βακτήριον

Baktḗrion in Greek is the diminutive of baktēria (a staff, a cane, a walking stick). The word was applied to microorganisms by Christian Gottfried Ehrenberg in 1828, who observed rod-shaped organisms under a microscope and named them for their appearance. The Greek walking stick became the name for an entire kingdom of life, even though many bacteria are spherical (cocci), spiral (spirilla), or comma-shaped (vibrios). The rod-shaped ones got there first.

Antonie van Leeuwenhoek first observed bacteria in 1676, calling them 'animalcules' (little animals). He saw them in rainwater, in scrapings from his teeth, in pepper-water infusions. He drew them with remarkable accuracy. But the connection between these tiny organisms and disease was not made for another two centuries. The germ theory of disease — the idea that bacteria cause infections — was established by Louis Pasteur in the 1860s and Robert Koch in the 1880s.

Koch's postulates (1890) set the standard for proving that a specific bacterium causes a specific disease: isolate it from a sick organism, grow it in pure culture, introduce it to a healthy organism and produce the same disease, then re-isolate it. Koch himself identified the bacteria responsible for tuberculosis (1882) and cholera (1883). The walking stick became the enemy.

The story reversed in the late twentieth century. Microbiome research revealed that the human body contains roughly as many bacterial cells as human cells — about 38 trillion. Most of these bacteria are beneficial. They digest food, produce vitamins, train the immune system, and protect against pathogens. The word 'bacteria' had spent a century meaning 'germs to be killed.' Now it also means 'partners to be preserved.' The small sticks turned out to be allies.