Cicada surface science

Photo of a clanger cicada by Arthur Chapman,
shared on Flickr under a Creative Commons licence.

The east coast of the USA is greeting the latest emergence of the Brood II 17-year cicadas this month, and it reminded me of some interesting science I read about earlier in the year.

The wingcases of the Australian clanger cicada have the special property of killing bacteria that touch them, using just their physical structure—that is, there's no chemical reaction going on. Now an international team of biophysicists have developed and tested a model for how this happens. Their paper was published in the Biophysical Journal this February.

It's been known for a while that the cicada's wingcases are covered in a hexagonal array of nanopillars, up to 200 nm high. You might expect that the pillars pierce the soft walls of the bacteria to kill them. In this model, however, the high surface area that they give the wingcase is more important. The bacterial cell wall adsorbs onto the surfaces of the nanopillars from the top down, stretching the sections of wall in between until they rupture. You can see how this works in the animation below.



Since the model predicts that bacteria with more rigid cell walls will be resistant to death-by-nanopillar, the team could test it by microwaving resilient strains of bacteria, to soften their walls, and observing their survival on the wingcase surface. As expected, softened bacteria were destroyed by the surface, even if the unsoftened strain could survive it.

This work should contribute to developing antibacterial surface treatments that don't need extra cleaning agents to be kept hygienic. I do wonder, though, just which bacteria would be affected by such a surface and which would be resilient against it. Altering the height and spacing of the nanopillars could make them effective against a wider range of germs, as could playing with the chemical makeup of the surface to increase bacterial adsorption.

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Some bonus cicada titbits:

• These insects are gluten-free, high in protein, and taste a bit like asparagus. If that tempts your tastebuds as much as it does mine, the National Geographic has a selection of recipes for you.
• After seventeen years underground, how do the members of Brood II know that the time is now to emerge? New research suggests that they count the spring blooms of the trees whose roots they snuggle up against.
• The Washington Post takes a look at the evolutionary pressures that led to North American periodical cicadas 13- and 17-year lifecycles.
• Scientific American covered the Biophysical Journal paper in March, with an article on nanopillars and a disinfected world.