Agrobacterium tumefaciens is a bacteria living in the soil. It is very common plant microbe that causes a disease know as a grown gall. Grown gall affects hundreds of plant species especially dicotyledonous (broad-leaved) plants. The disease is named from the large swellings (galls) that usually occur at the crown of the plant, just above soil level. Basically, the bacterium transfers part of its DNA to the plant, and this DNA changes the plant genome, causing the production of swellings and associated changes in plant metabolism. Agrobacterium is the only known example of the DNA transfer to eukaryotic cells in the interkindom DNA transfer. Even if agrobacterium is harmful to plants its is useful to scientist.
If you are interested in agrobacterium tumefaciens you can learn more about structure of the bacterium, the infection process and the uses of this bacteria from my web site.