Hi Bahir, 

In my experience, Sorbitol MAcConkey helps to isolate E. coli O157:H7 as long as your strain is a sorbitol negative. They will appear as colourless colonies. However, as you might be aware, there are some E. coli O157:H7 strains that ferment sorbitol, so you won't be able to distinguish them from other bacteria. 

I would suggest that you plate your samples in other selective agar besides Sorbitol MacConkey, such as CHROMagar for example.

Hope this helps.

You could also try  Fluorocult agar  to isolate colourless E. coli O157:H7 colonies.

Thank you Stefanos Petsios

 You 're welcome!

You could also try Rainbow Agar for

Detecting Escherichia coli O157:H7