Robert Koch, Microbiology on Potato Slice and understanding Experimental Design

The discussion on how bacteria will be surviving on boiled potato? It could be getting carbohydrates from the potato but from where it will amino acids? @PChitralekha explained how there are various biochemical pathways where after glcolysis glucose product pyruvate can enter into amino acid synthesis pathways, especially in anaerobic conditions. Lactic acid is also produced in anaerobic conditions, hence lactobacillus can grow on potato and many acid-sensitive bacteria will be eliminated. We also came to know Robert Koch cultured bacteria on potato slices.

@Enas explained her potato medium experiment using curd to isolate bacteria. Test slice 1 has some contamination. We can see an isolated colony on test slice 2. In Control similar contamination can be observed as test slice 1 indicating it is contamination. They could be yeast.
However, the test slice 2 has an isolated colony.
The role of control was understood and how we can use it to understand what is true colony from our curd and what is contamination not coming from curd.