09/27/2024
Fatty acids against antibiotic-resistant germs
An interdisciplinary team of researchers established a set of conditions under which sustainable fatty acid complexes can efficiently fight against dangerous, antibiotic-resistant bacteria.
The results are encouraging and inspire further research on the development of fatty acid-based antibiotics. They further demonstrate the value of neutron scattering in this kind of pharmaceutical research.
The discovery of penicillin in 1928 represented a huge milestone in the fight against bacterial infections. In the past years, the overuse of antibiotics has, however, lead to the emergence of antibiotic-resistant bacteria. A case in point is methicillin-resistant Staphylococcus aureus (MRSA), which is now responsible for approximately 150,000 infections per year in Europe alone.
In light of this critical situation, novel antibacterial agents are highly sought-after. Long-chain fatty acids (LCFA for short) have been established as one of the viable options. Fatty acids are an important constituent of many biological systems, including the cells that make up the human body. The anti-bacterial mechanisms of LCFA are manifold: they can inhibit bacterial protein synthesis, interfere with their DNA replication, and disrupt protective membranes.
In order to unfold their full bactericidal activity, LCFA molecules need to be solubilised. This is an important bottleneck in the development of LCFA-based bacteridices: the solubility of LCFA decreases with increasing chain length and is further limited by temperature and concentration. Below a certain temperature (called the Krafft temperature) LCFA crystallise; above a critical concentration, they associate into globular structures known as micelles. Both processes reduce the amount of active molecules available for attacking bacteria. Balancing these parameters to enhance the antibacterial performance of LCFA is thus an essential and delicate task.
While it is known that certain small molecules can play an important role in making LCFA more soluble, the potential of an important subclass of these molecules (so-called organic counterions) had, until recently, remained unexplored. A team of French researchers set out to tackle this challenge. By adding one such molecule (called choline, which is an essential nutrient for humans) to LCFA solutions, they were able to decrease the Krafft temperatures of all LCFA studied. Thereby, they managed to dissolve even those LCFA that previously crystallised in solution. Importantly, these LCFA-choline complexes are in fact plant-based soaps and therefore represent highly sustainable, eco-friendly bactericides.
This successful synthesis allowed for quantitative testing of the antibacterial properties of these LCFA. "Using an interdisciplinary set of methods and techniques, we were able to optimise and characterise the antibacterial activity of LCFAs against antibiotic-resistant germs," explains Anne-Laure Fameau, the principal investigator of the study, adding: "The results we obtained are encouraging and inspire further research on fatty acid-based antibiotics."
To achieve this goal, the researchers defined and determined the so-called Starting to Kill Concentration (SKC) of different LCFA, i.e. the fatty acid concentration at which the populations of bacteria are significantly reduced. The lower the SKC of a given LCFA solution, the better it is at eliminating bacteria. Overall, longer fatty acids were less efficient (i.e., their SKCs were higher) since they are less soluble (assembling into micelles faster) and a therefore a smaller amount of active molecules remains available in solution. Interestingly, the team found that the longest fatty acids (18 carbon atoms) were faster at eliminating bacteria than the shorter ones (12 carbon atoms).
The structure of the LCFA-choline solutions was investigated on a molecular level by performing small-angle neutron scattering (SANS) experiments on D22 at the ILL. "SANS allows probing structures on the nanometer to micrometer scale and is therefore an optimal tool for the molecules studied here", underlines Sylvain Prévost, ILL researcher and instrument responsible. Notably, the SANS instrument D22 at the ILL is very well suited for such experiments due to its high flux, which allows for systematic screening of a large number of sample conditions and parameters and thereby enables very time-efficient experiments.
The SANS data revealed clear signatures of elliptical micelles for all LCFA-choline complexes. Detailed analysis showed an increasing micelle-micelle distance and an increasing micellar aggregation with increasing fatty acid length, as well as a change in micellar shape. This detailed information on LCFA structure and behaviour in solution is valuable for the interpretation of the results of ongoing and further research on their antibacterial properties.
To verify the specificity of the antibacterial activity shown by the LCFA-choline complexes, the researchers also tested them against E. coli, a common pathogen. "Our data showed that the LCFA-choline complexes we tested were selectively efficient against methicillin-resistant S. aureus, but not against E. coli, which underlines their particular strength in the fight against novel antibiotic-resistant bacteria", say Elena Arellano and Tomasz Swebocki, the first authors of the study.
Source: Institut Laue-Langevin (ILL)