The 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.
Bacteria 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.
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.
Pogodin, S. et al. Biophys. J. 104, 835–840 (2013).