Experimental studies indicate that segmented filamentous bacteria (SFB) can recapitulate fundamental immune responses, particularly in reference to Th 17 and regulatory T-cell activity. The recent description of whole genomic DNA sequences of mouse and rat SFBs and the comparison between these sequences opens new important perspectives. In particular this knowledge allows to perform quali-quantitative studies in human beings regarding these yet unculturable anaerobic bacteria. The comparative analysis of rat and mouse SFB 16S rRNA sequences suggests the existence of highly conserved regions that could represent SFB-specific molecular targets potentially useful to develop PCR probes for study SFB in humans. These molecular tools may be of inestimable value to evaluate the possible role played by SFB in the immune system physiology, as well as its potential involvement in human chronic inflammatory and autoimmune diseases.