Top view of a Bacillus subtilis colony in conditions where extracellular matrix is produced, leading to biofilm formation. Photo courtesy of Hera Vlamakis.
research at Harvard explains how bacterial biofilms expand to form
slimy mats on teeth, pipes, surgical instruments, and crops.
experiment and mathematical analysis, researchers have shown that the
extracellular matrix (ECM), a mesh of proteins and sugars that can form
outside bacterial cells, creates osmotic pressure that forces biofilms
to swell and spread.
The ECM mechanism is so powerful that it can increase the radius of some biofilms five-fold within 24 hours.
The results have been published in the Proceedings of the National Academy of Science.
large colonies of bacteria that adhere to surfaces, can be harmful in a
wide range of settings, resulting in tooth decay, hospital infections,
agricultural damage, and corrosion. Finding ways to control or eliminate
biofilms is a priority for many industries.
order for a biofilm to grow, a group of bacterial cells must first
adhere to a surface and then proliferate and spread. When a vast number
of cells are present, this can translate into the creation of a filmy
surface spanning several meters.
work challenges the common picture of biofilms as sedentary communities
by showing how cells in a biofilm cooperate to colonize surfaces,” says
lead author Agnese Seminara, a research associate at the Harvard School
of Engineering and Applied Sciences (SEAS).
types of biofilms have been characterized based on composition and
antibiotic resistance, but until now it has not been clear what roles
the whip-like flagella and the ECM play in the outward movement of
the presence of a flagellum has traditionally been associated with
greater movement capability, the new research has found that a flagellum
actually confers little advantage in the formation of biofilms. In the
Harvard study, mutant bacteria lacking flagella were able to spread at
almost the same rate as the wild-type (natural) ones. Mutants that could
not secrete the ECM, however, showed stunted growth.
The team of physicists, mathematicians, chemists, and biologists examined the formation of biofilms in Bacillus subtilis,
a type of rod-shaped bacteria often found in soil. Their focus on this
particular species was led by Roberto Kolter, Professor of Microbiology
and Immunobiology at Harvard Medical School, an expert on biofilms and
the genomics of B. subtilis.
project establishes a link between the phenotype, the physically
observable traits of biofilm growth, and the genetic underpinning that
allows spreading to happen in B. subtilis,” notes co-principal investigator Michael Brenner, the Glover Professor of Applied Mathematics and Applied Physics at SEAS.
researchers had speculated about a possible connection between the
biofilm’s quest for nutrition and the process of spreading. Because
biofilms absorb nutrients through their exposed surface area, they can
only swell vertically to a certain point before the
surface-area-to-volume ratio makes it impossible to adequately nourish
every cell. At this point, the biofilm must begin to spread outward so
that the surface area increases along with the number of cells.
ECM, a complex mesh of proteins, sugars, and other components outside
of the individual cells, holds the key to one aspect of this movement:
it apparently increases osmotic pressure within the biofilm.
response to the increased pressure, the biofilm immediately absorbs
water from its surroundings, causing the entire mass to swell upward.
The final change in the shape of the biofilm is due to a combination of
this swelling and the horizontal spreading that follows.
and Brenner created a mathematical model that mirrored many of the
team’s physical observations. The model supported the experimental
observations; by considering the relationship between swelling and
spreading, they were able to find the “critical” time at which
horizontal outward motion begins.
work is led by theoretical predictions which were tested by experiment
and proved to be correct,” reflects co-principal investigator David
Weitz, Mallinckrodt Professor of Physics and Applied Physics at SEAS and
Co-Director of the BASF Advanced Research Initiative at Harvard. “The
results also demonstrate how simple physical principles can provide
considerable insight into the behavior of biofilms.”
motion of biofilms represents only a small part of a complex subject.
Further research will investigate how biofilms adapt and possibly
manipulate their environment. The ultimate goal is to alter biofilms’
behavior to minimize their harmful effects.
natural question at this point is: do cells actively control biofilm
expansion and can they direct it toward desired targets?” says Seminara.
“This is a first step toward understanding the striking evolutionary
success of these ubiquitous organisms, and it may open the way to
unconventional methods of biofilm control.”
Brenner, and Weitz worked with Thomas Angelini, an Assistant Professor
at the University of Florida and a former member of the Weitz lab; James
Wilking, a SEAS research associate in applied chemistry; Senan Ebrahim
’12, an undergraduate at Harvard; and Hera Vlamakis and Roberto Kolter
of Harvard Medical School.
research was supported by the BASF Advanced Research Initiative at
Harvard, the 7th European Community Framework Programme, the National
Institutes of Health, and the Harvard Materials Research Science and
Engineering Center, which is supported by the National Science