what is the definition for undersirable bacteria?

Answer 1

It usually refers to pathogenic bacteria (ones that can cause illness) or pathogenic levels of normal flora (bacteria normally harmless unless in significant numbers to cause an infection)

Answer 2

Undesirable bacteria is responsible for food spoilage. This type of bacteria may not make you sick, but they show themselves by the use of odors, sticky or slimy surfaces and discoloration.