Confessions of a "Molecular" Motorhead

Kansas "motorcycle roads" is a bit of an oxymoron, but there are a few really nice stretches of road in our state - though somewhat few and far between.  Riding west of Lawrence (KS) on US Highway-40 is one of those little gems “close to home” for me and thus, I find myself riding it frequently.   Just before the Douglas-Leavenworth County line the road starts to bend with decent regularity.   If you are like me, my senses begin to heighten just by the sight of the twisting road ahead.  Down shifting to engine-brake into the approach of a tight right turn, I look deep into the curve, apply a touch of front brake, counter-steer to lean the bike into the curve and hold steady on the throttle.   Once beyond the apex of the curve, I roll back hard on the throttle settling the rear suspension and lightening the front wheel as I pull away hard from the curve.  The deep, resonant exhaust note coming from the 1100 cc V-twin Moto Guzzi rumbles through my body and puts a grin on my face revealing my pure delight.  The 3-year-old in me says:…“Let’s do it again”!!

I’ve always loved motors and motor vehicles.  I think every kid loves to see things move and if they move under power of their own on-board motor, then this is extra special!   

Imagine my delight when, as an undergraduate student majoring in microbiology I learned that few, if any,  cellular processes occur without the intervention of motors – very tiny motors – molecular motors!  Motors composed of protein rather than metal, these sub-nanoscale mechanical devices function very similarly to the more familiar “conventional” motors that provide locomotion on the human scale.

Molecular Motor at the Base of a Bacterial Flagellum

There are a number of different molecular motors found in nature but my favorite has to be the one that powers bacterial locomotion.   The vast majority of bacteria (those that can move) are propelled by the rotation of a curved proteinaceous appendage called a flagellum (or flagella, pl.) that serves as a type of propeller.  The flagellum is driven by a reversible rotary motor at its base spinning at astounding rotational speeds as high as 1,700 revolutions per second or, 102,000 rpm!!!  Energy for motor rotation is supplied by transmembrane electrochemical gradients of either sodium ions or protons.   The electrochemical gradients are generated by a variety of mechanisms including oxidative metabolism, fermentation, and light, in the case of photosynthetic bacteria.    Imagine that! – solar-powered microbes!   The resulting electrochemical capacitor drives the motor that spins the curved flagellum, like a spinning corkscrew, and propels the bacterium at surprisingly stunning speeds.

BMW S1000RR

On a scale basis, the high rotational speed of the motor and the efficiency of the flagellum as a propeller translates into straight-line speeds of locomotion that far exceed that of the fastest of mammals. Speaking of speed, the BMW S100RR of the microbial world is the obligate predatory bacterium Bdellovibrio.   This tiny (1.4 microns in length) microbe possesses a single flagellum that propels it at speeds approaching 100 cell lengths per second.  If Bdellovibrio were the size of a typical motorcycle (~2 meters), it would be traveling at 200 meters per second or an incomprehensible 447 miles per hour!   

Bdellovibrio "T-Boning" an Unsuspecting E. coli at 100 Body Lengths/Second

Why would Bdellovibrio have evolved to become the “land speed record holder” of the microbial world?  Well, one answer might be the way that Bdellovibrio finds and eats it’s prey (other bacteria!).  Guided by chemotaxis, a form of “directional sniffing” Bdellovibrio finds its prey, propels itself towards the unsuspecting bacterial target at blinding speed and penetrates the target bacterial cell body by the sheer impact of the collision.  It’s somewhat analogous to a car being “T-boned” by a speeding sport bike…. Not a pretty sight! Once inside its prey, Bdellovibrio grows as a long filament approximately 20 times the length of a normal cell.  The filament separates into individual cells, each of which grows it’s own flagellum and the newly formed baby Bdellovibrio bust out of the prey carcass and speed off like hyper-sportbikes to slam into another bacterium thus, initiating a new lifecycle.  The high efficiency of the Bdellovibrio motor/flagella combination and its speed of locomotion are subjects of intense investigation.

So, the next time you are  zipping along your own favorite local US-40 enjoying the ride, grinning ear-to-ear, think about being passed by a motorcycle-sized bacterium traveling over 4-times your speed!!   -LD