Refer to the Figure

Refer to the figure. In the 1990s, stomach ulcers were shown by Koch's postulates to be caused by the bacterium Helicobacter pylori.Which of the following is not one of Koch's postulates used to identify pathogens?

A) It must be possible to grow a pure culture of the isolated microorganism.
B) Cultured samples must be able to produce disease symptoms in a newly infected host.
C) Isolated microorganisms must respond positively to Gram-staining procedures.
D) Newly infected hosts must yield the same suspect microorganism in pure culture.
E) The suspected microorganism must always be found in diseased hosts.