Collective motion and density fluctuations in bacterial colonies

H. P. Zhang, Avraham Be'er, E. L. Florin, Harry L. Swinney

Research output: Contribution to journalArticlepeer-review

461 Scopus citations


Flocking birds, fish schools, and insect swarms are familiar examples of collective motion that plays a role in a range of problems, such as spreading of diseases. Models have provided a qualitative understanding of the collective motion, but progress has been hindered by the lack of detailed experimental data. Here we report simultaneous measurements of the positions, velocities, and orientations as a function of time for up to a thousand wild-type Bacillus subtilis bacteria in a colony. The bacteria spontaneously form closely packed dynamic clusters within which they move cooperatively. The number of bacteria in a cluster exhibits a power-law distribution truncated by an exponential tail. The probability of finding clusters with large numbers of bacteria grows markedly as the bacterial density increases. The number of bacteria per unit area exhibits fluctuations far larger than those for populations in thermal equilibrium. Such "giant number fluctuations" have been found in models and in experiments on inert systems but not observed previously in a biological system. Our results demonstrate that bacteria are an excellent system to study the general phenomenon of collective motion.

Original languageEnglish
Pages (from-to)13626-13630
Number of pages5
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number31
StatePublished - 3 Aug 2010
Externally publishedYes


  • Active suspensions
  • Self-organization
  • Swarming

ASJC Scopus subject areas

  • General


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