The misuse and overuse of antimicrobials have driven the development of drug-resistant pathogens. Antimicrobial resistance is one of the top global public health and development threats and is estimated to have been directly responsible for 1.27 million deaths in 2019.

This growing crisis highlights the need for new approaches that can both treat infections and reduce the likelihood of resistance developing. One promising avenue is the use of metal nanoparticles due to their ability to target a broad range of bacterial species and lower potential for resistance. However, their clinical application is limited due to concerns surrounding solubility and stability.

A recent study published in the Open Access journal Antibiotics demonstrates copper-loaded starch nanoparticles that exhibit potent antibacterial activity against numerous bacterial strains. Equipped with a positive charge, the copper nanoparticles associate with negatively charged bacterial cells, while the starch-based matrix enables rapid nanoparticle release in the presence of amylase, an enzyme produced by certain species of bacteria.

Overall, the study demonstrates a targeted nanoparticle-based therapy that responds to bacterial metabolism. As lead author Dr. VanEpps explains:

“Copper-loaded starch nanoparticles act like Trojan horses. Some bacteria break down the starch, releasing antibacterial copper molecules.”

The rise of antimicrobial resistance

The causes of antimicrobial resistance are complex. Antibiotics don’t just act on the bacteria causing an infection, they also affect harmless gut microbes, as well as bacteria in wastewater and on hospital surfaces. When exposed to these drugs, bacteria are placed under pressure to survive, which drives them to evolve resistance.

To make the situation more complex, bacteria can perform horizontal gene transfer which allows them to share genetic material. They can do this without the need for reproduction and it can occur across different species.

The situation gets concerning when resistance genes spread to a human-associated strain of bacteria. These strains can then withstand treatments that would normally be effective, making infections harder to treat and allowing them to spread more easily through patients and hospitals.

There has been promise demonstrated using targeted antimicrobials. These are therapies that can directly target a specific strain of bacteria and deliver the drug, reducing collateral damage.

Design of the targeted antimicrobial

Targeted antimicrobial approaches can be categorised as passive and active:

• Passive – Therapeutics target non-specific properties of the environment, such as particle size or charge, to improve how treatments accumulate at infection sites.
• Active – Reagents are designed to specifically recognise a specific bacterium, improving efficacy of the drug.

In this study, the authors used a passive targeting approach. Most bacteria have a negative surface charge so they envisioned that equipping the nanoparticle with a positive charge should allow association through electrostatic interactions.

A further responsive element was also added. Coating them in starch improves the stability and solubility of metal nanoparticles. Starch-copper nanoparticles are responsive to the enzyme amylase, which breaks down starch and triggers the release of the antimicrobial copper nanoparticles.

Some bacteria, such as Bacillus subtilis, naturally produce amylase to break starch down. Although not usually a human pathogen, the researchers hypothesised that B. subtilis could act as a bystander amylase-producing species allowing for the release of copper nanoparticles, as shown in Figure 1.

Figure 1: Hypothesised mechanism of the targeted antimicrobial taken from the article. Amylase production causes starch degradation which releases antimicrobial copper nanoparticles.

Overall, the system consists of three key components:

• Copper nanoparticles – The antimicrobial agent.
• Starch – To stabilise copper nanoparticles and enable enzyme-triggered release.
• Positive charge – Promotes passive targeting through interaction with negatively charged bacterial cells.

A smarter way to kill bacteria

The authors synthesised the starch–copper nanoparticles and confirmed their size (~400 nm), positive charge (zeta potential of +22.7 ± 1.3 mV), and copper loading (0.34 ± 0.05 wt%). The nanoparticles were then tested against multiple bacterial strains.

They showed a limited efficiency against Gram-negative bacteria, likely because of its cell wall composition providing stronger resilience. However, strong antimicrobial activity was observed against Gram-positive bacteria, which includes common wound pathogens S. aureus and S. epidermidis. The starch–copper nanoparticles were effective at concentrations up to ten times lower than other copper–starch composites.

To understand this increased potency, S. aureus was incubated with the nanoparticles and imaged. Strong adhesion between bacteria and nanoparticles was observed, likely driven by electrostatic attraction between the positively charged particles and the negatively charged bacterial surfaces.

Further studies explored whether there was increased antimicrobial activity versus the amylase-producing B. subtilis compared to S. aureus. There was minimal difference in glucose supplemented media: however, in the absence of free glucose B. subtilis growth was more strongly inhibited. These results suggest that amylase production by B. subtilis is upregulated in glucose-limited environments, as the bacteria’s need to degrade more starch becomes greater.

To further explore the effects of amylase production, S. aureus was grown alongside increasing concentrations of B. subtilis; two effects emerged:

• Greater nanoparticle release due to increased starch degradation.
• Synergistic increase in antimicrobial activity.

Together, these results indicate that copper release, and therefore effectiveness, are highly dependent on the presence of amylase.

Early progress in the search for targeted antimicrobials

The work reports the development of a dual-action targeted antimicrobial therapy using amylase-responsive starch–copper nanoparticles. The system combines passive targeting through electrostatic attraction with a biologically triggered release mechanism, enabling high doses of copper delivery directly at bacterial sites.

While these results are promising, this work is still at an early stage. These experiments were carried out in media in a laboratory. It remains unseen how this therapy would perform in real infections. In the body, bacteria often exist within biofilms and are influenced by additional physiological factors that can reduce the effectiveness of antimicrobial treatments. As a result, further studies are needed to determine how this system performs in more clinically relevant conditions.

The system was also less effective against Gram-negative bacteria, responsible for diseases including cholera and salmonella and remain a major clinical challenge globally due to their high resistance to antibiotics. Further, factors such as long-term stability and toxicity from copper release still need to be explored.

As experimental models become more complex, the understanding of these targeted antimicrobials will continue to improve. This will be essential for determining whether this approach can be translated into clinical treatments.

More studies on antimicrobials and targeted therapeutics can be found across the Open Access journals Pharmaceuticals and Antibiotics. Alternatively, you can access the full MDPI journal list here.