No safe landing for bacteria
(appeared on 19th Feb 2014)

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Some kinds of skin are their own antiseptics, says S.Ananthanarayanan.

Bacterial infection is the reason for a great proportion of disease and is a risk that is there in all surgery. The need for antiseptics, to eliminate or limit bacterial population, adds to complexity and cost of assuring public health and even to contain some forms of corrosion in industry.

The success of scientists in institutes in Australia, with coworkers in Spain, in discovering that natural materials are able to combat bacteria, not by chemical barriers, but by physical structure, and then to mimic the structure, with its antibacterial features, in an artificial material, is hence an event of importance. Prof Elena P Ivanova and colleagues report in Nature Communcations that black silicon, a synthetic semiconductor, solar cell material with sharply rising nano-protrusions on its surface, so fashioned to absorb light more efficiently, shows bactericidal properties just like similar surfaces found in the natural world, the wings of the dragonfly, for instance. Prof Ivonaova’s group had confirmed only a few months earlier, in a paper in the journal, Small, that it was the physical surface structure of the wings of the cicada, and not any chemical coating, which made for its antibacterial quality.

Many surfaces in the natural world need to stay free of wet and dirt to be able to function. The legendary lotus leaf, like leaves of other plants that live in water, need to efficiently repel water, so that they do not get weighed down. Similarly, wings of insects need to be free of traces of wet for the insects to get about and survive. While water repelling leaves and insect skin is found to be coated with water repelling waxes, it has been found that the wings of the dragonfly, and others, also have a physical pattern, which breaks a drop of water that falls on them into smaller drops, to have greater rebound and to be lighter.

Many insects have evolved to have such strongly water repelling, or superhydrophobic surfaces. While water does not wet, and runs off these surfaces, water does wet specks of dirt and other contaminants and carries them away, to leave the surface clean, in addition to being dry, The observation that many superhydrophobic surfaces are also free of bacterial contamination was considered to be a case of the self-cleaning quality leading to clearing of biological material, an action that is called antibiofouling.

The first study of Prof Ivanova and group looked into the antibiofouling action of the wings of the Cicada, which are highly hydrophobic and self cleaning. The initial findings were that the wings were in fact not particularly effective in keeping away adhesion by a strain of common bacteria, as many of the cells were able to attach themselves to the wings. But then, it was found that once the bacterium landed on the wing, nano-pillars on the wing surface pierced the bacterium cell and caused cell components to spill out beneath the cell and between the nano-pillars. And further study showed that the cells which had landed on the surface were in fact dead! It was found that most cells were killed off within five minutes of attaching to the surface and there was a cycle of 20 minutes, of cells landing and getting killed, and getting cleared for more cells to land. Studies with Atomic Force Microscopy showed that the cells sank slowly into the wing surface for some 200 nanometers, in about three minutes, before there was a sudden downward movement which indicated rupture of the cell.

To eliminate the possibility of the cell death being due to some chemical component of the cicada wing surface, the researchers coated the surface with a 10 nano-metre thick gold film. The fine gold coating, which left the surface topography unchanged, was found to affect the hydrophobicity of the surface. But, despite the new surface chemistry of the surface, the bactericidal effect was found to stay unchanged. This demonstrated that it was the physical features, of sharp protuberances, that brought about the bactericidal effect, a conclusion of importance in the context of many bacteria developing resistance to biochemical agents, like antibiotics.

In the current study, reported in Nature Communications, Prof Ivanova and colleagues note that many surfaces with sharp protuberances at the microscopic scale display superhydrophbicity or biological activity at the molecular level. Taking off from what was seen with the cicada, the group looked at other surfaces which had similar structural characteristics. One such was a form of silicon, called black silicon, a form where the surface structure is of needle shaped single crystals of silicon. The effect is that the normal reflectivity of metallic silicon is greatly reduced and most light that falls on the surface is absorbed – hence the name, black silicon. Silicon being semiconductor that is used in photocells, this form, with the property of absorbing light, was developed for increasing the efficiency of photocells.

Microscopic studies showed that both black silicon as well as natural surfaces like, the cicada or dragonfly wing, had disordered nano-pillar systems, forming clusters and groups of clusters. Such grouping has also been shown to be a result of the forces that act when the structure is forming. While natural surfaces, in this grouping of spikes, consisted of fats and waxes, which are water repellent, the surface of silicon is only moderately water repellent. But the remarkable result was that the silicon structure was also equally bactericidal as the natural surfaces.

The bactericidal quality was assessed by culturing colonies of three different kinds of bacteria on the surfaces, of dragonfly wing, black silicon and plain glass and silicon, for comparison, for thirty hours. While the mix of bacteria that attached to the structured surfaces and the plan surfaces were different, the two kinds of structured surfaces were found to be lethal to the cells which attached to them. It can be seen in the table that despite differences of composition and hydrophobicity of natural materials and silicon, black silicon, which shares shape and construction, is as bactericidal as both cicada and dragonfly wings. The nature of the cicada wing structure limits its effectiveness to Gram negative cells, but black silicon, is like the dragonfly wing and is effective against all kinds of cells. Gold coating the black silicon and dragonfly wings again left the bactericidal property unchanged, like in the case of the cicada wings.

The results are significant, that nanomaterials, which can be readily fabricated, in quantity, can be used for control of bacteria growth. The structure of nanomaterials, in fact, can be optimized for exclusive antibacterial action, or for action along with rigidity, strength, etc, as required and could be better than as found naturally. This suggests that “novel antibacterial nanomaterials may open the way for new applications in the field of mechano-microbiology,” say the authors.

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