The Bacterial Flagellum Revisited: A Paradigm of Design

Going back to my undergraduate days, I have long been struck by the engineering elegance and intrinsic beauty of that familiar icon of intelligent design, the bacterial flagellar nano-motor. In tribute to this masterpiece of design, I have just published a detailed (31 pages, inclusive of references) literature review in which I describe the processes underlying its self-assembly and operations.

My essay also attempts to evaluate the plausibility of such a system having evolved by natural selection. Here’s a short excerpt to whet your appetite.

The bacterial flagellum is a reversible, self-assembling, rotary nano-motor associated with the majority of swimming bacteria. There exists a number of different models of this rotary motor (Pallen and Matzke, 2006; Soutourina and Bertin, 2003). Flagella are produced by a very tightly regulated assembly pathway (Chevance and Hughes, 2008; Macnab, 2003; Aldridge and Hughes, 2002), and the archetypical system for understanding flagellar assembly belongs to Salmonella enterica serovar Typhimurium, a rod-shaped gram negative bacterium of the family Enterobacteriaceae.

Flagella receive feedback from the environment by virtue of an elegant signal transduction circuit and can adjust their course in response to external stimuli by a mechanism known as chemotaxis (Baker et al., 2006 Bourret and Stock, 2002; Bren and Eisenbach, 2000). The most extensively studied chemotaxis system belongs to Escherichia coli.

By itself, the rotor is able to turn at a speed between 6,000 and 17,000 rotations per minute (rpm) but normally only achieves a speed of 200 to 1000 rpm when the flagellar filament (that is, the propeller) is attached. Its forward and reverse gears allow the motor to reverse direction within a quarter turn.

The bacterial flagellum, which has been described as a “nanotechnological marvel” (Berg, 2003), has long been championed as an icon of the modern intelligent design movement and the flagship example of “irreducible complexity” (Behe, 1996). But even biologists outside of this community have been struck by the motor’s engineering elegance and intrinsic beauty. As one writer put it, “Since the flagellum is so well designed and beautifully constructed by an ordered assembly pathway, even I, who am not a creationist, get an awe-inspiring feeling from its “divine’ beauty” (Aizawa, 2009).

The mechanistic basis of flagellar assembly is so breathtakingly elegant and mesmerizing that the sheer engineering brilliance of the flagellar motor — and, indeed, the magnitude of the challenge it addresses to Darwinism — cannot be properly appreciated without, at minimum, a cursory knowledge of its underlying operations. The purpose of this essay is to review these intricate processes, and evaluate the plausibility of such a system evolving by natural selection.

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