Source: Journal of Gerontology (Biol. Sci.) 63A (2008): 242-252
Article Type: Original Research
Authors: I Lenaerts et al.

When working with C. elegans myself (limited times though) I didn’t break my head too much about the pros and cons of feeding them with heat-inactivated E. coli as long as each plate contained the same amount of food.
Well, I maybe should have given it another thought, at least based on the latest article by Isabel Lenaerts and colleagues.

Here, the authors “…describe convenient ways to exert DR (dietary restriction) by culture on agar plates containing axenic (i.e., there is no microbial food source) medium. We used these to explore whether effects of axenic culture really reflect DR. Our results imply that major nutrient components of axenic medium, and overall caloric content, are not limiting for life span. However, adding growth-arrested Escherichia coli as an additional food source rescued the effects of axenic culture. We then sought to identify the component of E. coli that is critical for normal C. elegans nutrition using add-back experiments. Our results suggest that C. elegans has a nutritional requirement for live, metabolically active microbes or, possibly, an unidentified, heat-labile, nonsoluble component present in live microbes.”

Of note also, the addition of Daucus carota and Pisum sativum extracts to axenic medium, e.g., increased fecundity without affecting lifespan. In contrast, increasing concentrations of autoclaved or sonicated E. coli had no effect on offspring number.
Also, radiation-arrested but metabolically active E. coli rescued the effect of axenic medium. This, however, was just seen for low-dose radiated E.coli, indicating that “…as long as some metabolically active E. coli remained, rescue was possible. This finding suggests the surprising conclusion that metabolic activity in its microbial food source is a nutritional requirement for C. elegans.”

Almost everything known regarding C. elegans can be found here: wormbook.org