indole reaction of e coli

[cloudnine]

can indole(-) negative e coli exist?

[canalon]

Yes,
as for any biochemical test there is no 100% compliance. But then it's harder to confirm, so when you are isolating a lot of strains, you tend to ignore them.
For confirmation, tests like API strips, BBL crystal, or Biolog assays should work. rRNA sequencing is of course another option but maybe not the cheapest nor the fastest.

Cheers

Patrick