The inability to use citrate as an
energy source under oxic conditions has long been a defining
characteristic of E. coli as a species (35, 36). Nevertheless, E. coli
is not wholly indifferent to citrate. It uses a ferric dicitrate
transport system for iron acquisition, although citrate does not
enter the cell in this process (37, 38). It also has a complete tricarboxylic acid cycle, and can thus metabolize citrate internally during aerobic growth on other substrates (39). E. coli is able to ferment citrate under anoxic conditions if a cosubstrate is available for reducing power (40). The only known barrier to aerobic growth on citrate is its inability to transport citrate under
oxic conditions (41–43). Indeed, atypical E. coli that grow
aerobically on citrate (Cit) have been isolated from agricultural
and clinical settings, and were found to harbor plasmids, presumably
acquired from other species, that encode citrate transporters
(44, 45).
Other findings suggest that E. coli has the potential to evolve
a Cit phenotype. Hall (41) reported the only documented case
of a spontaneous Cit mutant in E. coli… Pos et al. (43) identified an operon in
E. coli K-12 that apparently allows anaerobic citrate fermentation,
and which includes a gene, citT, encoding a citrate–
succinate antiporter. High-level constitutive expression of this
gene on a multicopy plasmid allows aerobic growth on citrate,
but the native operon has a single copy that is presumably
induced only under anoxic conditions.

Despite this potential, none of the 12 LTEE populations
evolved the capacity to use the citrate that was present in their
environment for over 30,000 generations.

In the comment thread on the original post one of the grad students, Zachary Blount, showed up and gave a great answer to this question:

…citrate is commonly included in defined E. coli growth media (defined media are growth media for which we known exactly what is there and in what amount, as opposed to rich media such as Luria broth, trypticase soy broth, or brain heart infusion, which include enzymatic digests of yeasts and various proteins and, yes in the case of the third one, brains and hearts of cows which can vary in their exact constituents) just to make sure that the bacteria don’t starve for iron.

Also, when the media from which DM25 was developed were first formulated in the early to mid-20th century, it was common to keep them in 50x stocks that were then later diluted with water before use. At this concentration, the sodium citrate concentration was increased beyond what the organism strictly needed to prevent another component of the medium, magnesium sulfate, from precipitating out. As E. coli were not bothered by this, no other thought was given to the issue. I find this rather interesting, as it means that the citrate concentration in DM25 is also something of a matter of historical contingency – the niche for the Cit+ variant E. coli I study would not be nearly as large had not those bacteriologists of long ago not been so concerned about saving space!

I would need to check with Dr. Lenski as to precisely why he chose DM25, but I am pretty sure it was a medium he had worked with before. Certainly he saw the glucose level as more important, and it is very easy and non-troublesome to alter the amount of glucose in DM25.

You actually have some opinions about various people—who shall not be named here—in spite of the fact that the internal squabbles among bloggers never make it on to The Loom. That was refreshing. I was beginning to think you were perfect.

Thank you.

And now a not very germane question: if E. coli can’t metabolize citrate, why is it in the medium in the first place? Is it used for a buffer or something?

But just to add to your store of Irrelevant Facts For The Day, I could add to “Another speaker, Beth Shapiro, ancient DNA expert from Penn State, informed me that people in Siberia ride reindeer like horses, although she was thrown off one. ” Did you know that several abortive attempts have also been made to domesticate moose for riding in Scandinavia and Russia, in the high Middle Ages, in the 18th C., and again last century as late as the 60s? They are certainly strong enough, they are good in heavily forested areas, and they can handle swamps that horses won’t go near — but apparently they have a bad tendency to get too cranky to handle in the run-up to mating season, something I can well believe.

Very glad to hear Chautauqua Institute is still going strong after all these years. I grew up nearby but haven’t been back in a long time. It’s a fabulous cultural resource for southwestern NY & northwestern PA.

As I told Cava I’m not pushing anything, least of all Christianity or any other religion.

In your question, you make the assumption that science has made religion “irrelevant.” In the book, Dowd assumes that religion is still relevant and it is only by understanding science that this relevance can be fully appreciated.