Nanopillars and a Disinfected World


QDOTS imagesCAKXSY1K 8The microbial world is ever-present and unrelenting.  The enormity of it is hard to fathom, with facts like ‘there are 10  bacterial cells living in or on you for every one cell that is you’  and ‘estimates suggest there are five million trillion trillion bacteria  on this planet’, that’s hard to predict, it may be plus or minus  a few. Controlling our interactions with this world may seem futile  but we do so everyday.

750px-Algae_and_bacteria_in_Scanning_Electron_Microscope_magnification_2000xBacteria come in all shapes, sizes and types with  some beneficial, others pathogenic and others insignificant (to our  health at least) so being able to regulate our microbial environments  is vitally important. It is to our advantage to foster the beneficial  species and inhibit the species that are less so. We do this every day  by eating certain food, taking certain supplements and, of course, enlisting  the support of drugs and medications, all of which affect the bacteria  inside and on you. Controlling our own microbial microenvironments is  only part of the story though, what about controlling the bacterial  reservoirs we interact with, the tables, handrails, chairs, the surfaces  of our lives? That employs a whole range of other techniques.

Disinfecting a surface can be done in many ways. By  far the most common are the chemical disinfectant sprays and aerosols.  Disinfectant sprays contain active ingredients that effect either the  walls or metabolism of microbes. By disturbing the stability of bacterial  membranes or metabolic pathways they kill indiscriminately but they  have their drawbacks. Many bacteria sporulate and disinfectants are useless against them and to differences in virus  and fungus make-up they can also be less effective against these agents  too, but, most importantly, are often toxic.

Toxicity is not the only problem. Spreading these  agents around can cause a range of issues and as we have seen with antibiotics,  resistance to disinfecting agents can and is occurring. That ‘kills  99.9%’ label hides the problem of the 0.1% that survive, divide, and  pass on the ability to survive the disinfectant attack to their daughters.

An alternative to disinfectants is UV light. UV light  is very good at disinfecting solid surfaces. UV light mutates the nucleic  acids in DNA, which results in an inability to divide easily or continue  making important proteins. Having a surface disinfection system that  works by inducing mutations has its own problems and the known ability  of UV to cause mutations in any DNA means that this method has the potential  to cause cancers long term.

There is another problem shared by systems such as  spray disinfectants and UV lights, a reliance on continuing human involvement.  What would be really great would be a disinfection system that is included  as part of a products manufacture. Such systems exist and are part of  a growing field of ‘passive antimicrobial agents’.

Many metals are known to possess antimicrobial properties.  Products made with silver, despite there short shelf life, are thought  to be effective, although there are conflicting data on this. A particular  form of silver (a chelated form called silver dihydrogen citrate, SDC)  is thought to work in two main ways, by interfering with the way membrane  proteins work and by denaturing DNA after being taken-up by the bacterial  cells.

 

Another example is surfaces containing copper alloys.  Copper, in much the same way as silver, can interrupt protein form and  function as well as being able to interact with lipids and other cellular  architecture and by doing so inhibit bacterial population growth. Copper  also acts as a potent catalyst of redox reactions and so acts to increase  free radicals and oxidative stress.

 

With more support for copper than silver it seems  like the best way to go but copper is expensive. Reserves are dwindling  and some predictions suggest we could run out of economically viable  reserves within 60 years. The major reservoir of copper now lies in  recycled materials and these are increasingly re-used in electronics.  Dumping copper into surfaces is perhaps not the best use of it.

Passive antimicrobial surfaces have a new hero. Recent  work from Swinburne University in Australia has found that ‘nanopillars’  on the surface of the wings of an insect-like locust (the clanger cicada)  give it the ability to fight bacterial colonisation. The arrangement  of these hexagonal pillars is much like a bed of nails, as a bacterial  cell lies on top of them it spreads out and the pillars push against  the membrane. The parts of the membrane that sag between the pillars  are stretched and when weakened the bacterial membrane cannot keep the  liquid insides of the bacteria, well, inside. As the inside leaks out  the bacterial cell dies.

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This arrangement is mechanical, not chemical, and  so is completely non-toxic and safe for humans. Finding a cheap and  effective way to build these structures on surfaces would result in  a microenvironment imperceptible to us but lethal to bacteria that happen  upon it and inducing this microenvironment on hospital surfaces like  door handles, bed rails and tables can help prevent hospital-acquired  infections which are a huge issue in hospitals all around the world.  Being passive means it takes the risk of not quite cleaning that spot  out of the equation and being mechanical means it need never be replaced.

As the research pointed out, the more rigid a bacterial  membrane (rigidity was increased as a result of microwaving them) the  less effective this approach as the membrane doesn’t sag between the  pillars. This suggests that there may be a selectable trait for evolving  around this strategy long-term but as it is the only mechanical antimicrobial  surface structure to be observed so far it presents an interesting opportunity  to think differently about disinfection.

About the Author: Dr James Byrne has a PhD in Microbiology and works as a science communicator at the Royal Institution of Australia (RiAus), Australia’s unique national science hub, which showcases the importance of science in everyday life. Follow on Twitter @JB_blogs.

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