Yeah friendly ones that break everything up

E. coli is an exmaple of one of the many intestinal bacteria that you could NOT live without. All part of the human bodies normal flora.

another quite interesting bacterial organism that poses good and bad effects on humans i Enteroccocus faecalis. I lives in the upper colon but also can be an infectious pathogen. It is vancomycin-resistant making it difficult to treat with anti-biotics.

Yes. "Microorganisms can form an endosymbiotic relationship with other, larger organisms. For example, the bacteria that live within the human digestive system contribute to gut immunity, synthesise vitamins such as folic acid and biotin, and ferment complex indigestible carbohydrates." Source and further information: http://en.wikipedia.org/wiki/Microorganism#Human_digestion "Human bacterial flora and human health: Bacteria are vital for the maintenance of human health, but some pathogenic bacteria also pose a significant health threat by causing diseases. Large numbers of bacteria live on the skin and in the digestive tract. Their growth can be increased by warmth and sweat. Large populations of these organisms on humans are the cause of body odor and thought to play a part in acne. There are more than 500 bacterial species present in the normal human gut and are generally beneficial: they synthesize vitamins such as folic acid, vitamin K and biotin, and they ferment complex indigestible carbohydrates. Other beneficial bacteria in the normal flora include Lactobacillus species, which convert lactose and other sugars to lactic acid in the gut. The presence of such bacterial colonies also inhibits the growth of potentially pathogenic bacteria (usually through competitive exclusion) and some beneficial bacteria are consequently sold as probiotic dietary supplements." Source and further information: http://en.wikipedia.org/wiki/Human_flora#Human_bacterial_flora_and_human_health