Rapid detection of bacteria is essential due to the rise in antibiotic-resistant microbes, the global food trade, and their application in pharmaceutical, bioremediation, and food production. Optical detection techniques have piqued the curiosity of researchers due to their potential for fast, high-throughput, non-destructive, amplification-free identification.
Developments in Bacterial Detection Techniques
Several bacterial species are useful for enhancing safety and quality of life in medication, food and energy production; yet, some bacteria are dangerous.
Bacterial identification tests performed in the food, environmental and medical field must satisfy selectivity, sensitivity, cost, and speed standards. Recent years have seen extensive research into the development of bacterial testing, ranging from absorption, luminescence, or current response-based detections to the integration of spectroscopy or microscopy and deep learning.
While these advancements have numerous 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 various obstacles.
Culturing
Culturing is a popular method for identifying bacterial species based on their biological activity. However, results take at least one day because of the culture time.
Gram staining
Gram staining can be done 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 a microscope. It has issues related to intensity adjustment and limited fluorescence lifespan.
Lateral flow test
The lateral flow test uses antibody-conjugated gold nanoparticles (AuNP) to label target bacteria for naked-eye detection. The label does not fade, based on the AuNP's localized surface plasmon resonance. Stable inspection is easier with the lateral flow test than fluorescent labeling. However, due to the label's low optical intensity, the considerable antigen must be cultured to see the color.
Using Nanometer-Scaled Metal−Organic Hybrids to Detect Bacteria
Researchers used the optical properties of nanometer-scaled metal-organic hybrids to identify various bacteria in their study.
Metal nanoparticles (NPs) are valuable for optical detection and have strong affinities to biological components. Darkfield microscopy (DFM) was utilized to investigate scattering light induced by target substances due to its ability to observe metal nanoparticles smaller than the theoretical resolution limit.
A reaction system that autonomously controls nanostructure production was developed using aniline and metal ions to produce organic metal NHs.
E. coli O157, E. coli O26, and S. aureus were added to the mixture to generate an assessment solution with 13% bacteria density of the total cells. The capacity of NHs to mark individual cells was investigated using sample suspensions isolated from rotting chicken mince.
A dark field microscope and a field emission scanning electron microscope helped examine the mixture of the antibody-conjugated NH dispersion and bacterial solution. Images from a dark field microscope were captured using an optical microscope equipped with a halogen lamp, darkfield condenser, and a charge-coupled device camera.
The light-scattering spectra were recorded with a small grating spectrometer connected via an optical fiber to the dark field microscope. Focusing on the NH labels' light-scattering features helped identify the bacterial species.
Important Findings of the Study
Organic metal NHs are an excellent identification tool, facilitating quantitative and qualitative investigations of bacterial species in the same reaction area.
The optical properties of the nanohybrid structures (NHs) depend heavily on the individual metal elements of nanoparticles.
The rate of false negatives was estimated to be around 6%, while false positives were not confirmed.
Integrating antibodies into NHs leads to the binding of antigens to the cells, allowing bacteria to be identified by light scattering. Multiple bacterial species deposited on a slide were recognized within one field of view of a dark field microscope using scattered light colors.
Future Developments
There are currently no rapid techniques for detecting several bacterial species in a small number of samples. However, the proposed approach will allow for the simultaneous identification of many bacterial species in a single reaction area, which is not currently attainable with current technology.
The advancement of bacterial testing methods will improve quality and safety in various sectors of our life, 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 Multiple Bacterial Species Using Nanometer-Scaled Metal–Organic Hybrids. Analytical Chemistry. https://pubs.acs.org/doi/10.1021/acs.analchem.2c01188
Source: AZO Optics.