Why is Ecoli unable to peptonise milk?

Answer 1

The E. coli is an enteric bacteria and milk is a proteinful medium and there is a very much competition for nutrients and IE. coli canot stand the competition

Answer 2

E. coli lacks the enzymes needed to break down the proteins in milk into peptides and amino acids. Specifically, E. coli does not produce the enzyme protease, which is essential for peptonization of milk proteins. As a result, E. coli is unable to effectively digest milk protein and peptonize milk.