Stiffness and Oligomer Content Affect the Initial Adhesion of Staphylococcus aureus to Polydimethylsiloxane Gels

The growing prevalence of methicillin-resistant Staphylococcus aureus (S. aureus) infections necessitates a greater understanding of their initial adhesion to medically relevant surfaces. In this study, the influence of the mechanical properties and oligomer content of polydimethylsiloxane (PDMS) gels on the initial attachment of Gram-positive S. aureus was explored. Small-amplitude oscillatory shear rheological measurements were conducted to verify that by altering the base to curing (B:C) ratio of the commonly used Sylgard 184 silicone elastomer kit (B:C ratios of 60:1, 40:1, 10:1, and 5:1), PDMS gels could be synthesized with Young's moduli across four distinct regimes: ultrasoft (15 kPa), soft (30 kPa), standard (400 kPa), and stiff (1500 kPa). These as-prepared gels (unextracted) were compared to gels prepared from the same B/C ratios that underwent Soxhlet extraction to remove any unreacted oligomers. While the Young's moduli of unextracted and extracted PDMS gels prepared from the same B:C ratio were statistically equivalent, the associated adhesion failure energy statistically decreased for the ultrasoft gels after extraction (from 25 to 8 J/mm²). The interactions of these eight well-characterized gels with bacteria were tested by using S. aureus SH1000, a commonly studied laboratory strain, as well as S. aureus ATCC 12600, which was isolated from a human lung infection. Increased S. aureus inactivation occurred only when the bacteria were incubated directly on top of the unextracted gels prepared at high B:C ratios (40:1 and 60:1), whereas none of the extracted gels (no unreacted oligomers) had significant levels of inactivated bacteria. S. aureus adhered the least to the stiffest extracted PDMS gels (no unreacted oligomers) and the most to soft, unextracted PDMS gels (with ∼17% unreacted oligomers). These findings suggest that both unreacted oligomers and Young's moduli are important material factors to consider when exploring the attachment behavior of Gram-positive S. aureus to hydrophobic elastomer gels.