Modulating the Stress Response of E. coli at GaN Interfaces Using Surface Charge, Surface Chemistry, and Genetic Mutations.

The surface properties of inorganic materials can be used to modulate the response of microorganisms at the interface. We used the persistent photoconductivity properties of chemically treated gallium nitride substrates to evaluate the stress response of wild-type, ΔfliC, and ΔcsgG mutant E. coli exposed to charged surfaces. Substrate surface characterization and biological assays were used to correlate the physiological response to substrate surface charge. The physiological response was evaluated by measuring the intracellular levels of reactive oxygen species (ROS) and Ca2+ cations using fluorescent probes. We evaluated the response 1, 2, and 3 h after a short exposure to the surfaces to determine generational effects of the initial exposure on the physiology of the bacteria. In general, the ROS levels 1 h after exposure were not different. However, there were differences in Ca2+ levels in E. coli 1 h after the initial exposure to charged GaN surfaces, primarily in the wild-type E. coli. The differences in Ca2+ levels depended on the substrate surface chemistry and genetic mutation that suggests the involvement of multiple factors for modulating the interactions of bacteria at interfaces.

Oxidative Stress Transcriptional Responses of Escherichia coli at GaN Interfaces.

Microorganisms regulate their interactions with surfaces by altering the transcription of specific target genes in response to physicochemical surface cues. To assess the influence of surface charge and surface chemistry on the transcriptional oxidative stress response, we evaluated the expression of three genes, oxyS, katE, and sodB from the Gram-negative bacterium, Escherichia coli, after a short exposure to GaN interfaces. We observed that both surface charge and surface chemistry were the factors regulating the transcriptional response of the target genes, which indicates that reactive oxygen species (ROS) generation and the ROS response at the GaN interfaces were affected by changing surface properties. The changes in transcription did not correlate to the surface charge in all cases, indicating that there was an influence from multiple interfacial properties on the interactions. Alteration of the bacterial morphology also was a critical factor in these transcriptional responses to the surface cues. When compared to wild-type E. coli bacteria, bacteria missing either flagella or curli exhibited altered transcriptional profiles of the three oxidative stress genes when exposed to GaN materials. These results indicate that the bacterial flagella and curli modulated the oxidative stress response in different ways. The results of this work add to our understanding of the interactions of microbes at interfaces and will be useful for guiding the development of electronic biointerfaces.

Modification of the Surface Properties of Al x Ga1-x N Substrates with Gradient Aluminum Composition Using Wet Chemical Treatments.

The surface properties of biomolecular gradients are widely known to be important for controlling cell dynamics, but there is a lack of platforms for studying them in vitro using inorganic materials. The changes in various surface properties of an Al x Ga1-x N film (0.173 ≤ x ≤ 0.220) with gradient aluminum content were quantified to demonstrate the ability to modify interfacial characteristics. Four wet chemical treatments were used to modify the surface of the film: (i) oxide passivation by hydrogen peroxide, (ii) two-step functionalization with a carboxylic acid following hydrogen peroxide pretreatment, (iii) phosphoric acid etch, and (iv) in situ functionalization with a phosphonic acid in phosphoric acid. The characterization confirmed changes in the topography, nanostructures, and hydrophobicity after chemical treatment. Additionally, X-ray photoelectron spectroscopy was used to confirm that the chemical composition of the surfaces, in particular, Ga2O3 and Al2O3 content, was dependent on both the chemical treatment and the Al content of the gradient. Spectroscopic evaluation showed red shifts in strain-sensitive Raman peaks as the Al content gradually increased, but the same peaks blue-shifted after chemical treatment. Kelvin probe force microscopy measurements demonstrated that one can modify the surface charge using the chemical treatments. There were no predictable or controllable surface charge trends because of the spontaneous oxide-based nanostructured formations of the bulk material that varied based on treatment and were defect-dependent. The reported methodology and characterization can be utilized in future interfacial studies that rely on water-based wet chemical functionalization of inorganic materials.