Analysis Of Pathogenic Bacteria Based On Magnetic Relaxation Sensing Of Iron Oxide Nanoparticles And MRI Imaging Of Self-Assembly Gadolinium Contrast Agent

Infectious diseases caused by pathogenic bacteria are serious threats to human health.Due to the interference of background signals from complex biological samples,Colorimetric,fluorescent,and scattered light methods exhibit deficiencies in the rapid analysis of pathogenic bacteria in complex biological samples?such as whole blood,living tissue,etc.?.Therefore,the development of rapid and accurate method for detecting pathogenic bacteria in complex samples is of great significance for diagnosis and treatment of infectious diseases.The unique magnetic separation and magnetic resonance properties of superparamagnetic iron oxide nanomaterials make it widely used in the enrichment and magnetic relaxation sensing analysis of small molecules,proteins,nucleic acids,cells,etc.Due to its high magnetic relaxation efficiency,paramagnetic material of gadolinium chelate is often used as a magnetic resonance imaging?MRI?contrast agent for the imaging and diagnosis of tumors,cardiovascular diseases,etc.In this thesis,a magnetic relaxation sensor for the detectin of pathogenic bacteria?using Staphylococcus aureus?S.aureus?as model pathogen?in complex samples was constructed.The sensor is based on the principle that the T₂ relaxation time of water proton is related to the concentration change of magnetic iron oxide nanoparticles after they are first bond with target bacteria and then magnetic separation.Besides,a targeted and self-assembled MRI contrast agent based on antibiotics,functionalized peptides and gadolinium agent was fabricated and then used for imaging analysis of pathogenic bacteria in complex samples.The main contents are as follows:Part ?:Magnetic relaxation sensor was constructed based on iron oxide nanoparticles to detect S.aureus.T₂ signal probe(IgG-MB_30nm)was prepared by modified porcine IgG with iron oxide nanoparticle?with particle size of 30 nm?.Capture probe(Ab-MB_{200 nm})was prepared by modified peptidoglycan specific antibody with iron oxide nanoparticle?with particle size of 200 nm?.Both the signal probe(IgG-MB_30nm)and capture probe(Ab-MB_{200 nm})could specifically bind with S.aureus due to the dual recognition of IgG and antibody to S.aureus.After magnetic separation,the concentration of IgG-MB_30nm in the solution is decreased.Therefore,S.aureus were quantitatively detected based on the change in the T₂ value caused by the change in the concentration of IgG-MB_{30 nm} in the solution.The enhanced T₂ value was correlated with the concentration of S.aureus in the range of 10⁵-10⁷ cfu/mL with a detection limit of 10⁴ cfu/mL.And non-target bacteria such as Escherichia coli,Salmonella typhimurium,Bacillus subtilis does not interfere with the detection signal.The recovery rate of S.aureus in the whole blood complex sample is 94.3%-103.3%,which has a high recovery rate.At the same time,the sensor can be used for the detection of S.aureus in mouse abscess samples.Since the magnetic signal can ignore the scattering,absorption and autofluorescence background interference of the biological sample,it is expected to detect S.aureus in other complex biological samples based on the magnetic relaxation sensor,and provide technical support for the diagnosis of pathogenic bacteria infectious diseases.Part ?:Targeted and self-assembled MRI contrast agent was constructed based on antibiotics,functional peptides and gadolinium for imaging of S.aureus in vitro.Since vancomycin can specifically binds to the cell wall component D-alanyl-D-alanine of S.aureus,and the polypeptide?FFYEGK?can be induced by S.aureus to form a self-assembly on its surface by means of phenylalanine-phenylalanine?-?stacking.gadolinium contrast agent?Gd-FFYEGK-Van?prepared by Van and self-assembling polypeptide?FFYEGK?.Studies have shown that the relaxation rate of the prepared Gd-FFYEGK-Van contrast agent was 5.74 mM^-1·s^-1,which makes it excellent MRI imaging performance.At the same time,S.aureus can be targeted imaging by the contrast agent in vitro.Besides,the contrast agent can self-assemble to form aggregates on the surface of S.aureus.The contrast agent is expected to amplify the MRI detection signal of the Gram-positive infection site.Therefore,the contrast agent has potential clinical application value in imaging and diagnosis of deep tissues caused by bacterial infectious diseases.