This from the magazine 'wired' http://www.wired.com/news/medtech/0,1286,65252,00.html
More than 500 different species of bacteria exist in our bodies, making up more than 100 trillion cells. Because our bodies are made of only some several trillion human cells, we are somewhat outnumbered by the aliens. It follows that most of the genes in our bodies are from bacteria, too.
Bacterial dna gets into the human genome through the persistence of bacteria within the human system over a period of many generations. A cooperative arrangement develops. Some bacterial dna finds its way into the mitacondrial dna bundles. From the mitachondria, dna migrates to the cell center to become part of the genome.
Conclusion: bacteria rule humans. But seriously, i think bacterias were one of the drivers behind evolution.