A Bacillus subtilis biofilm, the light-colored, slimy, bacterial coating on top, displays unprecedented liquid repellency as it beads up a drop of ethanol solution that has been placed on it. |
By rethinking what happens on the surface
of things, engineers at Harvard
Univ. have discovered
that Bacillus subtilis
biofilm colonies exhibit an unmatched ability to repel a wide range of liquids—and
even vapors.
Centimeters across yet only hundreds of
microns thick, such slimy bacterial coatings cling to the surfaces of
everything from pipes to teeth and are notoriously resistant to antimicrobial
agents. The researchers now suspect they know the secret to a biofilm’s
resiliency.
Published in the Proceedings of the National Academy
of Sciences (PNAS),
the study holds promise for both creating bio-inspired non-wetting materials
and developing better ways to eliminate harmful biofilms that can clog pipes,
contaminate food production and water supply systems, and lead to infections.
“By looking at biofilms from a materials
perspective rather than a cellular or biochemical one, we discovered that they
have a remarkable ability to resist wetting to an extent never seen before in
nature,” says lead author Alex Epstein, a graduate student at the Harvard
School of Engineering and Applied Sciences (SEAS). “In fact the biofilm
literally resisted our initial efforts to study it.”
The finding came about serendipitously, as
the original intention of the researchers was to study the structure of the
biofilm. To image the interior of the biofilm, the team had to soak it with
liquids such as ethanol and acetone, which normally spread and seep easily into
a surface.
“But to our surprise, it was impossible.
The liquids kept beading up on the surface and wouldn’t infiltrate the
colonies,” says Epstein, who is a member of the laboratory of Joanna Aizenberg,
Amy Smith Berylson Professor of Materials Science at SEAS; Susan S. and Kenneth
L. Wallach Professor at the Radcliffe Institute; and a core member of the Wyss
Institute for Biologically Inspired Engineering at Harvard.
As the Aizenberg lab studies materials and
wetting, the engineers immediately recognized the significance of what they
were observing. It turns out that biofilm has an unprecedented liquid-repellent
surface, thereby revealing a critical clue to what may be responsible for its
broad antimicrobial resistance.
Nature offers numerous examples of
water-resistant surfaces, such as the lotus leaf, a longstanding inspiration
for creating synthetic materials. Until now, however, no model natural systems
have been found for broadly repellent materials.
While such surfaces can be manufactured,
the top-down process is costly, labor intensive, and reliant on toxic chemicals
and brittle structures. A biofilm, however, is living proof that only the
simplest and most natural of components are required—namely, a resilient meshwork
made from proteins and polysaccharides assembled into a multi-scale,
hierarchical structure.
At the same time, the finding offers a
completely new perspective on how biofilms are immune to so many different
types of biocides. Even the most sophisticated biochemical strategy will be
ineffective if a biocide cannot enter the slime to reach the bacteria. In
short, the antimicrobial activity of alcohols and other solvents becomes
compromised by the strongly non-wetting behavior at clinically relevant concentrations.
The team expects that their newfound
knowledge will help alert researchers to the need to consider this requirement
when designing ways to destroy harmful biofilms.
“Their notorious resistance to a broad
range of biocide chemistries has remained a mysterious and pressing problem
despite two decades of biofilm research,” says Aizenberg, a pioneer in the
field of biomimicry. “By looking at it as a macroscopic problem, we found an
explanation that was just slightly out of view: antimicrobials can be
ineffective simply by being a non-wetting liquid that cannot penetrate into the
biofilm and access subsurface cells.”
Aizenberg and her colleagues speculate
that such strong liquid repellence may have evolved in response to the
bacteria’s natural soil environment where water can leach heavy metals and
other toxins.
Moreover, the property may underlie the
recent success of the use of biofilm as an eco-friendly form of biocontrol for
agriculture, protecting plant roots from water-borne pathogens.
Looking ahead, the Harvard team plans to
investigate precisely how the biochemical components of biofilms give rise to
their exceptional resistance and to test the properties of other bacterial
species.
“The applications are exciting, but we are
equally thrilled that our findings have revealed a previously undocumented
phenomenon about biofilms,” says Aizenberg. “The research should be an
inspiring reminder that we have only scratched the surface of how things really
work.”
Just as with biofilm, she adds, “It has
been a challenge to get deep into the core of the problem.”
Epstein and Aizenberg’s co-authors
included Boaz Pokroy, a former postdoctoral fellow in Aizenberg’s group and now
a faculty member at Technion (the Israel Institute of Technology), and Agnese
Seminara, a postdoctoral fellow at SEAS and participant in the Kavli Institute
for Bionanoscience and Technology at Harvard Univ.
The research was funded by the BASF
Advanced Research Initiative at Harvard
Univ.
