Harmless strains [of E Coli] are part of the normal flora of the gut, and can benefit their hosts by producing vitamin K2,[3] or by preventing the establishment of pathogenic bacteria within the intestine.[4][5]
http://en.wikipedia.org/wiki/E_coli

Since a healthy vagina is colonized by a mutually symbiotic flora of microorganisms that protect its host from disease-causing microbes, any attempt to upset this balance [soap, douching, feminine hygiene products] may cause many undesirable outcomes, including but not limited to abnormal discharge and yeast infection. The acidity of a healthy vagina due to lactic acid secreted by symbiotic microorganisms retards the growth of many strains of dangerous microbes. http://en.wikipedia.org/wiki/Vagina#Sexual_health_and_hygiene