Amazing stuff!
"... In the new open-access study ... identifies the specific chemicals that a key neuron in C. elegans senses, both in the bacteria that it eats and in the bacteria that it needs to avoid ingesting. ...
C. elegans a “bacterial specialist” because the tiny, transparent worm has evolved to eat bacteria as its diet, while also needing to avoid pathogenic bacteria that can prove to be its undoing. This has led it to develop a nervous system especially well-attuned to sorting out what is food and what is foe. ...
what the ion channels are detecting in the bacteria. To get started, they exposed worms to 20 different kinds of bacteria the worms are known to encounter and found that they all activated NSM activity to varying extents. Then they broke the bacteria down into more and more specific chemical components to see which one or ones triggered NSM. The experiments ruled out many components, including DNA, lipids, proteins, and simple sugars, and instead found that it’s specifically the polysaccharide sugars that coat many bacteria that drive NSM activation. In particular, in gram-positive bacteria, a chemical called peptidoglycan activated NSM. In gram-negative bacteria, a different polysaccharide was apparently in play. ...
Having shown what exactly triggers the worms to recognize their bacterial food, the researchers wondered whether they could also pinpoint a danger sign the worm finds in harmful bacteria.
For these experiments, they carefully used Serratia marcescens, a bacterium that’s also infectious for humans. Some strains of the bacteria have a red color, while others do not. The red ones, which have a pigment called prodigiosin, tend to be much more lethal for worms. In their testing, the researchers found that when NSM detected the non-pigmented bacteria, the neuron still activated and the worms still ingested the bacteria, but when prodigiosin was present, NSM did not activate and the worm did not pump it in or slow down to eat. ..."
From the highlights and abstract:
"Highlights
• The enteric sensory neuron NSM is activated by ingestion of diverse bacteria
• Bacterial polysaccharides, including peptidoglycans, are sufficient to activate NSM
• Bacterial polysaccharides drive serotonin-dependent changes in foraging behaviors
• Prodigiosin, produced by pathogenic S. marcescens, inhibits NSM activity
Summary
The bacterial microbiome influences many aspects of animal health and disease. Bacteria can have beneficial functions, for example providing nutrients, whereas others can act as pathogens.
Bacteria are sensed by host cells to induce adaptive changes in physiology and behavior. While immune and intestinal cells detect bacterial signals through well-characterized mechanisms, recent studies indicate that neurons can also directly sense bacteria.
However, the bacterial sensory mechanisms in neurons are less well understood. In Caenorhabditis elegans, the enteric sensory neuron NSM innervates the pharyngeal lumen and is directly activated by bacterial ingestion; in turn, NSM releases serotonin to induce feeding-related behaviors.
However, the molecular identities of the bacterial signals that activate NSM are unknown.
To identify them, we probed bacterial macromolecules from nutritive bacteria using biochemical approaches. We find that polysaccharides from bacteria are sufficient to activate NSM.
We further identify peptidoglycans from Gram-positive bacteria as specific components capable of activating NSM. NSM responses to polysaccharides require the acid-sensing ion channels DEL-3 and DEL-7, which localize to NSM's sensory dendrite in the pharyngeal lumen.
Ingestion of bacterial polysaccharides enhances feeding and reduces locomotion, matching the known effects of NSM on behavior.
We also examine signals produced by pathogenic bacteria. This approach identifies prodigiosin, from pathogenic Serratia marcescens, as a metabolite that prevents NSM activation by nutritive bacterial signals.
This study identifies molecular signals that underlie neuronal recognition of nutritive bacteria in the alimentary canal and competing signals from a pathogenic bacterial strain that can mask this form of recognition."
How neurons sense bacteria in the gut | MIT News | Massachusetts Institute of Technology "Neural interaction with bacteria has important effects on animal brains. A new study investigates how neurons sense bacteria by revealing, in nematodes, the bacterial signals that a key neuron detects."
Figure 1 Bacterial polysaccharides activate the enteric sensory neuron NSM
Figure 5 A Serratia marcescens metabolite, prodigiosin, inhibits NSM activity and associated behaviors
