Well, C. elegans is not really a meat eater, but if you read the recipe for Nematode Growth Medium (NGM) , you would be forgiven for thinking so. Bacto-peptone, contrary to what its name implies, is not a bacterial extract but is made from animal tissue. It is an enzymatic digest of beef and/or pork meat. Peptones were introduced into microbiology around 1880 by Naegeli and have become a standard ingredient for bacteriological culture media ever since.

It is not hard to imagine that bacto-peptone is not made out of prime steak or sirloin, but from meat that doesn’t sell. Because of the possible danger of bovine spongiform encephalopathy (BSE) coming from these tissues, BD Biosciences (a major bacto-peptone producer) began tracking the origin of their meat sources and requiring all bovine meat to come from BSE-free countries. Based on catalogue and lot numbers, a certificate of analysis can be requested that states the animal source and the different tissues that went into the production of your specific lot of bacto-peptone. One batch my lab used a couple of years ago had the lot-number 8052930 and was made out of bovine and porcine tissue from animals farmed in the US. It contained tissue categories I, II and IV which include: bone, bone marrow, mucosa, pancreas, brain, retina, pituitary gland, pineal gland, optic nerve, peripheral nerves, albumin, adipose tissue, cartilage, milk, prostate, ovary and plasma.

Incidentally, many of the tissues out of which this particular lot was made from, and probably most batches are made from, are tissues known to be involved in endocrine signaling. Not surprisingly, significant amounts of estrogen and estradiol have been found in peptone (Feldman and Krishnan, 1995) as well as at least 9 different bile acids (Kamekura et al., 1988), a class of acids to which dafachronic acids belongs (Motola et al., 2006).

How does this affect day-to-day C. elegans experiments? Using solvents in combination with NGM may have unintended consequences. The most widely used solvents, besides water, are ethanol and dimethyl sulfoxide (DMSO). DMSO is used in drug discovery and pharmacology because of its ability to pass through biological membranes and to carry other molecules with it. It is to be expected that DMSO will carry some of the constituents of bacto-peptone into C. elegans and potentially have some unintended consequences. If you use solvents like DMSO in your experiments in combination with NGM you should be at least aware of this possibility. Ideally, use a culture method that does not contain bacto-peptone. Otherwise, you may make your worms “eat some meat”, carnivorous indeed.