Microorganisms are not inherently dangerous, but rather based on specific parameters some microorganisms may pose a greater risk than others.
There are hundreds of other bacteria which are dangerous on human. One of the most dangerous ones is tuberculosis, it hides within the immune cells and takes over in the lungs. There is the Neisseria genus which has species which cause gonorrhea and meningitis (Neisseria gonorrhoeae and Neisseria meningitidis). There is B. abortus, causing abortion of one's fetus once the placenta has been infected. Escherichia coli is by far not the most dangerous, the coliform bacteria are part of the normal microflora of the intestines. There are many many microorganisms dangerous for humans, animals or plants, but they can be killed off by antibiotics, si we can heal the infections.