In Search of the Next-Generation of Antimicrobials: Dr. Sajal Ghosh from the Physics Department at the Shiv Nadar Institution of Eminence

Antibiotic resistance is one of the World Health Organisation’s top global concerns. In 2022, the reported rates for methicillin-resistant Staphylococcus aureus in 76 countries was found to be 35%. Against this backdrop, it has become increasingly important to find ways to combat bacterial infections without using antibiotics. In recent years, there has been a growing interest in ionic liquids—organic salts that are liquid at room temperature—as antibacterial agents that can circumvent the problem of antibiotic resistance. Dr. Sajal Ghosh, Associate Professor at the School of Natural Sciences, Shiv Nadar Institution of Eminence (SNIoE), has been making great strides in the field of ionic liquid research for the past decade. He has published numerous papers on the influence of imidazolium-based ionic liquids on lipid membranes, which are present in bacterial cells.

“It has been found that some ionic liquids with long hydrocarbon chains are antibacterial in nature,” explains Dr. Ghosh. “They can affect the growth of bacteria. They can also inhibit enzyme activities. These characteristics of ionic liquids have opened the possibility to utilise them as controlled and selective antimicrobial agents. Our research focuses on imidazolium salts as they exhibit stronger adverse effects on micro-organisms than other ionic salts like morpholinium salts.” His work on the subject has been published in prestigious journals such as the American Chemical Society’s Langmuir, Biochimica et Biophysica Acta (BBA) – Biomembranes, Pharmaceutical Research, and the Royal Society of Chemistry’s Materials Advances.

Most plasma membranes found in nature are made up of lipids, specifically phospholipids. Phospholipids have a hydrophilic (water-loving) head and two hydrocarbon chains that make up the hydrophobic (water-rejecting) tail. In biological membranes, these phospholipids have a tendency to arrange themselves into a bilayer—a double layer configuration that can be understood as head-tail-tail-head, given that both the insides of cells, as well as the environment outside them, are usually hydrophilic. It is this bilayer that ionic liquids disrupt. Describing the mechanism of action of imidazolium-based ionic liquids on bacteria, Dr. Ghosh says, “When a salt is placed in water, it tends to dissociate into an anion and cation. The same happens to an ionic liquid. Due to dissociation of the tiny anion of an imidazolium-based ionic liquid, the bulky cationic part of the molecule becomes positively charged with a head imidazolium group attached to the hydrocarbon chain. Because of the long-range electrostatic interaction between this cation and the cellular membrane, the ionic liquid initially adsorbs onto the cellular membrane. Later, due to the van der Waals interaction between the hydrocarbon chain and the hydrophobic core of the lipids of the membrane, the ionic liquid inserts into the membrane. Such an insertion influences the structures and dynamics of cellular membrane leading to cell death.”

To better understand how Dr. Ghosh and his team deciphered this mechanism of interaction, he explains the primary technique deployed in his studies, “You may have heard of X-ray diffraction, which is used to characterise solid crystals. Cell membranes have a dynamic structure, so they are not exactly solid, hence, to quantify them, we use another technique called X-ray reflectivity. This technique needs a highly specialised piece of equipment called a synchrotron-Xray source.” In an X-ray reflectivity study, a thin film is irradiated with X-rays. The X-rays that are reflected from the film are captured and their intensity recorded. “So, for example, if the hydrocarbon chain of the lipid is in its compressed form, we get a certain intensity profile. If it has been straightened out due to the presence of another molecule, like say, cholesterol, we get another intensity profile. Using that we can make certain calculations to understand what is taking place within the membrane.”

Bacterial cell death is only one of the many applications of imidazolium-based ionic liquids studied by Dr. Ghosh. He has also investigated the impact of these ionic liquids on mammalian cells. One of the fundamental differences between a bacterial cell and a mammalian cell is the presence of cholesterol. Cholesterol molecules insert themselves between the hydrophobic tails within the lipid bilayer, lending a degree of robustness to the membrane, which makes them less susceptible to disruption by an ionic liquid. “The presence of cholesterol in mammalian cell makes it compact and rigid. It becomes hard for a foreign molecule to insert into the membrane. On the other hand, there is no cholesterol in bacterial cell membrane. Our research shows that a suitably chosen ionic liquid with controlled dose may not be harmful to a human cell but can be harmful to a bacterial cell.” His research on mammalian cells can also be used to understand the tumour-restricting behaviour of ionic liquids described in other literature. “Our research indicated that such results are due to two effects of an ionic liquid, one being the enhanced delivery of drug to the tumour cell and the other being the insertion of the ionic liquid itself into the cell membrane, a structure that is compromised in a tumour cell as compared to a healthy mammalian cell,” explains Dr. Ghosh.

Dr. Ghosh’s work on ionic liquids also holds valuable results in the field of drug delivery. He says, “The interaction of the drug delivery vehicle and the targeted cellular membrane is one of the focus areas of research as the membrane is the main barrier to deliver any molecules into the cell. In 2022, we quantified the thermodynamics of interaction and the in-plane viscoelasticity of a lipid layer floating at an air–water interface, i.e., of a single lipid layer. The result described in the article will be useful in designing liposomes composed of suitable lipids with required physical properties, such as viscoelastic flexibility and enough strength of interaction with the targeted cell membrane. The added ionic liquid, on the one hand, can enhance fluidity of the liposome, and on the other hand, it can cause long range strong electrostatic interaction with the targeted cell membrane.”

Earlier this year, Dr. Ghosh and his colleagues also published a study on the use of ionic liquids as a coating material to create antimicrobial surfaces. “These novel surfaces have three-fold antibacterial properties. First, the chosen metal oxide surfaces themselves are highly reactive and thus reduce bacterial growth. Second, nanostructures are developed on these surfaces, which also prevent bacterial growth. Finally, imidazolium-based ionic liquids are coated on them to add an extra antibacterial layer,” describes Dr. Ghosh. The growth and formation of biofilms on metallic surfaces is a serious problem across a number of industries, from food and beverage packing to marine industries.

Speaking on the future directions of his research, Dr. Ghosh indicates an interest in developing antimicrobial peptides that can be used to treat antibiotic-resistant bacteria. “We are also investigating the assembly of macromolecules at liquid–liquid interfaces in our lab for relevant applications in the food industry,” he adds.

The field of biophysics in India has produced many world-class discoveries. Although research was previously clustered in only a select few institutes before, Dr. Ghosh and his stellar work in the soft matter physics research group at SNIoE are clear evidence that these barriers are slowly dissolving. “However, as a large country, we still have much more to do. Instead of confining research in a few centres, it must be initiated in state and central universities. The cost of research in biophysics is relatively high and, hence, both the government and private organisations must support the research,” says Dr. Ghosh. He also adds that physicists need to make stronger contributions to the field of biology. “Though there are examples of contributions by physicists to understand biological phenomena better, there are many more opportunities to apply the basic principles of physics to make quantitative predictions of biological phenomena,” he concludes.

Congratulations to Dr. Ghosh on all his excellent work! We are eagerly waiting to see what he works on next!