C. elegans worms with an activated unfolded protein response (in green) strongly prefer avoiding low-quality, heat-killed bacteria (right of the yellow line) in comparison to live bacteria (left of the yellow line). Image credit: Pengfei Liu (CC BY 4.0)
We quickly learn to steer clear of eating the moldy apple, the foul-smelling piece of chicken or the leftovers that taste a little ‘off’. This survival instinct is shared across most animal species – even those with extremely simple and limited visual or taste systems, like the tiny worm Caenorhabditis elegans. Indeed, assessing the safety and quality of available food items can also rely on cells activating built-in cascades of molecular reactions. However, it remains unclear how these ‘cellular stress response programs’ actually help guide feeding behaviors.
To better understand this process, Liu et al. conducted a series of experiments using C. elegans worms exposed to heat-killed bacteria, which are devoid of many nutrients essential for growth. After initially consuming these bacteria, the worms quickly started to avoid feeding on this type of low-quality food. This suggests that mechanisms occurring after ingestion allowed the worms to adjust their feeding choices.
Further work showed that the consumption of heat-killed bacteria triggered two essential stress response pathways, known as the unfolded protein response and the innate immune response. The activation of these pathways was essential for the animals to be able to change their behavior and avoid the heat-killed bacteria. These biochemical pathways were particularly active in the worms’ nerve cells, highlighting the importance of these cells in sensing and reacting to food. Finally, Liu et al. also found that adding sugars like lactose and sucrose to the low-quality food could prevent the activation of the stress response pathways. This result suggests that specific nutrients play a central role in how these worms decide what to eat.
These findings shed light on the complex systems that ensure organisms consume the nutritious food they need to survive. Understanding these processes in worms can provide insights into the broader biological mechanisms that help animals avoid harmful food.
