(549a) Surface Stiffness Affects Bacterial Adhesion, Growth and Antibiotic Susceptibility of Attached Cells

Bacterial biofilm formation is a leading cause of chronic infections, which is involved in 80% of all microbial infections in humans. Biofilm associated chronic infections are very difficult to eradicate because the biofilm cells are up to 1000 times more resistant to antimicrobial agents than planktonic cells of the same species. The chemical and physical properties of surfaces play important roles in biofilm formation. However, compared to the well documented effects of surface chemistry, roughness, and charge, little is known about the role of surface stiffness. In this study, Escherichia coli RP437 and Pseudomonas aeruginosa PAO1 were used as the model strains to investigate the early stage biofilm formation on poly(dimethylsiloxane) (PDMS) surfaces with varying stiffness in the range of 0.3-3 MPa, which were prepared by controlling the degree of crosslinking. The number of biofilm cells was determined by counting colony forming units (CFU) and the surface coverage was quantified by analyzing microscopic images using COMSTAT. The length of biofilm cells was determined by analyzing images obtained with optical microscopy and scanned electron microscopy (SEM). Antibiotic susceptibility was evaluated by quantifying CFU before and after antibiotic treatment (Ofloxacin, ampicillin and tobramycin were tested.). An inverse correlation between cell adhesion and substrate stiffness was observed for both species. Interestingly, it was found that the cells attached to relatively stiff substrates (3 MPa) were significantly smaller than those on relatively soft substrates (0.3 MPa) and the growth rate of attached cells on relatively stiff substrates (3 MPa) is much slower than that on relatively soft substrates (0.3 MPa). In addition to the difference in size, the cells on 3 MPa substrates were also found to be less susceptible to antibiotics than those on 0.3 MPa substrates, regardless of the types of antibiotics. These results reveal that surface stiffness is an important material property that influences the attachment, growth, and stress tolerance of biofilm cells.