Your appendix is a small dead-end tube connected to a section of your large intestine. It has long been thought to be a vestigial remnant of some other organ, but there is little evidence for an appendix in our evolutionary ancestors. Few mammals have any appendix at all, and the appendices of those that do bears little resemblance to the human one.
The appendix is isolated from the rest of the gut, with an opening smaller than a pencil lead. In times of trouble, such as an infection that flushes the system, these commensal bacteria could hide out there, ready to repopulate the gut when the danger is past.
Biofilms, colonies of beneficial microbes, form in your large intestine. They aid digestion and protect against infection, while enjoying the protection and nutrition of the human host. Researchers have found biofilms on the epithelial lining of the appendix as well.