Chapter 9 – Emerging Biophysics Techniques  427

by white blood cells in the host organism immune response. There is therefore enormous

potential to probe the various mechanisms of biofilm chemical defense using next-​generation

biophysical tools.

However, there is now emerging evidence that biofilms exhibit significant mechanical

robustness. This compounds the difficulty in healthcare treatment of biofilm infections since

they are often very resistant to mechanical removal. This is not just at the level of needing

to brush your teeth harder, but presents issues in treating infections in vivo, since to com­

pletely eradicate a biofilm infection potentially needs a combination of killing outer cells

and weakening outer biofilms structures to the extent that, for example, viscous drag forces

for in vivo fluid flow might then be sufficient to completely dislodge a biofilm from the sur­

face it is infecting. Experimental measurement of the mechanical shear stress failure levels

for different biofilms using a range of biophysical force probe approaches (see Chapter 6)

suggests that typical physiological fluid flow levels, for example, those found in blood and

urine, are in general not sufficient to dislodge biofilms.

At a coarse-​grained level of approximation, the mechanical properties of a biofilm can be

modeled as that of a densely packed hydrogel. As discussed previously (see Chapter 8), the

variation of elastic modulus of hydrogels varies as ~C^2.25 where C is the concentration of

the effective “biopolymer.” C here is more a metric for the local cell density in this case, and

the implication is that under the tightly packed conditions inside a biofilm, the equivalent

elastic modulus is very high. New methods to disrupt the EPS to reduce its mechanical

robustness are under development currently, and a key feature of these studies is biophysical

force probe investigations.

9.5.3  FROM ORGANISMS TO ECOSYSTEMS

An ecosystem is defined as the combination of a population of interacting individual

organisms, whether of the same or different types, and of the environment in which these

organisms reside. The connections between biophysics and ecological phenomena have only

been directly investigated relatively recently, principally along the theme of ecomechanics.

That is, how do the organisms in a population interact mechanically with each other and with

their environment, and how do these mechanical interactions lead to emergent behaviors at

the level of an ecosystem.

A number of model ecology systems have emerged to probe these effects. For example,

the mechanics of mollusks clinging to rocks on a seabed. Modeling the mechanical forces

experienced by a single mollusk is valuable of course; however, population-​level modeling

is essential here to faithfully characterize the system since the close packing of individual

mollusks means that the effect on the local water flow due to the presence of any given

mollusk will be felt by neighboring mollusks. In other words, the system has to be modeled

at an ecomechanics level.

This level of organism fluid dynamics cooperativity is also exhibited by swimming

organisms, for example, fish and swimming mammals such as whales create trailing vor­

tices in the water as they move, which has the effect of sucking surrounding water into

the wake of a swimmer. This in effect results in the local fluid environment being dragged

into different marine environments by the swimmer. In other words, populations of large

swimming organisms have an effect of increasing the mixing of sea water between different

environments. This is relevant to other ecosystems since this could, for example, result in a

spread in ocean acidification along migratory swimmer routes, which is greater than would

be expected in the absence of this fluid drag effect.

There is also a sensible argument that this drift behavior in a school of fish results in greater

hydrodynamic efficiency of the population as a whole, that is, by sticking close together, each

individual fish exerts on average less energy, though a counterargument applies in that fish

toward the front and edges of a school must expend more energy than those in the middle

and at the back. Thus, for the theory to be credible, there would need to be something equiva­

lent to a social mechanism of sharing the load, that is, fish change places, unless the fish at

the front and edges benefit in other ways, for example, they will be closer to a source of food