  Bacteria suspension exhibits a wide range of collective phenomena arising
from interactions between individual cells. Here we show Serratia marcescens
cells near an air-liquid interface spontaneously aggregate into dynamic
clusters through surface-mediated hydrodynamic interactions. These long-lived
clusters translate randomly and rotate in the counter-clockwise direction; they
continuously evolve, merge with others and split into smaller ones.
Measurements indicate that long-ranged hydrodynamic interactions have strong
influences on cluster properties. Bacterial clusters change material and fluid
transport near the interface and hence may have environmental and biological
consequences.
