Coliform bacteria are organisms that are present in the environment and in the feces of all warm-blooded animals and humans. Coliform bacteria will not likely cause illness. However, their presence in drinking water indicates that disease-causing organisms (pathogens) could be in the water system. (Washington State Department of Health)
What does it do to your yoghurt?
If your yoghurt becomes contaminated with coliform bacteria, it is likely to have lots of little bubbles on the surface.
There are many kinds of coliforms, and most of them do not cause illness; however, if there are large numbers of coliforms in your yoghurt, there is a higher chance of E. coli being in it, and that can be a severe problem.
Should I eat this yoghurt?
The short answer is no. While the coliform bacteria itself may not harm you, it is an indication of possible contamination by other bacteria that can cause illness. Yeast contamination looks very similar, and the yoghurt may have a slightly yeasty smell to it. The taste will also be adversely affected.