Autumn is drawing close - at least here, where I live -, and with it come increased concerns about a possible swine-flu pandemia. Some people choose to wash their hands more often, the more cautious will go and buy masks and some of them, according to this morning’s news on my local radio station, are stocking up on colloidal-silver sprays. That would be sprays containing really small tiny particles of metallic silver. I am not advertising so no hyperlink.
As a former „nanotech-gal“ I am delighted that a nanoengineered product is experiencing market-success. The question is, however, weather nano-silver is the ultimate weapon to fight the flu pandemic or will it just make the viral surroundings look shiny and disco-like?.
The health benefits of silver are known since the times of ancient civilisations (hence the greeks used silver beverage recipients). In some sources you might also find an interesting hypothesis on the wide spreading of silverware use – apparently this was due to the empirical observations that rich families, who afforded these objects, were less prone to sickness and infections. Historians have probably a better view of this, so I will focus on the scientific aspect of the question.
The antibacterial properties of silver ions, mainly from silver nitrate solutions, started to be investigated after WW2 and, short after, a number of products hit the market . For example, a good review on their applications in the treatment of burns can be found here. (1) In the year of grace 2008 however, people still investigate the mechanism trough which these ions manage to effectively kill bacteria, without having nasty systemic side effects and inducing no resistance. The widest spread theory is that the said silver ions, by their chemical name Ag+ , mess up with the thiol group of respiratory enzymes, making the latest' functioning impossible and thus leading to bacterial death (figure 1). Other paths involve the agglomeration of silver ions in the bacterial vacuoles and cell walls – obviously bacteria don’t appreciate this, they experience change in morphology (both membrane and cytoplasmic contents) and they die. Finally, silver could speed up oxidative degradation and also lead to bacterial death.
Figure 1. An artist' view of a bacterium with nanoparticles stuffed up its ahem, nose
Similar mechanisms are also proposed for the action of silver nanoparticles, and some groups go to great lenghts to demonstrate this. The nanoparticles are however a lot more effective, since one can achieve an important antibacterial effects using concentrations in the nanomolar range. This is easy to understand if we think what a huge increase in surface (and hence virtual silver concentration) can be achieved by colloidal dispersion. Interesting enough this works very well for gram-negative bacteria and far from it for the gram-positive type. The major difference between the two is the thickness of the an intermediate layer in the cell wall. Gram negative ones have a thinner layer. An easy hypothesis would be to assume that this leads to an easier penetration of the Ag nanoparticles. Oh, well, more work for nano-bio PhD students.
So, now that we know bacteria hate silver - and more or less why, or better said how -, what’s the deal with viruses?
Well, here it gets tricky. There have been some sudies regarding the interaction of silver nanoparticles with viruses (some recent ones are reviewed in the introduction of this paper ) and it seems to be all about the scale. Nanoparticles cannot penetrate virus capsids for many reasons, a simple one being a size problem. Nanoparticles and viruses happily share the same tens-of-nanometer dimension. For comparisons bacterias have a couple of microns, so they can eat a lot of particles. Second, viruses don’t have respiratory enzymes which can be messed by silver. At most they could be degraded through a silver-catalysed oxidation process, but usually they are tough little buggers. However, suitably functionalised nanoparticles can bind to viruses, block the groups that they use to attach to cells and thus making them „die“ by solitude, or at least keeping them busy until the next good handwash. I should have mentioned before probably that viruses need a host to do their nasty stuff - like reproduction. So no home, no reproduction, no flu or HIV or Hepatitis.
I couldn’t find any other sources (credible sources that is, not blogs selling colloidal generators :)) proposing a different, universal mechanism through which silver nanoparticles would destroy all types of viruses. For all we know it could also be that the nanoparticles mess up with the cells so bad, that not even viruses have the desire to reproduce inside anymore.
One could therefore conclude that there are a lot of colloidal-generator-sellers out there that profit of the lay men ignorance concerning the differences between viruses and bacterias (aren't they all small evil things that we don’t see ?) and that buying silver-based products for swine-flu profilaxis is just a waste of money. There is a consolation prize however. A very real problem lies in the susceptibility to bacterial infections while having the flu, because the immune system has other priorities. And it is usually these infections which are difficult to treat and can become life threatening – as it is the case for complicated pneumonias. So a good dose of nanosilver could come in handy to prevent these last ones.
This being said, I will just resume myself to washing my hands regularly and not kissing strangers :).
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