The human vermiform (“worm-like”) appendix is a 5 to 10 cm long and 0.5 to 1 cm wide pouch that extends from the cecum of the large bowel. The architecture of the human appendix is unique among mammals, and few mammals other than humans have an appendix at all. The function of the human appendix has long been a matter of debate, with the structure often considered to be a vestige of evolutionary development despite evidence to the contrary based on comparative primate anatomy. The appendix is thought to have some immune function based on its association with substantial lymphatic tissue, although the specific nature of that putative function is unknown. Based (a) on a recently acquired understanding of immune-mediated biofilm formation by commensal bacteria in the mammalian gut, (b) on biofilm distribution in the large bowel, (c) the association of lymphoid tissue with the appendix, (d) the potential for biofilms to protect and support colonization by commensal bacteria, and (e) on the architecture of the human bowel, we propose that the human appendix is well suited as a “safe house” for commensal bacteria, providing support for bacterial growth and potentially facilitating re-inoculation of the colon in the event that the contents of the intestinal tract are purged following exposure to a pathogen.Their points (a) to (e) are fine, but the conclusion depends on the totally unjustified assumption that a severe bout of diarrhoea eliminates commensal bacteria from the colon. Of course diarrhoea can reduce the number of bacteria in the colon, and cholera is likely to disrupt its epithelial biofilms. But even the most severe diarrhoeal infections are very unlikely to sterilize it, largely because there will be so many microenvironments within the colon that retain at least part of their biofilm. Biofilms are notoriously difficult to remove and to sterilize.
- What about animals that don't have an appendix? Only humans and anthropoid apes have appendices, but they are certainly not the only animals that get diarrhoeal infections. The authors argue that humans in Western cultures don't need their appendices because they're so rarely at risk of contracting the severe diarrhoeal diseases that the authors postulate necessitate reinoculation from the appendix. But animals are much more like humans in primitive cultures than like affluent humans.
- Some of the news reports (but not the paper itself) suggest that humans in primitive cultures need their appendix for reinoculation because populations are sparse and reinoculation from other people would not be reliable. But humans always live in social groups, and one thing severe diarrhoea does is spread intestinal bacteria around the environment, making reinoculation even more likely that it would otherwise be.