social critters. Bacteria exist in unfathomable diversity, and horizontal gene transfer — movement of genes between different strains — complicates the evolutionary picture.

Through horizontal transfer, Polz says, “A benign bacterium can pick up genes that turn it bad.” The transferred gene may turn a benign bug rogue — even if it has normal functions, such as attaching to a surface or protecting against antibiotics.

Rapid rogue evolution

Genes that are critical to survival “are under selection to travel fast between microbes,” Polz says. “Imagine an arms race: If a population starts producing antibiotics, the co-existing population becomes resistant, the antibiotic gene loses effectiveness and a gene for a new antibiotic may come in and spread through the population.”

This arms race is not limited to the lab: Antibiotic-resistant bacteria are almost literally eating up antibiotics — which used to be called “wonder drugs” before resistance became so common. Genes transferred horizontally can render difficult pathogens even more resistant to antibiotics.

This is no abstract threat. An infectious disease expert from the University of Florida recently called the rapid growth of resistance to antibiotics “the single biggest problem we face in infectious disease today.”

And so this revolutionary view of gang warfare and bacterial specialization is clearly something we need to check out.