How did Robert Koch connect specific microorganisms to specific diseases?

Robert Koch established the causal link between specific microorganisms and diseases through rigorous experimental methods and the development of systematic criteria known as Koch’s postulates.

His approach involved several key innovations:

• Pure-Culture Techniques: Koch perfected methods to isolate pathogenic bacteria and grow them in pure culture outside the body, using various media to ensure he was studying a single type of organism.12
• Microscopic Observation: He utilized advanced techniques, such as the hanging-drop method and photomicrography, to observe the growth, behavior, and life cycles of microorganisms.1
• Experimental Inoculation: By inoculating healthy animals with isolated cultures, he demonstrated that the same disease could be reproduced, providing convincing proof of a specific causal relationship.12

These efforts culminated in his four basic criteria for establishing causality:
1. The specific microorganism must be consistently associated with the disease.
2. The organism must be isolated and grown in pure culture.
3. The cultured organism must induce the same disease when introduced into a healthy host.
4. The organism must be re-isolated from the newly infected host and confirmed as identical to the original.1

Through these methods, Koch identified the causative agents of anthrax, tuberculosis, and cholera.3