Probiotics are live bacteria, formulas of single or multiple species, provided to help establish a healthy gastrointestinal tract.  Probiotics are a hot topic these days, and a potent marketing tool. Despite their popularity, we often don't know if they really work to relieve intestinal problems.  And for those probiotics that do work, we almost never know their mechanism(s) of action.   

Olier and colleagues have taken us a step closer to understanding the mechanism of probiotic action.  They've been studying a particular strain of Escherichia coli, Nissle 1917, already marketed and available as a probiotic (Mutaflor).  Studies have shown this probiotic to be highly effective in treating a variety of intestinal disorders, especially ulcerative colitis.  However, this particular strain also causes DNA damage in host cells, which is of concern because of the potential for colorectal cancers to form.  Olier et al. were trying to separate the good from the bad: to keep the beneficial aspects of E. coli Nissle while dropping the icky bad DNA damage-causing trait.  Once this was done, the probiotic would not have any harmful side effects.

Not surprisingly, biology turned out to be more complicated than hoped.  (Well, in this case, it is less complicated for the bacteria and more complicated for us.)  As it turns out, Olier et al. found that the same set of genes is likely to be responsible for the good and bad traits of E. coli Nissle, such that separating the good from the bad may not be possible. Interestingly, these genes encode a machine to make the toxin Colibactin.  But the way in which this toxin mediates the relief of intestinal disorders, or causes DNA damage in host cells, remains to be seen.