Rice University researchers have developed visible-light-activated molecular machines that kill antibiotic-resistant bacteria in minutes by targeting the bacteria’s membranes. This approach to treating drug-resistant infections, which uses antimicrobial nanomaterials rather than conventional antibiotics, could help save as many as 10 million lives per year by 2050, according to the Rice team. Synthetic molecular machines are molecular structures that rotate in a controlled manner in response to stimuli, resulting in a mechanical action — in this case, the breaching of the bacterial membrane. Bacteria have no natural defense against molecular machines and are unlikely to develop resistance to them, the researchers said. Among the stimuli that can activate molecular machines, light is appealing because of its nonchemical, noninvasive nature and its ease of control, the researchers added. The original version of the single-molecule nanomachines, introduced by the researchers in 2017, was activated by ultraviolet (UV) light. Because extended exposure to UV light can be harmful to humans, the researchers developed a version that is activated with light at the 405-nm wavelength. Blue light (400 to 490 nm) is believed to have antibacterial properties in its own right. According to professor James Tour, who led the research, the molecular machines strengthen these properties. 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. Courtesy of Matthew Meyer/Rice University. The researchers 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,” researcher Dongdong Liu said. When the molecular machines are activated, the molecules’ rotors spin unidirectionally at the rate of 2 to 3 million times per second. Their fast rotation propels individual molecules through the membrane, punching a hole in the membrane. Subsequent leakage of cell contents and loss of membrane potential eventually culminate in bacterial cell death. Six molecular machine variants were tested. At therapeutic doses, the synthetic molecular machines were activated by visible light to kill Gram-positive and Gram-negative bacteria, including methicillin-resistant staphylococcus aureus (MRSA). The bacteria were killed after as little as 2 min of light activation. In further tests, the researchers eliminated persister cells and established bacterial biofilms, which become dormant to evade the effects of 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.” In addition, the researchers found that sublethal treatment with molecular machines boosted the potency of conventional antibiotics, as a result of molecular machine-induced membrane permeabilization and enhanced antibiotic access to intracellular targets. Two variants of light-activated molecular machines developed at Rice University that drill into and destroy antibiotic-resistant bacteria. The machines could be used to fight infectious skin diseases. Courtesy of Tour Research Group/Rice University. The researchers tested the molecular machines on an in vivo model of burn wound infection. Burn wounds of G. mellonella were infected with two bacterial pathogens typically associated with burn wounds, A. baumannii and S. aureus. Treatment with visible-light-activated molecular machines mitigated the mortality associated with infection by both pathogens. The mortality mitigation demonstrated by the molecular machines — up to 83% — was similar or superior to that of conventional antibiotics. Bacterial infections like those suffered by burn victims and patients with gangrene are expected to be early targets for the light-activated, mechanically based therapy. The molecular machines developed by the Tour lab are based on the Nobel Prize-winning work of Bernard Feringa, who developed the first molecule with a rotor in 1999 and got the rotor to spin reliably in one direction. The researchers are working on ways to hone in on bacteria better with the molecular machines. They plan to link bacteria-specific peptide tags to the drills to direct them toward pathogens of interest. This will minimize damage to mammalian cells. The research was published in Science Advances (www.science.org/doi/10.1126/sciadv.abm2055).