The composition can vary with each brewing process. In terms of numbers, the bacteria (mainly lactic acid bacteria and acetic acid bacteria) make up the largest proportion in milk kefir.
For example, the following genera of bacteria are included: Lactobacillus, Bifidobacterium and Acetobacter.
A variety of yeasts are also included, for example: Saccharomyces cerevisiae and Candida lipolytics...and a lot more :)