When Would Koch's Postulates Be Utilized

When would Koch's postulates be utilized?

A) To determine the cause of a patient's illness in a hospital microbiology lab
B) To develop a new antibiotic in a pharmaceutical lab
C) To determine the cause of a new disease in a microbiology research lab
D) To formulate a vaccine against a new pathogen in a genetic engineering lab
E) Whenever the scienti?c method cannot be used to investigate a microbiological problem