Here is an interesting article from the Smithsonian about the history of discovering viruses (with EMs) and they discuss Koch too: https://www.smithsonianmag.com/science-nature/what-are-viruses-history-tobacco-mosaic-disease-180974480/

Here is a paper about Koch's postulates satisfied for Sars https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7095368/

Here is a paper with an EM of a bacteriophage https://www.researchgate.net/figure/Electron-micrograph-of-a-bacteriophage_fig2_307863150

Good luck.