(402d) Variations In Molecular Interaction Forces Measured Between Virulent And Avirulent Listeria Monocytogenes Strains And A Model Surface Of Silicon Nitride

Listeria monocytogenes is perhaps the most significant food-borne pathogen present in soil. Pathogenic L. monocytogenes can cause fatal listeriosis, and abortion of pregnant women. Whilst many strains of L. monocytogenes have pathogenic potential and can result in disease and mortality, others have limited capability of establishing infections and relatively avirulent. The pathogenicity of the strain is largely dependent on the composition of the surface biopolymers of the bacterial mutant. Although very important, the question of how the composition of the bacterial surface and the properties of bacteria vary between strains that have different level of virulence at the molecular level needs to be answered. To answer this question, interaction forces between five different L. monocytogenes mutants (EGDe, ATCC 51776, ATCC 19112, ATCC 19113, ATCC 19114, and ATCC 15313) that vary in their virulence level and a model surface of silicon nitride were investigated using atomic force microscopy (AFM). Silicon nitride was chosen as our model surface since it has a similar zeta potential to soil. Adhesion between a pathogenic Listeria mutant (ATCC 51776) and silicon nitride were measured to be 0.7 ± 0.4 nN (n =1517). In all adhesion events captured, 20% of the events were registered on proteins while 80% of the events were captured on polysaccharides. We hypothesized that the larger probability of pulling polysaccharides as compared to protein from the bacterial surface by the AFM tip was mainly due to a larger concentration of polysaccharides on the bacterial surface. To test this hypothesis, colorimetric assays of proteins and carbohydrates concentrations on the bacterial surfaces were performed. The results showed that the protein concentration in mg/cfu was 4.2 ± 0.2 times larger than that of polysaccharides concentration in mg/cfu. Therefore, and since these results did not confirm our hypothesis, we attributed the higher probability of pulling polysaccharides compared to proteins to a difference in the conformation of the molecules on the surface. Testing this hypothesis is currently ongoing. In addition to adhesion measurements, the elasticity and molecular surface characteristics of L. monocytogenes' mutants were investigated via atomic force microscopy (AFM). By applying the Hertz model to force?indentation data, we determined that the Young's modulus of the pathogenic ATCC 51776 strain was 0.67 MPa in water. The thickness of and the grafting density of the biopolymer brush layer on the bacterial surface were quantified using a steric model to be 260 nm and 1.6 × 10¹⁵ per m², respectively, in water. Experiments on intermediate virulence mutants and avirulent mutants are currently ongoing. Successful completion of these experiments will improve our understanding of the main molecular differences between virulent and avirulent strains of L. monocytogenes. Such findings would be very important; because it will allow reducing unnecessary recalls of food products.