Bacillus spores are highly resistant dormant cells formed
in response to starvation. The spore is surrounded by a
structurally complex protein shell, the coat, which protects the genetic material. In spite of its dormancy,
once nutrient is available (or an appropriate physical
stimulus is provided) the spore is able to resume metabolic activity and return to vegetative growth, a
process requiring the coat to be shed. Spores dynamically
expand and contract in response to humidity, demanding
that the coat be flexible. Despite the coat’s critical biological functions, essentially nothing is known about
the design principles that allow the coat to be tough
but also flexible and, when metabolic activity resumes,
to be efficiently shed. Here, we investigated the hypothesis that these apparently incompatible characteristics
derive from an adaptive mechanical response of the
coat. We generated a mechanical model predicting the
emergence and dynamics of the folding patterns uniformly seen in Bacillus spore coats. According to this
model, spores carefully harness mechanical instabilities
to fold into a wrinkled pattern during sporulation.
Owing to the inherent nonlinearity in their formation,
these wrinkles persist during dormancy and allow the
spore to accommodate changes in volume without
compromising structural and biochemical integrity.
This characteristic of the spore and its coat may inspire
design of adaptive materials.