If the world cannot be persuaded to use antibiotics carefully enough to avoid harmful bacteria acquiring resistance we will have to come up with alternatives. Most proposals involve borrowing from nature to find enough new molecules to keep one step ahead of our tiny foes, but one set of scientists have a more radical suggestion – squeeze the little blighters until they pop.
The idea is not as mad as it seems. Mechano-bactericidal materials have surfaces shaped so that bacteria that settle on them get punctured, for example by having “pillars” a few tens of nanometers wide but hundreds of nanometers high. These spikes are not much use for killing an infection already inside your body, perhaps, but great for when sterile equipment is required. Moreover, certain nanoscale materials can kill bacteria in liquids, although it is debated whether this is primarily a physical or chemical effect.
However, the details of how these surfaces and particles work are poorly understood, which makes it hard to refine them. Dr Cristina Flors of the Madrid Institute of Advanced Study In Nanoscience set out to resolve this situation by working out what force will put holes in a bacterium's cell wall, exposing its innards to everything the walls usually keep out.
In ACS Applied Materials and Interfaces, Flors describes using a fluorescent marker that reveals whether an individual cell is viable or not and then applying an atomic force microscope to E. coli until they weren't.
Unsurprisingly, there is no single answer to the question posed. Flors found pushing softly on a bacterium's cell wall repeatedly could reduce its resilience, inducing something analogous to metal fatigue, leading the wall to eventually break under lower pressure than would otherwise be required.
This, Flors thinks, explains why colloidal nanomaterials can kill bacteria: frequent low-level collisions with tiny particles in suspension weaken the cell walls and eventually, even modest pressure breaks them. "Our work showcases how the development of advanced microscopy techniques can play a part to quantitatively understand the interactions between bacteria and nanomaterials," Flors said in a statement.
On average, a force of about 20 nanoNewtons is required to break E.coli's cell wall, Flors found, or about 5,000 times less than the weight of a fly.
Bacteria are immensely variable, so it is unlikely one figure applies universally. Being gram-negative, E. coli has thinner cell walls than gram-positive bacteria. Previous research has found gram-positive bacteria are less vulnerable on mechano-bactericidal surfaces, suggesting more force is required to kill them.