Members of the are the most abundant bacteria present in the large intestine of humans and contain unusual in their cell membranes.

Bacteroides

Sphingolipids.

Explanation:

Bacteroides are symbiotic bacteria which populates  GI tract  of their hosts,where  they maintain general well being of the host  and in return gain shelter and nutrients  from the host.

However,in  order to survive the fluctuating environmental factors of chemical, temperature, of the intestinal medium,these  bacteria had   developed   highly sensitive structures to quickly  detect the fluctuations, and a sequence of   response  mechanisms to respond accordingly to survive.

The sensor or danger signaling and response  structure in the membranes of these bacteria for this  purpose is called Sphingolipids. The structure  sensed fluctuation in the intestinal environments and created a  response pathways for the bacteria to evade the unpleasant environments,This structure was formally thought to be present in eukaryotes only.Thus why the word unusual in the question.

that is true   : ) hope this you