Biofilms grow on various surfaces and in many different environments, a phenomenon that constitutes major problems in industry and medicine. Despite their importance little is known about the viscoelastic properties of biofilms and how these depend on the chemical microenvironment. Here, we find that the mechanical properties of Pseudomonas aeruginosa (P.a.) biofilms are highly robust towards chemical perturbations. Specifically, we observe that P.a. biofilms are able to fully regain their initial stiffness after yielding is enforced, even in the presence of chemicals. Moreover, only trivalent ions and citric acid significantly affect the biofilm elasticity, the first of which also alter the texture of the material. Finally, our results indicate that biofilm mechanics and bacteria viability inside the biofilm are not necessarily linked which suggests that targeting bacteria alone might not be sufficient for biofilm removal strategies.