Rapid nanoparticle-mediated monitoring of bacterial metabolic activity and assessment of antimicrobial susceptibility in blood with magnetic relaxation

Abstract

Considering the increased incidence of bacterial infections and the emergence of multidrug resistant bacteria at the global level, we designed superparamagnetic iron oxide nanoparticles as nanosensors for the assessment of antimicrobial susceptibility through magnetic relaxation. In this report, we demonstrate that iron oxide nanosensors, either dextran-coated supplemented with Con A or silica-coated conjugated directly to Con A, can be used for the fast (1) quantification of polysaccharides, (2) assessment of metabolic activity and (3) determination of antimicrobial susceptibility in blood. The use of these polysaccharide nanosensors in the determination of antimicrobial susceptibility in the clinic or the field, and the utilization of these nanoprobes in pharmaceutical R&D are anticipated.

Figure 1. Proposed model for the assessment of antimicrobial susceptibility using dextran-coated polysaccharide nanosensors and Concanavalin A (ConA).

In this competition assay, the dextran on the surface of the iron oxide nanoparticles and the starch in solution compete for binding to Con A. This results in changes in the degree of Con-A induced magnetic nanoparticle clustering upon bacterial metabolic uptake of starch.

Figure 2. Nanoparticle-mediated sensing of polysaccharide levels and monitoring of bacterial metabolic activity.

(A) Quantification of starch in sterile MH broth, and (B) determination of starch consumption due to bacterial metabolism in MH broth, using the dextran-coated polysaccharide nanosensors.

Figure 3. Antimicrobial susceptibility screening in MH broth with dextran-coated polysaccharide nanosensors.

(A) Determination of E. coli's minimum inhibitory concentration in MH broth, using the changes in spin-spin relaxation times (ΔT2, upper panel) (Means±SE; p<0.05) and the turbidity method (lower panel). (B) Identification of S. marcescens' resistance to ampicillin via ΔΤ2 (upper panel, Means±SE) and the turbidity assay (lower panel). The corresponding amount of ampicillin is indicated in the graphs and pictures of the bacterial cultures.

Figure 4. Dextran-coated polysaccharide nanosensor-mediated determination antimicrobial susceptibility in blood.

(A) Assessment of E. coli's ampicillin MIC, and (B) identification of S. marcescens' ampicillin resistance in blood (Means±SE; p<0.05).

Figure 5. Schematic representation of the assessment of antimicrobial susceptibility using Concanavalin A-conjugated polysaccharide nanosensors.

The use of silica-coated nanoparticles and the direct conjugation of Con A to the capping matrix result in a non-competition-based assay format, which may potentially provide faster readout times.

Figure 6. Kinetic profiles of the competition and non-competition assay formats.

Point A represents the end-point of the competition assay utilizing the dextran-coated polysaccharide nanosensors, whereas Point B corresponds to the end-point of the non-competition assay based on the Con A-conjugated polysaccharide nanosensors.

Figure 7. Antimicrobial susceptibility in blood using Con A-conjugated polysaccharide nanosensors.

(A) Determination of E. coli's ampicillin MIC in blood, and (B) determination of Serratia marcescens' drug resistance in blood with the Con A-conjugated polysaccharide nanosensors, five minutes after addition of the bacterial aliquot into the nanoparticle solution (Means±SE; p<0.05).