Identifying Bacteria Using Optical Properties of Metal-Organic Hybrids at Nanometer Scale

A study recently published in Chemical analysis proposed a strategy for optical detection of several bacterial species based on the optical properties of the nanohybrid structure of polymer-coated metal nanoparticles.

Study: Simultaneous Optical Detection of Several Bacterial Species Using Nanometer-Scale Metal-Organic Hybrids. Image Credit: Yurchanka Siarhei/Shutterstock.com

Rapid detection of bacteria is critical because of the rise of antibiotic-resistant microbes, the global food trade, and their applications in pharmaceuticals, bioremediation, and food production. The optical detection technique has piqued the curiosity of researchers because of its potential for rapid, high-throughput, non-destructive, and amplification-free identification.

Development of Bacterial Detection Techniques

Several species of bacteria are useful for improving safety and quality of life in medicine, food production and energy; however, some bacteria are harmful.

Bacterial identification tests carried out in the food, environmental and medical fields must meet the standards of selectivity, sensitivity, cost and speed. The last few years have seen extensive research in the development of bacterial assays, from current absorption, luminescence, or response-based detection to the integration of spectroscopy or microscopy and deep learning.

While these advances have many advantages, they require adequate development time to replace conventional approaches.

Challenges of Conventional Bacterial Detection Techniques

Although conventional bacterial tests offer advantages, they also present a number of obstacles.

culture

Culture is a popular method for identifying bacterial species based on their biological activity. However, results take at least a day due to culture time.

gram stain

Gram stain can be performed more quickly than culture. It distinguishes between gram negative and gram positive bacteria under a microscope but cannot distinguish bacterial species.

Fluorescent labeling

Fluorescent labeling detects dye-conjugated antibody-labeled bacteria using flow cytometry or microscopy. It has problems related to the adjustment of the intensity and limited fluorescence lifespan.

Lateral flow test

Lateral flow assays use antibody-conjugated gold nanoparticles (AuNP) to label target bacteria for naked eye detection. Labels do not fade, based on AuNP local surface plasmon resonance. Stable inspection is easier with the lateral flow test than with fluorescent labeling. However, due to the low optical intensity of the label, sufficiently large antigens must be cultured to see the color.

Using Organic Hybrid Nanometer Scale Metal to Detect Bacteria

The researchers used the optical properties of nanometer-scale metal-organic hybrids to identify various bacteria in their research.

Metal nanoparticles (NPs) are valuable for optical detection and have strong affinity for biological components. Darkfield microscopy (DFM) is used to investigate the scattering of light caused by the target substance due to its ability to observe metal nanoparticles smaller than the theoretical resolution limit.

A reaction system that independently controls the production of nanostructures was developed using aniline and metal ions to produce organic metal NH3.

E. coli O157, E. coli O26, and S. aureus were added to the mixture to produce an assessment solution with a bacterial density of 13% of the total cells. The capacity of NH to characterize individual cells was investigated using a sample suspension isolated from rotting chicken meat.

Dark-field microscopy and field emission scanning electron microscopy assist in examining mixtures of antibody-conjugated NH dispersions and bacterial solutions. Images from a dark-field microscope were captured using an optical microscope equipped with a halogen lamp, a dark-field condenser, and a charge-coupled device camera.

The light scattering spectra were recorded with a small grating spectrometer connected via optical fiber to a dark field microscope. Focusing on the light scattering features of the NH label helps identify bacterial species.

Key Findings from the Study

Organic metal NH is an excellent identification tool, facilitating the quantitative and qualitative investigation of bacterial species in the same reaction area.

The optical properties of the nanohybrid structures (NHs) are highly dependent on the individual metallic elements of the nanoparticles.

The false negative rate is estimated to be around 6%, while false positives are not confirmed.

Integrating the antibody into the NH causes the binding of the antigen to the cell, allowing the bacteria to be identified by light scattering. Several bacterial species deposited on slides were recognized in one field of view of a dark-field microscope using scattered light colors.

Future Development

There are currently no rapid techniques for detecting multiple bacterial species in a small number of samples. However, the proposed approach would allow the simultaneous identification of many bacterial species within a single reaction area, which is currently not achievable with current technology.

Advances in bacterial testing methods will improve quality and safety in many sectors of our lives, including food production, medicine and energy harvesting.

Reference

Tanabe, S., Itagaki, S., Matsui, K., Nishii, S., Yamamoto, Y., Sadanaga, Y., & Shiigi, H. (2022). Simultaneous Optical Detection of Several Bacterial Species Using Metal-Organic Hybrids on a Nanometer Scale. Chemical analysis. https://pubs.acs.org/doi/10.1021/acs.analchem.2c01188

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