What is the significance of Tobacco mosaic virus in the history of microbiology?

• A. It was the first virus to be studied, helping define viruses as a distinct class of infectious agents
• B. It proved viruses are larger than bacteria
• C. It demonstrated that tobacco plants can be infected by bacteria
• D. It disproved germ theory

Answer: (D)

Tobacco mosaic virus helped establish viruses as a separate category of infectious agents. Early researchers showed that the disease could be transmitted by sap that had been filtered through a filter designed to trap bacteria, indicating the causal agent was smaller than bacteria and not a typical bacterium. Beijerinck’s work described this agent as a contagious living fluid that could replicate only within living cells, leading to the coinage of the term virus and the birth of virology. This shifted microbiology from thinking only about bacteria to recognizing a distinct class of subcellular pathogens, expanding germ theory to include non-bacterial infectious agents.

So, the significance isn’t about viruses being larger than bacteria, nor about bacteria causing tobacco disease, and it certainly doesn’t disprove germ theory. It’s about establishing viruses as a unique, filterable, and replication-dependent form of infectious agent that opened a new field of study.