Friday, 10 January 2014

Bleach: past fortunes and modern molecular biology

Thinking about the results of your experiments on the inhibition of bacterial growth by antibiotics, ampicillin and chloramphenicol, led me to look a little further into the chemistry and biology of bleach. What I hadn't realized is that bleach manufacture is something we do ourselves to protect against microbial attack! So the industries that grew up around Merseyside at the dawn of the industrial revolution (a life size model of a bleach packer is shown on the right, from the Catalyst Museum at Widnes) have been going on inside our cells for a long time before then. This is one of those "dangerous" processes that provides a defence for the body, but is also potentially hazardous. Just as the bleach packers from 150 years ago "held their lives in their hands", so do we when we selectively release activated chlorine via hypochlorous acid production from blood neutrophils to combat infection.

So what is the chemistry of bleaching? Y12 chemistry classes this week will cover the reaction through which sodium hypochlorite (usually in conjunction with the alkali sodium hydroxide) generates chlorine and atomic oxygen, both of which have anti-microbial properties. The effects are not however, non specific (like corrosive compounds such as strong acids and alkalis): it appears that bacterial proteins become unfolded in the presence of the hypochlorous acid and this activates a class of protein called molecular chaperones (left). These proteins were originally identified as heat shock proteins: they are switched on in response to environmental stress. They protect our key enzymes and other proteins for a short while until the danger has passed. And of course the danger does pass when the agent is a gas. So when the bleaching effect of chlorine subsides some microbes begin growing again. This is whey we see growth in the cultures to which bleach was added, but not in the cultures where the antibiotics were added. We really need to do a systematic study of the concentrations of bleaching agent, the number of cells in the inoculum and the growth rates. Let's do this over the next couple of weeks

Since this discovery, it has become clear that Neutrophils (cells that form part of our immune system) produce hypochlorous acid from chloride ions (found everywhere in the body: think of salt) using the enzyme myeloperoxidase and hydrogen peroxide (another popular hair bleaching agent that parents may know!). The enzyme has a molecule of haem at its centre, similar to that found in haemoglobin, so it is a coloured protein, but unlike haemoglobin, it is green. In fact the green colour of pus is due to secreted myeloperoxidase!

What next in the bleach story? Well Ursula Jakob (left) at the University of Michigan has discovered a gene sensor called the Reactive chlorine receptor, that switches on the genes that encode the molecular chaperones above. This means that bacteria defend themselves against bleach until they become overwhelmed. So this is another example of a "biological arms race" that we find in Nature and is a great example of evolutionary adaptation. However I think this observation suggests that bleach resistance could emerge and this would cause more problems for human health! Little did the bleach manufacturers of old realise when they set out on the road to develop disinfectants and solutions for the textile industry.  I think we should take a further look at antimicrobials such as bleach in more detail this term! Any thoughts?

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