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Engineering Photoluminescent Bacteria through Gold Nanoparticle Studs

Bacteria-mediated theranostics is a new paradigm in medical technology. In an article recently published in the journal ACS Applied Biomaterials, researchers synthesized gold (Au) nanoclusters on the Lactobacillus rhamnosus cell wall that rendered them photoluminescence properties. ​​​​​​​

Engineering Photoluminescent Bacteria through Gold Nanoparticle Studs​​​​​​​

​​​​​​​Study: Hierarchical Passage of Gold Nanoclusters in Living Bacteria. Image Credit: Kateryna Kon/Shutterstock.com

The team observed that the nanoclusters were passed down the generation. However, the new generations have lost luminescence with the agglomeration of the nanoclusters. The present work discusses the role of the bacterial cell wall in the aggregation of nanoclusters.

Bacteria-mediated Theranostics

An ideal vehicle in drug delivery either encapsulates cargo or may act as a therapeutic agent by itself. Additionally, the drug vehicles addressed the challenges of host immune response, targeting, and specificity. Genetically engineered bacteria and nanomaterial-based drugs are effective in killing tumors. The live bacteria loaded with nanoparticles are considered nonviral gene delivery systems. Near-infrared (NIR) radiation-induced gene expression was possible by employing photothermal metal nanoparticles.

Attenuated facultative anaerobic bacteria help in developing programmed delivery vehicles that can promote cytotoxicity at the tumor site. Listeria monocytogenes, Bifidobacterium, and Salmonella typhimurium are a few bacterial species used as self-propelling delivery systems. Moreover, gut bacteria have therapeutic importance as they influence the tumor microenvironment and affect the treatment outcomes.

Bacteria-mediated therapy allows the combination of the nanomaterials with molecular drugs for effective theranostics. A convenient way to develop such therapeutic agents is to synthesize nanoparticles on the bacterial cell wall and later load drug molecules into nanoparticles. However, peptidoglycan prevents the nanoparticles from bonding on the surface of the bacterial cell wall. Hence, there is a need for a robust strategy to synthesize nanoparticles on the bacterial cell wall and an understanding of their fate in new generations of bacteria.

Au nanoclusters on Lactobacillus rhamnosus 

In the present work, the authors synthesized Au nanoclusters on Lactobacillus rhamnosus (MTCC 1408) via chemical reduction. They observed the luminescent cluster formation on the bacterial cell walls while keeping the bacteria alive. Further, the clusters agglomerated into 100 to 200-nanometer-sized spherical structures without transforming into plasmonic Au nanoparticles on the bacterial cell wall. The team studied up to six subcultures thoroughly to understand the implication of nanoparticle-studded bacteria in the medical field.

Research Findings

Synthesis of Au nanoclusters on the cell wall of bacteria was observed under an ultraviolet (UV) transilluminator. The nanoclusters appeared as luminescent, orange-colored structures. As the bacteria divided, the color systematically decreased from the first subculture to subsequent cultures. Observing the bacterial cultures under UV-visible (vis) spectrum revealed the presence of a broad peak at 200 to 400 nanometers and the absence of a peak at 520 nanometers which is characteristic of Au nanoparticles.

The amino acid residues of bacterial proteins showed a peak at 420 nanometers and were persistent throughout the subcultures. The peak at 580 nanometers corresponding to Au nanoclusters disappeared in the first and succeeding subcultures of bacteria.

X-ray photoelectron spectroscopy (XPS) of Au nanocluster-studded parent cells (Lac_AuNC) revealed the presence of Au (0) peaks at 83.2 and 87.2 electronvolts, respectively. Confocal laser scanning microscopy (CLSM) of Lac_AuNC supported the cluster formation and revealed a characteristic orange emission under 405-nanometer lasers.

The authors observed that the individual bacterium appeared as luminescent orange structures, suggesting the attachment of Au nanoclusters to bacteria. The progeny bacteria showed luminescence for a minimum of 12 hours, which was lost in the first and subsequent subcultures.

Transmission electron microscopy (TEM) images of the product medium’s drop-cast revealed nanoparticle-studded bacteria with a particle size of 1.3±0.4 nanometers. Moreover, the Au nanoclusters spread over the bacterial body were revealed by employing elemental mapping performed on energy dispersive X-ray spectroscopy (EDS). 

Field emission scanning electron microscope (FESEM) studies on product medium revealed the presence of bacteria, and the Au nanoclusters on bacterial cell wall showed characteristic photoluminescence that was absent in control Lactobacillus rhamnosus.

Furthermore, atomic force microscopy (AFM) studies revealed higher roughness in Lac_AuNC than in the control bacteria, which suggests the presence of nanoclusters on the surface of the bacterial cell wall.

Conclusion

In conclusion, the authors developed a novel technique to generate photoluminescent Au nanoclusters on the living bacteria’s outer cell walls. They observed that the subsequent generations that received the nanoclusters passed as agglomerated non-luminescent Au-containing particles.

The results revealed that the cell wall’s peptidoglycans received by the next generation bacteria helped carry the nanoclusters. The cell wall elongation during bacterial division led to the formation of larger particles in new generations.

During the bacterial elongation and division, the luminescence of nanoclusters is lost without plasmonic Au nanoparticle formation. Through the present study, the authors demonstrated a new path to understanding the passing of nanoparticles to the next generation and explored their biomedical applications.

Reference

Debasmita, D., Ghosh S.S., Chattopadhyay, A. Hierarchical Passage of Gold Nanoclusters in Living Bacteria. ACS Applied Bio Materials (2022).https://pubs.acs.org/doi/10.1021/acsabm.2c00315https://pubs.acs.org/doi/10.1021/acsabm.2c00315

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Bhavna Kaveti

Written by

Bhavna Kaveti

Bhavna Kaveti is a science writer based in Hyderabad, India. She has a Masters in Pharmaceutical Chemistry from Vellore Institute of Technology, India, and a Ph.D. in Organic and Medicinal Chemistry from Universidad de Guanajuato, Mexico. Her research work involved designing and synthesizing heterocycle-based bioactive molecules, where she had exposure to both multistep and multicomponent synthesis. During her doctoral studies, she worked on synthesizing various linked and fused heterocycle-based peptidomimetic molecules that are anticipated to have a bioactive potential for further functionalization. While working on her thesis and research papers, she explored her passion for scientific writing and communications.

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