Selected metal ions protect Bacillus subtilis biofilms from erosion

Many problems caused by bacterial biofilms can be traced back to
their high resilience towards chemical perturbations and their
extraordinary sturdiness towards mechanical forces. However, the
molecular mechanisms that link the mechanical properties of a
biofilm with the ability of bacteria to survive in different
chemical environments remain enigmatic. Here, we study the erosion
stability of Bacillus subtilis (B. subtilis) biofilms in the
presence of different chemical environments. We find that these
biofilms can utilize the absorption of certain metal ions such as
Cu2+, Zn2+, Fe2+, Fe3+ and Al3+ into the biofilm matrix to avoid
erosion by shear forces. Interestingly, many of these metal ions
are toxic for planktonic B. subtilis bacteria. However, their toxic
activity is suppressed when the ions are absorbed into the biofilm
matrix. Our experiments clearly demonstrate that the biofilm matrix
has to fulfill a dual function, i.e. regulating both the mechanical
properties of the biofilm and providing a selective barrier towards
toxic chemicals.