The new study estimates that there are at least 5,600 different species or strains inhabiting the colon, substantially more than the 500 previously estimated.
A technique known as pyrosequencing helped identify myriad strains which exist in only small numbers to gain a full appreciation of the diversity seen in this off-putting area of the human body.
Many of the intestinal microbes feed off complex carbohydrates which humans' own digestive enzymes are impervious too.
Others perform useful functions like producing vitamin K, but their collective contribution is what is most significant.
By occupying intestinal real estate and devouring its useless-to-us, tasty-to-them contents, friendly gut organisms prevent pathogens from taking over in the same way a house filled with legal residents deters squatters.
The research provides some good news for those worried that antibiotics might be having a negative impact on this unusual ecosystem.
A five-day treatment of the broad-spectrum antibiotic ciprofloxacin as part of an eight-month monitoring period showed recovery within a few weeks.
"The bacterial ecosystems were knocked for a loop, but they snapped back quickly," researcher Les Dethlefsen said.
The study will be published online Nov. 18 in the journal Public Library of Science-Biology.