Microbial Synthesis of Reduced Graphene Oxide Nanosheets by Proteus mirabilis MT378377 and Antibacterial Activity against Multidrug Resistant Bacteria

In the world of modern materials, few substances are as celebrated as graphene. Imagine a sheet of carbon atoms arranged in a perfect, single-layer honeycomb; it is incredibly strong, conducts electricity with ease, and is so thin that it is essentially two-dimensional. However, creating this pristine material often requires harsh chemicals, high temperatures, or expensive equipment that can be difficult to scale up for everyday use. Scientists have long sought a gentler way to make these carbon sheets, one that mimics the efficiency of nature. A related material, called graphene oxide, is easier to produce but carries extra oxygen atoms on its surface that make it less conductive and more water-soluble. To get the best of both worlds, researchers try to strip away these oxygen groups to create "reduced graphene oxide," a material that retains the useful properties of graphene while being easier to handle. The challenge lies in finding a method to remove these oxygen groups without resorting to toxic chemicals or extreme heat, leading scientists to look toward the microscopic world of bacteria for a solution.

A team of researchers at Lahore College for Women University in Pakistan decided to test whether a specific bacterium could perform this chemical transformation. They began by collecting soil samples and isolating various bacterial colonies, screening them to see which ones could naturally reduce graphene oxide. After testing several candidates, they identified a single strain that stood out. Through genetic analysis, they confirmed this bacterium was Proteus mirabilis, a common microbe found in the environment. They named their specific strain MT378377 and set out to understand exactly how it worked and under what conditions it performed best. The goal was to create a clean, eco-friendly process that could produce these carbon nanosheets in large quantities, potentially offering a greener alternative to traditional chemical manufacturing.

The researchers discovered that the bacterium acted as a living factory for these nanomaterials. When they mixed the bacteria with a solution of graphene oxide, the microbes began to strip away the oxygen groups, turning the mixture a deep black color, which signaled the formation of reduced graphene oxide. To find the perfect recipe for this biological process, the team adjusted various conditions, much like a chef tuning a dish. They found that the bacteria worked best at a warm temperature of 37 degrees Celsius, which is the standard body temperature for many organisms. They also discovered that the acidity of the water mattered; a slightly acidic environment with a pH of 5 allowed the bacteria to produce the most material. Adding a tiny amount of salt, specifically sodium chloride, helped the process along, as did adding a specific surfactant called Tween 80, which helps mix substances that usually don't blend well. The most effective concentration of the starting graphene oxide was one milligram per milliliter, and the bacteria worked best when kept in the dark rather than in the light.

Once the optimal conditions were established, the team measured how much material was produced and what it looked like. Under the right conditions, the bacteria generated a significant amount of reduced graphene oxide, which the researchers confirmed using light absorption tests. When they examined the final product under powerful microscopes, they saw that the bacteria had successfully created thin, sheet-like structures. These sheets were roughly 91 nanometers in size, a scale so small that a million of them would fit across the width of a human hair. The material also carried a negative electrical charge on its surface, which helps keep the sheets from clumping together and ensures they stay suspended in water. Chemical analysis showed that while the bacteria removed many of the oxygen groups from the original graphene oxide, some oxygen remained attached to the surface, giving the material unique chemical properties.

The researchers then tested whether these newly created nanosheets could fight off dangerous bacteria, specifically looking at multidrug-resistant strains of Escherichia coli, Staphylococcus aureus, and Salmonella typhi. They compared the performance of the original graphene oxide, the new reduced graphene oxide, and standard graphite. The results were surprising. The original graphene oxide proved to be a potent weapon, creating clear zones where bacteria could not grow, with the effect becoming stronger as the concentration increased. However, the reduced graphene oxide produced by the bacteria showed no ability to kill these resistant pathogens at the concentrations tested. The standard graphite also failed to stop the bacteria. The team concluded that while the bacteria successfully created the material, the specific chemical structure of the reduced graphene oxide made it less effective at killing bacteria than the original, un-reduced version.

Despite the lack of antibacterial activity in the final product, the study highlights a successful and environmentally friendly method for manufacturing nanomaterials. The researchers noted that the reduced graphene oxide they produced is non-toxic and retains functional groups that could be useful for other applications, such as carrying drugs or genes into cells for medical treatments. The material's large surface area also suggests it could be useful in industrial settings, perhaps as a lubricant for machinery. While this specific bacterial strain did not produce a material that kills multidrug-resistant bacteria on its own, the process itself offers a promising, low-energy pathway to create nanomaterials that could be combined with other agents to fight infections or serve as advanced delivery systems in medicine. The work demonstrates that nature can indeed be harnessed to build complex materials, provided the right conditions are met.