Some of the bacterial diseases that plague us come from animals—Anthrax, Salmonella, and Lyme disease are just a few examples. Known as zoonotic diseases, their transmission is driven by the ability of bacteria, which are constantly evolving, to adapt to and colonize new hosts.
But how are bacteria able to establish themselves in a diverse range of species in the first place?
Unlike specialist bacteria, which require a narrow range of environmental conditions to survive, generalists, have evolved to cope in a wide range of environments—inside the human gut, lungs, or even on our skin—by possessing a large and variable repertoire of genes that can take advantage of different nutrients available around them. As a result, scientists expect generalists to have larger genomes.
Some generalists, however, have another trick up their sleeve to enable them to survive in a range of host conditions. Researchers from the University of Edinburgh, UK, found that some bacteria, in theory, can infect a variety of hosts by tweaking the environment around them to make it more uniform, and were more likely to have smaller genomes.
“I was initially surprised by our findings,” said Luke McNally, the lead researcher of the study. “While we thought an environmental modification strategy was a possibility for zoonosis, it does run counter to previous suggestions that a big genome is the key to generalism for bacteria.”
The study focused on bacterial qualities that drive evolution rather than host-specific factors, such as human population density or diversity of wildlife, that have been the center of previous studies.
Like humans, bacteria can modify their environment by working together. When large numbers of them are present, they release protein-based secretions outside their cells, making their external environment, more favorable for themselves, and inhospitable for competing bacteria.
These secretions have many roles: Some are toxins or antibiotics to kill competitors, some are biofilms that shield themselves from toxins of competitors, or smother their competitors, and some are enzymes that digest nutrients outside to standardize the host environment. Ultimately, the sole purpose of these secretions is to enable them to survive, and increase their own growth rate relative to their competitors.
To find out the role of secretions in expanding host range, the team analyzed the genome sizes and secretions of 191 species of disease-causing bacteria in humans to see if they could predict whether a species is zoonotic.
They found that bacteria with a larger number of secretions were more likely to be zoonotic, whereas those with a bigger genome were less likely to be zoonotic. Disease-causing bacteria appear to adapt to varied hosts—both animals and humans—mainly by environmental modification.
But producing secretions are costly for bacteria, consuming precious resources that could have been used for other purposes, such as growth. So why would they harness such a strategy over the more classical generalist method?
McNally’s team predict, using modeled scenarios of simultaneous infections of specialists, classical generalists, and environmental modifiers, that modifiers can invade specialist populations in a wider range of environments compared with classical generalists. By cooperatively secreting proteins that modify their environment, modifiers can thrive because they can lower the growth rate of competing specialists simultaneously present.
However, some of these secretions, they acknowledge, though originally intended to adapt to a specific environment, such as in the soil, for example, could have coincidentally given them the advantage of being able to infect new hosts.
And what about “cheaters” who capitalize on the modified environment without actually secreting? “Cheating can often lead to the collapse of cooperation, said Dr McNally. “However, there are a number of factors like spatial structure that can help cooperators thrive against cheaters. It’s not an easy problem to solve but many species of bacteria have clearly done so.”
Like many antibiotic resistance genes, bacteria may share the genes for secretions with their fellows. It may, therefore, be useful to monitor such transfer of genes to predict which species of bacteria are more likely to infect humans, the authors suggest.
The team are now looking at how environmental modification can cause disease during infections and testing whether secretions can contribute to standardizing the environment.
McNally, L. et al. Cooperative secretions facilitate host range expansion in bacteria. Nat. Commun. 5:4594 doi: 10.1038/ncomms5594 (2014).