The commensal E. coli strains that inhabit the large intestine of all humans and warm-blooded animals comprise no more than 1% of the total  bacterial biomass.

The E. coli flora is apparently in constant flux. One study on the distribution of different E. coli strains colonizing the large intestine of women during a one year period (in a hospital setting) showed that 52.1% yielded one serotype, 34.9% yielded two, 4.4% yielded three, and 0.6% yielded four.  The most likely source of new serotypes of E. coli is acquisition by the oral route.

To study oral acquisition, the carriage rate of E. coli carrying antibiotic-resistance plasmids (R factors) was examined among vegetarians, babies, and nonvegetarians. It was assumed that nonvegetarians might carry more E. coli with R factors due to their presumed high incidence in animals treated with growth-promoting antimicrobial agents. However, omnivores had no higher an incidence of R-factor-containing E. coli than vegetarians, and babies had more resistant E. coli in their feces than nonvegetarians. No suitable explanation could be offered for these findings. Besides, investigation of the microbial flora of the uninhabited Krakatoa archipelago has shown the presence of antibiotic-resistant E. coli associated with plants.

The bottom line seems to be that most of us have more than one strain of E. coli in our gut, and intestinal strains tend to displace one another about three or four times a year.