Molecular machines that kill infectious bacteria have been engineered to function with a more clinically useful light power source. The latest iteration of these synthetic nanoscale drills, or molecular machines (MM), developed by scientists at Rice University, are activated by visible light rather than by ultraviolet (UV), as in earlier versions. Tests on burn-related bacterial infections in live preclinical models confirmed that the new MMS can effectively kill bacteria.
Six variants of molecular machines were successfully tested by Rice chemist James Tour, PhD, and team. All of them were able to punch holes in the membranes of Gram-negative and Gram-positive bacteria in as little as two minutes. And as bacteria have no natural defenses against such mechanical invaders, it’s unlikely that they will develop resistance, the researchers believe, offering up a strategy that could be used to defeat bacteria that have become resistant to standard antibacterial treatments. “I tell students that when they are my age, antibiotic-resistant bacteria are going to make COVID-19 look like a walk in the park,” Tour said. “Antibiotics won’t be able to keep 10 million people a year from dying of bacterial infections. But this really stops them.”
As with earlier versions, the new molecular machines could also help to improve the effectiveness of antibacterial drugs. “Drilling through the microorganisms’ membranes allows otherwise ineffective drugs to enter cells and overcome the bug’s intrinsic or acquired resistance to antibiotics,” said co-researcher Ann Santos, PhD.
Tour, together with colleagues including Rice alumni and first author Santos and Dongdong Liu, PhD, reported on their development in Science Advances, in a paper titled, “Light-activated molecular machines are fast-acting broad-spectrum antibacterials that target the membrane,” in which they concluded that at therapeutic doses the synthetic MMs were capable of “vastly outperforming conventional antibiotics.” The team wrote in conclusion, “Visible light–activated MMs represent a new antibacterial mode of action by mechanical disruption at the molecular scale, not existent in nature and to which resistance development is unlikely.”
Antimicrobial resistance (AMR) represents one of the greatest challenges that humans face, the authors wrote. “AMR is currently responsible for 700,000 deaths/year. By 2050, 10 million lives/year worldwide will be at risk from drug-resistant infections.” The problem is becoming increasingly urgent as drug-resistant bacteria continue to thwart existing antibiotics, while the development of new antimicrobial agents “has nearly stagnated,” the team continued. “No new class of antibiotics against Gram-negative bacteria has been approved since the late 1980s, and only one in four antibiotics under clinical development is a novel drug class or acts via a new mechanism of action.” And with most antibiotics under development potentially susceptible to the same resistance mechanisms that are rendering existing drugs ineffective, there is “an urgent need” to develop safe and effective new antimicrobials that can help to prevent the development of resistance, while preserving the viability of existing antibiotics.
The schematics show two variants of light-activated molecular machines developed at Rice University that drill into and destroy antibiotic-resistant bacteria. The machines could be useful to fight infectious skin diseases. [Tour Research Group/Rice University]Synthetic molecular motors, or molecular machines, are molecular structures that can rotate in one direction in response to stimuli, resulting in a mechanical action, the authors explained. “Among the stimuli that can activate MMs, light is particularly appealing because of its nonchemical and noninvasive nature and ease of control,” they noted. When irradiated by the right wavelength the molecule rotates in one direction, resulting in a fast drilling-like motion that can propel it through a lipid bilayer.
But while MMs have shown promise for applications ranging from drug delivery to chemo- or antimicrobial therapy, the ultraviolet (UV) radiation needed to activate them has limited their clinical utility, because extended exposure to UV can be damaging to humans.
The Rice lab has been refining its MM technology for years. The machines are based on Nobel Prize-winning work by Bernard Feringa, PhD, who developed the first molecule with a rotor in 1999 and got the rotor to spin reliably in one direction. Tour and his team introduced their advanced drills in a 2017 Nature paper.
The new version gets its energy from visible, but still-blueish light, at 405 nanometers, spinning the molecules’ rotors at two to three million times per second. The team achieved visible light activation by adding a nitrogen group. “The molecules were further modified with different amines in either the stator (stationary) or the rotor portion of the molecule to promote the association between the protonated amines of the machines and the negatively charged bacterial membrane,” said Liu, who is now a scientist at Arcus Biosciences.
The Rice lab’s first tests with the new molecules on burn wound infection models confirmed their ability to quickly kill bacteria, including methicillin-resistant Staphylococcus aureus, a common cause of skin and soft tissue infections that was responsible for more than 100,000 deaths in 2019. “… at therapeutic levels, MMs mitigated mortality associated with infection by different bacterial strains (Acinetobacter baumannii and S. aureus) in a burn wound infection model,” the scientists stated.
A transmission electron microscope image shows Escherichia coli bacteria in various stages of degradation after exposure to light-activated molecular drills developed at Rice University. The machines are able to drill into the membranes of antibiotic-resistant bacteria, killing them in minutes. [Image by Matthew Meyer/Rice University]The researchers also found that the new MMs will effectively break up biofilms and persister cells that become dormant to avoid antibacterial drugs. “Even if an antibiotic kills most of a colony, there are often a few persister cells that for some reason don’t die,” Tour said. “But that doesn’t matter to the drills.”
The authors further stated: “Persister cells are defined as transiently antibiotic-tolerant fractions of bacterial populations that are metabolically inactive or dormant. MMs were also able to significantly reduce the cell number and biomass of established biofilms of P. aeruginosa and S. aureus.”
It’s been suggested by other researchers that light at the wavelength used for the new MMs has mild antibacterial properties of its own, but the addition of molecular machines supercharges it, said Tour, who suggested bacterial infections like those suffered by burn victims and people with gangrene will be early targets.
As with earlier versions, the new machines might also be used to revive antibacterial drugs that are otherwise considered ineffective. “…by permeabilizing the membrane, MMs at sublethal doses potentiate the action of conventional antibiotics,” the Rice team pointed out. “Drilling through the microorganisms’ membranes allows otherwise ineffective drugs to enter cells and overcome the bug’s intrinsic or acquired resistance to antibiotics,” said Santos, who’s on the third year of the postdoctoral global fellowship that brought her to Rice for two years and is continuing at the Health Research Institute of the Balearic Islands in Palma, Spain.
The lab is working toward better targeting of bacteria to minimize damage to mammalian cells by linking bacteria-specific peptide tags to the drills to direct them toward pathogens of interest. “But even without that, the peptide can be applied to a site of bacterial concentration, like in a burn wound area,” Santos said.
Summarizing their reported studies, the authors wrote, “Together, these results indicate that, under the experimental conditions examined, MM-induced antibacterial effects can be attributed to the rapid drilling-like unidirectional rotation of MMs following light activation, whereby the rotor portion of the molecule spins around the central olefinic bond, propelling the molecule through the membrane. Subsequent leakage of cell contents and loss of membrane potential eventually culminate in bacterial cell death.”
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