The present invention generally relates to the field of intestinal health, disorders associated with aberrant microbiota and/or archaea-deficiency, and early-life immune system development. The present invention more specifically relates to M. smithii and components thereof for use as a biomarker, to methods for detecting M. smithii and/or for monitoring colonization status of M. smithii, biosensors and kits for detecting M. smithii or markers thereof, and M. smithii for promoting immune system homeostasis and intestinal health and for treating disorders associated with aberrant microbiota and/or archaea-deficiency.