Differential protection from tobramycin by extracellular polymeric substances from Acinetobacter baumannii and Staphylococcus aureus biofilms.

We investigated biofilms of two pathogens, Acinetobacter baumannii and Staphylococcus aureus, to characterize mechanisms by which the extracellular polymeric substance (EPS) found in biofilms can protect bacteria against tobramycin exposure. To do so, it is critical to study EPS-antibiotic interactions in a homogeneous environment without mass transfer limitations. Consequently, we developed a method to grow biofilms, harvest EPS, and then augment planktonic cultures with isolated EPS and tobramycin. We demonstrated that planktonic cultures respond differently to being treated with different types of EPS (A. baumannii versus S. aureus) in the presence of tobramycin. By harvesting EPS from the biofilms, we found that A. baumannii EPS acts as a "universal protector" by inhibiting tobramycin activity against bacterial cells regardless of species; S. aureus EPS did not show any protective ability in cell cultures. Adding Mg(2+) or Ca(2+) reduced the protective effect of A. baumannii EPS. Finally, when we selectively digested the proteins or DNA of the EPS, we found that the protective ability did not change, suggesting that neither has a significant role in protection. To the best of our knowledge, this is the first study that demonstrates how EPS protects pathogens against antibiotics in a homogeneous system without mass transfer limitations. Our results suggest that EPS protects biofilm communities, in part, by adsorbing antibiotics near the surface. This may limit antibiotic diffusion to the bottom of the biofilms but is not likely to be the only mechanism of protection.