Dr. Boisselier, one of the experts we met as part of our Human Practice, informed us that gold nanorods might be best suited for our purposes because their shape absorbed the best infrared light compared to standard gold nanoparticles. Nanorods can also be synthesized biologically, although by less organisms [1]. For simplicity’s sake, gold nanoparticles will be used as the placeholder term throughout this page and our whole wiki. Gold nanoparticle size should lean toward the smaller side, as their toxicity is more strongly correlated with size than with quantity. Smaller nanorods also demonstrate enhanced photothermal conversion and higher PTT efficiency. Literature suggests that particles in the 20–50 nm range achieve good cellular uptake, with uptake peaking around 50 nm. In vivo, clearance is most efficient for very small nanorods (~7 nm), compared to larger ones (>14 nm). Ultimately, the balance between therapeutic efficiency and toxicity would need to be experimentally validated before selecting the optimal nanorod size for our complex [2].

Aerobactin production

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We attempted to genetically engineer E. coli to produce aerobactin. However, this endeavour wasn’t very successful due to multiple cloning issues. For more details, check out our Engineering success page.

Assembly of gold-nanoparticles and aerobactin

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Once synthesized, aerobactin and our gold nanoparticles would be assembled using chemical processes. To ensure stability and strong binding between gold nanoparticles and aerobactin, we draw inspiration from Ha and al. (2025) [3] who used N-heterocyclic carbene (NHC) gold-nanoparticles complexes. These complexes possess strong covalent bonds that surpass the strength of typical noncovalent bonds used to stabilize other nanoparticle types. In addition, the NHC ligands prevent particle aggregation because of its steric hindrance. This makes them very stable under a variety of conditions, including in biological systems [3]. For research purposes, we plan to add a fluorescent tag (such as cyanine 7) to our complex so we could track its entry in K. pneumoniae.

Storing methods

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Au-NSC were stored at 4 °C for thirty days and the solution retained its original color. Its absorption peak also remained unchanged. Further investigations involving exposure to light from a fluorescent lamp and an 808 nm laser with recording of the UV–vis spectra showed that the characteristic absorbance peak at 520 nm for AuNP_NSC was preserved, indicating their great stability [3].

Modes of administration

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We consider three primary routes of administration for our complexes: topical, intravenous and in situ of deep tissue infections. Topical delivery would be appropriate for superficial skin infections, such as diabetic foot ulcers. Several vehicle strategies are possible, but semisolid formulations, such as gels, creams and ointments tend to be the preferred vehicle because they deliver the drug over an extended period of time. The choice between them would depend on the infection site, drug concentration and solubility as well as ease of application, in line with dermatological best practices [4]. For infections involving internal organs or deep tissues, intravenous administration would be the preferred route, as it enables rapid systemic delivery while avoiding hepatic first-pass metabolism. In situ administration by injection of the complex directly in the deep tissue site of infection is also considered in cases with abscess. The common treatment of an abscess is to drain the pus out of the site to lessen the amount of infected fluid and to induce healing. It is common in this procedure to use a catheter to drain the pus from the infected site [5]. We aim to take advantage of the already-open access to the infection site to inject directly in the infected tissue the complex. That way, the complex can directly diffuse into the infected tissue and, we hope, lessen the time between administration of the complex and irradiation. The catheter could also be used as an entry point for an optical fiber for irradiation of the infection site. Of course, these parameters will need to be confirmed during clinical trials.

Penetration into Infectious Sites and Wait Time Before Irradiation

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In the original study underlying this year’s project [3], skin wounds treated with the siderophore–gold nanoparticle complex were irradiated with NIR light 30 minutes after administration. Based on this, we propose that treatments of skin wounds follow a similar timeline from administration to irradiation. However, experiments in mice using our completed siderophore–gold nanoparticle complex will be necessary to determine the optimal time for penetration into bacteria and effective treatment.

For deep tissue infections, whether in organs or muscles, the time required for the siderophore–gold nanoparticle complex to reach the infection site can vary widely depending on factors such as tissue location, vascularization, and inflammation. Literature reports that smaller nanoparticles penetrate tissues more rapidly [6]. Therefore, the interval between intravenous administration and NIR irradiation generally needs to be longer for infections located deeper from the circulatory system. Patient-specific factors, including inflammation levels and vascular quality, may also influence penetration. Our siderophore–gold nanoparticle complex is relatively small, approximately 1–2 nanometers, which is expected to allow efficient diffusion and rapid tissue penetration. Nonetheless, mice trials will be necessary to confirm these expectations and to guide treatment strategies for infections at different sites.

Irradiation time and Methods

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Near-infrared (NIR) light is one of the most promising sources of excitation energy for gold nanoparticles because of its relatively deep penetration into biological tissues. In particular, the “second biological window” (NIR-II, ~1000–1700 nm) is of growing interest due to reduced scattering and lower absorption by endogenous chromophores, which allows greater treatment depth and minimizes superficial heating and burns [7].

The excitation efficiency of gold nanoparticles depends strongly on the structure of the siderophore–nanoparticle complex. Different configurations alter the plasmon resonance spectrum, meaning that the optimal excitation wavelength can shift depending on the complex’s geometry and local environment. Ideally, complexes would be designed to exhibit peak absorption within the NIR-II range (1000–1700 nm) to maximize penetration and therapeutic efficiency.

However, not all effective systems require such long wavelengths. For example, Ha et al. (2025) [3] successfully used an 808 nm laser—within the first biological window (NIR-I)—to activate siderophore-conjugated gold nanoparticles, achieving strong bactericidal activity while minimizing collateral damage to surrounding tissue. This suggests that shorter NIR wavelengths may still be appropriate in certain contexts.

Ultimately, the choice of wavelength and irradiation parameters must be determined experimentally, balancing penetration depth, nanoparticle resonance characteristics, and safety considerations to identify the conditions that maximize therapeutic efficacy while minimizing harm.

Another important consideration is the laser-induced temperature at the target site. Thermal effects on cells and tissue depend strongly on both temperature and exposure time, and published thresholds vary by tissue type and experimental conditions. Reviews of thermal damage summarize commonly used ranges: mild hyperthermia (≈41–43 °C) can induce heat-shock responses and reversible protein denaturation; sustained exposures in the ≈42–46 °C range increasingly cause irreversible protein denaturation and cell death (necrosis or apoptosis) depending on exposure duration; higher temperatures (≈46–55 °C) are associated with rapid protein denaturation, microvascular damage and thrombosis that produce ischemia; and temperatures above ~60 °C typically cause almost instantaneous coagulative necrosis. These ranges are approximations — the precise boundary between reversible and irreversible injury is time-dependent and tissue-specific [8].

Although it might seem advantageous to use a higher temperature, the kinetics of thermal injury follow Arrhenius-type models. In practice, this means that tissue damage increases exponentially with temperature: even a small rise can dramatically shorten the exposure time needed to cause a given level of injury. Because of this exponential dependence, both peak temperature and exposure duration (the thermal dose) must be carefully controlled. Otherwise, higher temperatures can increase the risk of unintended injury to surrounding healthy tissue [9].

In addition to purely thermal mechanisms, photothermal irradiation can produce mechanical effects such as microcavitation or rapid thermal expansion that cause direct mechanical disruption of cells and extracellular matrix. These non-thermal, cavitation-driven effects have been shown to contribute to microbial and cellular damage in several photothermal and ultrasonic modalities and may increase the risk of collateral mechanical injury if not carefully controlled [10].

The physiological effects of photothermal therapy (PTT) are temperature-dependent in cancer models, and the same temperature dependence plausibly applies to bacteria — but there are important differences. Bacteria can respond to sublethal heat and oxidative stress by activating stress responses, upregulating heat-shock proteins, altering gene expression, and in some cases increasing mutation rates or adaptive phenotypes. Sublethal treatments can sometimes select for more thermotolerant or biofilm-forming cells, so achieving a bactericidal thermal dose (or combining PTT with another lethal modality such as photodynamic production of ROS) is important to avoid selecting survivors [11],[12].

Different infection sites require different irradiation strategies. For superficial infections, such as those affecting the skin, external near-infrared (NIR) sources (lamps or lasers) may be sufficient. For deeper infections, however, more invasive approaches have to be considered. Endoscopic or endovascular procedures could be employed to deliver an optical fiber directly to the infection site within hollow organs or other accessible body cavities. Once in place, the fiber can transmit NIR light precisely to the targeted area. Optical fibers remain the most practical delivery system because they allow high transmission efficiency, spatial precision, and compatibility with endoscopic procedures [13].

An alternative approach would be to modify the gold nanoparticles themselves into so-called “smart gold nanoparticles.” These particles are termed “smart” because they can activate autonomously in response to specific physiological triggers, such as acidic pH. In oncology, smart nanoparticles are increasingly explored, and some have even been engineered to synthesize their own photothermal therapy (PTT) agents in situ, eliminating the need for external irradiation [14]. Nevertheless, this remains largely experimental. For infectious disease applications, it is not yet clear whether such nanoparticles would activate at a sufficient rate, accumulate selectively at infection sites, penetrate bacterial cells, or achieve robust bactericidal activity. At this stage, it may even be more straightforward to develop smart nanoparticles directly engineered to target antibiotic-resistant bacteria, with or without siderophore conjugation.

Regardless of the irradiation or nanoparticle strategy employed, combining these approaches with antibiotics could provide synergistic benefits and improve overall treatment outcomes.

Clinical applicability

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One of our human practices experts, infectiologist Dr. Dumaresq made us remark that superficial skin infections, such as diabetic foot ulcers, generally respond well to current antibiotics due to the high dosages used. He encouraged us to explore additional applications for our project such as sterilization of inert surfaces, treatment of deep tissue infections, sepsis, and infections related to medical implants (e.g., hip prostheses). These types of infections are often caused by Gram-positive bacteria like Staphylococcus aureus which the team did not focus on because siderophore entry is less well studied in Gram-positive than in Gram-negative bacteria. These suggestions remain valuable for guiding future research, and Dr. Dumaresq reassured us that K. pneumoniae is a highly relevant and concerning pathogen, making it a valid focus for our work.

Patient impact

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For patients, a potential disadvantage of implementing this project is the need for short-term hospitalization—ranging from one to several days—in more severe cases that require invasive surgery. Endovascular interventions can allow same-day discharge, but full recovery may take from a few days up to a month in more complex cases [15]. In contrast, procedures for skin infections and endoscopic treatments typically allow same-day discharge within a few hours, although mild discomfort may persist for several days [15],[16],[17].

The potential benefits include improved survival outcomes and a reduced risk of complications associated with severe bacterial infections. Additionally, minimizing or eliminating the use of antibiotics could help preserve the microbiome—a system with extensive whole-body interactions—whose disruption has been linked to a higher risk of developing a wide range of disorders, including Clostridioides difficile infection, metabolic disease, depression, and autoimmune diseases [18],[19],[20].