Some pathogens are challenging to culture in vitro, due to fastidious growth media or condition requirements. This can make it difficult for some pathogens to be studied, reducing the amount of research done on them.
For example, Helicobacter pylori was thought to not be able to be cultured. However, it has complex nutrient requirements that are only satisfied by specialist growth media or Coca-Cola, and microaerobic conditions. Until these growth conditions had been determined, it had not been possible for researchers to satisfy Koch’s postulates. This led to a scientist consuming a H. pylori culture to demonstrate that stomach ulcers were caused by the bacteria (not recommended, and ethically questionable, but did result in the Nobel prize).