At a young age, most children are taught that washing their hands is hygienic because it kills and washes away bugs they cannot see, that otherwise might make them ill. This attack on ‘unfriendly bacteria’, while standard and a healthy precaution, is possibly a bit light on detail about the underlying process on a molecular level.
Certainly it is well established that clean hands contribute to clean food, which means diarrhoea/colds/influenza etc., are not transferred from person to person so easily as once they were. Soap, as we also know, provides an excellent way of doing this: it allows a lot of matter that is not water-soluble, to be washed away, bacteria included. What you might not expect is that a good detergent will also dissolve cellular membranes. Perhaps this is not so surprising when we consider that both the surfactants in soap and lipids respond physically to water. We know that both surfactants and lipids self-assemble, and that the physical forces that govern this process in both are the same. So, it is not too much of a leap to wonder that the two might interact if they are lathered up together on the palm of your hand after you have been chopping something up. And indeed it is the case: certain detergents are able to dissolve the membranes of cells (mainly prokaryotic and animal cells).
This is an excellent anti-biotic property, as without a membrane, the cell no longer exists. In one, relatively mild, movement, the bacterial population takes a trouncing. This is often part of what is at the basis of advertisements for cleaning products that state a killing off of “99·9% of bacteria”. This sounds pretty comprehensive. In fact, it is only a difficulty in proving in a legal sense that 100% of bacteria are killed, that prevents a less modest claim being made.
If we stop and think for a moment, and consider the wider picture, an interesting question emerges. The evidence we have suggests that bacteria were one of the first recognisable life-forms to have existed on Earth. Currently, they survive in a bewildering array of environments, from boiling acid to the frozen poles. If that were not enough, mammals are entirely dependent upon them for digestion. They form a crucial part of the carbon and nitrogen cycles, as they are part of the process of decay. Why, then, are they scuppered by something as simple as a surfactant without having realised it?
Part of answering that question is of course, the laws of physics alluded to above. If the detergent is a good one, and there is enough of it relative to the mass of bacterial cells, there is not much the bacterium can do. What it could do though, is exploit the properties associated with the surfactants to benefit itself.
Recent evidence from a Flemish laboratory suggests that in fact, some do. Fauvart et al. show convincing evidence that a bacterium commonly found in human environments, Pseudomonas aeruginosa, use a relatively unusual type of lipid, called a rhamnolipid (Figure 1) in order to colonise a given location and food source. There is now evidence to suggest that in a process known as swarming, this bacterium uses a surfactant to change the surface properties of an environment in order to allow them to flourish.
Interestingly, and again, perhaps not to the advantage of the bacterium, if we know what makes them swarm best, we can exploit this to destroy them. Medical therapies and anti-biotic cleaning agents based upon this are some way off. However, a further defence against bacterial attack from ‘unfriendly bacteria’ may yet be possible with a surfactant-based approach.
M. Fuavert , P. Phillips, D. Bachaspatimayum, N. Verstraeten, J. Fransaer, J. Michiels, J. Vermant, Soft Matter, 2012, 8, 70-76. http://pubs.rsc.org/en/content/articlelanding/2012/sm/c1sm06002c