How much vitamin C has a noticeable effect on E.coli k12
I found a 'short communication' paper by Akira Murata et.al where they did In Vitro testing of Vitamin C on L. casei showing no inhibition of growth, and also tested on E.coli and B. subtilis and found that Vitamin C inhibited the growth. Also I believe Suzanne Humpries stated the same thing somewhere in this 1.5 hour long video, indicating she may posses more knowledge as to studies.


  Ascorbic acid did not inhibit the growth of L. casei even at high
  concentrations, and rather stimulated the growth at certain
  concentrations (2~3 X 10- 2 M). This observation indicates that
  ascorbic acid can be used as an antiviral agent.
  
  In E. coli and B. subtilis, unlike L. casei, a high concentration
  of ascorbic acid acted so as to lessen their growth rates (Fig. 3).
  Particularly, in B. subtilis W23, cell lysis was observed about 100
  min after the contact with ascorbic acid and, moreover, when a drop of
  the lysate was placed on a plate seeded with B. subtilis 168 and
  incubated overnight at 30oC, there appeared a clear zone around the
  placed lysate. Since B. subtilis W23 is known to possess an inducible
  defective phage PBSZ able to kill B. subtilis 168,3 ' 41 the observed
  result maybe ascribed to the induction of the defective prophage from
  B. subtilis W23 by action of ascorbic acid. Lwoff and Siminovitch51
  reported once that the lysogenic phage in B. megaterium was induced by
  treatment with ascorbic acid.