The hysteria among molecular biologists about our old friend ethidium bromide has long been an irritation to me. Researchers are rightly wary of this potential carcinogen. More recently this wariness has been whipped up into a witch hunt by companies touting “safer” alternatives and disposal methods. While I don’t for a minute think that we should all throw our gloves away and bathe in the stuff, I think that it’s time for an informed reality check about the dangers, and the myths about ethidium bromide.
Ethidium bromide is genotoxic, a frame-shift mutagen and teratogen. This is fact, determined by in vitro tests on various cultured cell lines and embryo systems that showed ethidium bromide can cause things like frame-shift mutations, chromosomal recombination, arrested cell division and developmental problems. This information is summarized in an excellent report from the National Toxicology Program.
These in vitro tests, which comprise the entire body of evidence upon which the ethidium bromide hysteria is built, don’t provide any evidence that ethidium bromide can exert a genotoxic effect in anything more complicated than a single cell or an unprotected embryo. In fact there is no direct evidence implicating ethidium bromide as a carcinogen in any animal.
For many years, ethidium bromide has been routinely administered for the treatment of African Sleeping Sickness in cattle. For this purpose, ethidium bromide is administered via subcutaneous or intramuscular injection with no reported increase in incidence of tumor formation or birth defects in the treated cattle. This suggests that ethidium bromide is far less genotoxic to animal systems than is presumed from the in vitro data.
The recommended, apparently non-toxic, dose of ethidium bromide is 1mg/kg of body weight in cattle. In comparison to this, the standard concentration used in molecular biology (around 1 microgram/litre), is low. Rosie Redfield puts it into perspective:
A 50kg researcher would need to drink 50,000 liters of gel-staining solution to get even the non-toxic dose used in cattle.
From this, the risks posed to a scientist handling a very weak solution of ethidium bromide, with a gloved hand (remember the cattle are injected with the stuff) are put into perspective.
A real concern is that the irrational and ill-informed fear of ethidium bromide drives us to solutions that are more dangerous than ethidium bromide itself. What could be more dangerous than ethidium bromide?
More concentrated ethidium bromide. The method of choice for ethidium bromide disposal at the moment seems to be absorption onto charcoal followed by incineration. You have a 1 microgram/litre solution of ethidium bromide, a concentration that appears to be of low toxicity according to the data from cattle. To me, it seems counter-intuitive to make it MORE concentrated by absorbing onto charcoal. What about just diluting it in water, reducing the toxicity even further?
More toxic reaction products of ethidium bromide. Another disposal method is to react it with phosphoric acid, HCl, bleach… but these are fairly dangerous chemicals themselves, and can produce reaction products that are even more toxic than ethidium bromide itself.
Safer DNA dyes. Beware of clever marketing and don’t believe everything you read. For example, as Rosie points out in her excellent article, “SYBR safe” has a higher acute toxicity in mice than ethidium bromide.
My take home message on this would be to forget all of the hype and myths you have read about ethidium bromide, get real and do what a scientist does best; read the articles I have cited, arm yourself with the known data. Then make your own decision on how to handle ethidium bromide, a decision based on fact… not hysteria.
This is the first in a three part series on the transformation of E.coli. By the end of this you should be an expert on E.coli transformation and on which strains to choose for different applications. If you’re already an expert, I hope it’ll be an enjoyable refresher for you. In either case, please comment […]
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