◄ WORLD V · SONNY 5DART 596 · a helldive into the living

THE CHEMOTAXIS how a bacterium finds food blind

E. coli is far too small to sense which way food lies — the concentration barely differs across its body. So it does something cleverer: it runs in a straight line, then tumbles to a random new direction, and biases the timing by comparing concentration now versus a moment ago. Improving? Run longer. Worsening? Tumble sooner. A biased random walk with a memory, and it reliably climbs the gradient it cannot see.

THE TECHNIQUE temporal compare → extend runs uphill

The demo makes the run/tumble decision from a temporal comparison of concentration: live demo







  
HISTORY & CREDIT Adler, Berg · 1970s

  
“Bacteria steer toward food.” — they cannot sense direction; they bias a random walk by comparing concentrations over time, extending favorable runs. cited

  
run & tumble · swim straight, then randomly re-orient.
the memory · compare concentration now vs before — improving → suppress the tumble, run longer.
1970s · Julius Adler & Howard Berg — temporal, not spatial, sensing.

  
Climbing a gradient you are too small to see. biophysics






  
RECOMMEND FOR I-13 the decision, on the compiler

  
On i-13, rising concentration extends the run; falling triggers a tumble — the biased walk climbs:

  
$ i13 run lf_chemotaxis.i13
RUN OK · 36 step(s)
run_uphill(12>8) = 1   tumble_downhill(5<8) = 0
biased_walk = 1  climbs_gradient = 1

  
Recommend as a NULL — a pinned decision rule (B39). The run/tumble bias is a fixed function of the temporal comparison; the walk itself needs randomness (B47). NULL — how a bacterium finds food blind.