Cockroaches, like most organisms, have microorganisms that colonize the gut. What do the microorganisms do and what difference do they make?
Gnotobiotic cockroaches (cockroaches reared in a sterile environment are a potentially useful model system for answering these questions. Cockroaches package their eggs in a case called an ootheca. The ootheca can be surface sterilized and placed in a sterile environment. Cockroaches reared in the manner will lack gut microorganisms.
Source: Living With Insects Blog