  The envelopes covering bacterial cytoplasm possess remarkable elastic
properties. They are rigid enough to resist large pressures while being
flexible enough to adapt to growth under environmental constraints. Similarly,
the virus shells play an important role in their functions. However, the
effects of mechanical properties of the outer shell in controlling and
maintaining the sizes of bacteria or viruses are unknown. Here, we present a
hydrodynamic "bubbles with shell" model, motivated by the study of bubble
stability in fluids, to demonstrate that shell rigidity and turgor pressure
control the sizes of bacteria and viruses. A dimensionless compliance
parameter, expressed in terms of the elastic modulus of the shell, its
thickness and the turgor pressure, determines membrane response to deformation
and the size of the organisms. By analyzing experiment data, we show that
bacterial and viral sizes correlate with shell elasticity, which plays a
critical role in regulating size.
