Flagella and motility represent two of the richest subjects in microbiology, involving not only bacterial genetics, molecular biology, and physiology but also bioenergetics, hydrodynamics, structural analysis, and macromolecular assembly.
Our knowledge that bacteria actively move goes as far back as the discovery of bacteria themselves (7). To quote from an article by Howard Berg in 1975 (4), written not long after the modern era of investigation of bacterial flagella, motility, and chemotaxis had begun:
When Antony van Leeuwenhoek looked through a single-lens microscope in 1676 and observed man’s first recorded glimpse of bacteria, it was their motion that most delighted him: “I must say, for my part, that no more pleasant sight has ever yet come before my eye than these many thousands of living creatures, seen all alive in a little drop of water, moving among one another, each several creature having its own proper motion.”
Leeuwenhoek goes on to say, in a charming phrase: “…I can make out no paws…[yet] I am persuaded that they too are furnished with paws withal.”
The bacterial “paw,” more commonly known as the flagellum, is a structure with a very long (ca. 10-μm), thin (ca. 20-nm-diameter) external filament. Besides its extreme thinness and length, the first thing that strikes one about the flagellar filament is its “waviness.” The active propagation of this wave during motility was evident from early high-speed movies, so there was no doubt that flagella were the organelles of bacterial motility. Cells typically displayed more than one type of movement: in some cases, simple forward and backward swimming and in other cases (e.g., Salmonella), swimming and tumbling.
Around 1970, the major questions about bacterial motility could be summarized as follows: (i) What is the shape of the wave, and is it intrinsic to the flagellar structure? (ii) How is the waveform propagated? (iii) What is the nature of the motor? (iv) What is responsible for the two types of motility (swimming and tumbling in the case of Salmonella)? (v) What is the energy source? In only a few years, the answers to these questions were obtained, at least in broad outline.