Modeling Bacterial Attachment Mechanisms on Superhydrophobic and Superhydrophilic Substrates.

Superhydrophilic and superhydrophobic substrates are widely known to inhibit the attachment of a variety of motile and/or nonmotile bacteria. However, the thermodynamics of attachment are complex. Surface energy measurements alone do not address the complexities of colloidal (i.e., bacterial) dispersions but do affirm that polar (acid-base) interactions (ΔGAB) are often more significant than nonpolar (Lifshitz-van der Waals) interactions (ΔGLW). Classical DLVO theory alone also fails to address all colloidal interactions present in bacterial dispersions such as ΔGAB and Born repulsion (ΔGBorn) yet accounts for the significant electrostatic double layer repulsion (ΔGEL). We purpose to model both motile (e.g., P. aeruginosa and E. coli) and nonmotile (e.g., S. aureus and S. epidermidis) bacterial attachment to both superhydrophilic and superhydrophobic substrates via surface energies and extended DLVO theory corrected for bacterial geometries. We used extended DLVO theory and surface energy analyses to characterize the following Gibbs interaction energies for the bacteria with superhydrophobic and superhydrophilic substrates: ΔGLW, ΔGAB, ΔGEL, and ΔGBorn. The combination of the aforementioned interactions yields the total Gibbs interaction energy (ΔGtot) of each bacterium with each substrate. Analysis of the interaction energies with respect to the distance of approach yielded an equilibrium distance (deq) that seems to be independent of both bacterial species and substrate. Utilizing both deq and Gibbs interaction energies, substrates could be designed to inhibit bacterial attachment.

Quantifying interfacial substrate interactions via surface energy analyses.

Determination of a substrate's surface energy profile is a facile and inexpensive method to indicate the substrate's interfacial thermodynamics with another substance (e.g., microorganisms, biomacromolecules, medical devices, etc). The following protocol details a goniometric method to calculate a substrate's surface energy profile which (1) directly correlates to a substrate's interfacial Gibbs energy (ΔG) and (2) predicts the interfacial interactions with other substances. We also provide a calculation template using advanced mathematics to expedite surface energy profile determination. For complete details on the use and execution of this protocol, please refer to Cavitt et al. (2020).

Thermodynamic Surface Analyses to Inform Biofilm Resistance.

Biofilms are the habitat of 95% of bacteria successfully protecting bacteria from many antibiotics. However, inhibiting biofilm formation is difficult in that it is a complex system involving the physical and chemical interaction of both substrate and bacteria. Focusing on the substrate surface and potential interactions with bacteria, we examined both physical and chemical properties of substrates coated with a series of phenyl acrylate monomer derivatives. Atomic force microscopy (AFM) showed smooth surfaces often approximating surgical grade steel. Induced biofilm growth of five separate bacteria on copolymer samples comprising varying concentrations of phenyl acrylate monomer derivatives evidenced differing degrees of biofilm resistance via optical microscopy. Using goniometric surface analyses, the van Oss-Chaudhury-Good equation was solved linear algebraically to determine the surface energy profile of each polymerized phenyl acrylate monomer derivative, two bacteria, and collagen. Based on the microscopy and surface energy profiles, a thermodynamic explanation for biofilm resistance is posited.