Identification of Pseudomonas aeruginosa exopolysaccharide Psl in biofilms using 3D OrbiSIMS.

Secondary ion mass spectrometry (SIMS) offers advantages over both liquid extraction mass spectrometry and matrix assisted laser desorption mass spectrometry in that it provides the direct in situ analysis of molecules and has the potential to preserve the 3D location of an analyte in a sample. Polysaccharides are recognized as challenging analytes in the mass spectrometry of liquids and are also difficult to identify and assign using SIMS. Psl is an exopolysaccharide produced by Pseudomonas aeruginosa, which plays a key role in biofilm formation and maturation. In this Letter, we describe the use of the OrbiTrap analyzer with SIMS (3D OrbiSIMS) for the label-free mass spectrometry of Psl, taking advantage of its high mass resolving power for accurate secondary ion assignment. We study a P. aeruginosa biofilm and compare it with purified Psl to enable the assignment of secondary ions specific to the Psl structure. This resulted in the identification of 17 peaks that could confidently be ascribed to Psl fragments within the biofilm matrix. The complementary approach of the following neutral loss sequences is also shown to identify multiple oligosaccharide fragments without the requirement of a biological reference sample.

The Role of Nanoscale Distribution of Fibronectin in the Adhesion of Staphylococcus aureus Studied by Protein Patterning and DNA-PAINT.

Staphylococcus aureus is a widespread and highly virulent pathogen that can cause superficial and invasive infections. Interactions between S. aureus surface receptors and the extracellular matrix protein fibronectin mediate the bacterial invasion of host cells and is implicated in the colonization of medical implant surfaces. In this study, we investigate the role of distribution of both fibronectin and cellular receptors on the adhesion of S. aureus to interfaces as a model for primary adhesion at tissue interfaces or biomaterials. We present fibronectin in patches of systematically varied size (100-1000 nm) in a background of protein and bacteria rejecting chemistry based on PLL-g-PEG and studied S. aureus adhesion under flow. We developed a single molecule imaging assay for localizing fibronectin binding receptors on the surface of S. aureus via the super-resolution DNA points accumulation for imaging in nanoscale topography (DNA-PAINT) technique. Our results indicate that S. aureus adhesion to fibronectin biointerfaces is regulated by the size of available ligand patterns, with an adhesion threshold of 300 nm and larger. DNA-PAINT was used to visualize fibronectin binding receptor organization in situ at ∼7 nm localization precision and with a surface density of 38-46 µm-2, revealing that the engagement of two or more receptors is required for strong S. aureus adhesion to fibronectin biointerfaces.

Development of a Label-Free LSPR-Apta Sensor for Staphylococcus aureus Detection.

The risk of foodborne diseases has increased over the last years. We have developed a simple, portable, and label-free optical sensor via aptamer recognition of Staphylococcus aureus at nanostructured plasmonic elements. The developed aptamers conjugated to a localized surface plasmon resonance (LSPR) sensing device were applied in both pure culture and artificially contaminated milk samples enabling a limit of detection of 103 CFU/mL for S. aureus in milk. There was no need for a pre-enrichment step, and the total analysis time decreased from 30 min to 120 s. Finite-difference time-domain was used to simulate the experimentally measured optical responses for a range of different sensor designs (100 and 200 nm disks), addressing the role of the near field and intrinsic refractive index sensitivity. A comparison of the aptamer to antibody-based recognition approaches showed that the thickness of the sensing layer was critical with a significantly larger response for the thinner aptamer layer. Comparison of differently sized metal nanostructures showed a significantly higher sensitivity for 200 nm diameter compared to 100 nm diameter disk structures resulting from both increases in bulk refractive index sensitivity and the extent to which the local field extends out from the metal surface. These findings confirmed that the developed gold nanodisk-based LSPR sensing chips could facilitate sensitive detection of S. aureus in food samples.