​Antibiotic resistance – when bacteria develop resistance to bactericidal drugs – is a major global public health threat making it difficult to treat common bacterial infections effectively. Antimicrobial peptides, i.e. molecules produced by many tissues and cell types as a first line of defence, have been touted as alternative therapeutic agents that could be used in the design of new antibiotics. But first, researchers must understand their molecular structure and function in different environments such as liquids and organic solvents.  

This is exactly what Zimbabwean-born scientist Dr Cathrine Pfukwa tried to do in her recent doctoral study at Stellenbosch University (SU). She graduated with her doctorate in Physics from SU on Thursday 16 December 2021 at SU's December graduation.

Pfukwa focused on a specific antimicrobial peptide gramicidin S that is effective against a wide range of pathogens. It has been used in medicine, such as in lozenges and eyedrops, and for treating wound infections.

As part of her study, Pfukwa combined experimental and analytical techniques from Physics, Biochemistry and Chemistry to determine how the peptide's structure and activity change in different environments, for example, in liquids and organic solvents.

She says it is important to gain a better understanding of the peptide's structure in environments that are representative of membrane environments as this sets the initial stages for understanding the peptide behaviour and interactions which modulate peptide activity.

“Understanding the structure and function of gramicidin S in different environments is of fundamental importance to determine how it can be used in the design of next-generation antibiotic drugs."

According to Pfukwa, her study showed that exposing gramicidin S to different environments and temperatures led to different perturbations in its structure, thus helping us to see how these conditions influence its structure and activity.

“My work sheds light on the peptide's structure, the effects of the amino acid residues on its conformation (the spatial arrangement of atoms in the molecule) and activity, as well as how it interacts with cell membranes. It also contributes towards the development of initial predictions of the peptide structure through the use of computer simulations and theoretical models. This information could be particularly relevant in advancing the development of antimicrobial agents.

“It allows us to explore possible replacements or structural modifications which can be made to the properties of gramicidin S, while still maintaining its stability and antimicrobial activity," she adds.

Pfukwa says gramicidin S has antimicrobial activity against the cell membrane as well as inside the cell. It targets a bacterium's cell membrane in such a way that it compromises the structural integrity of the double layer of fatty molecules in the cell. This causes pores to form which leads to cell leakage and ultimately to cell death.

“With its broad spectrum of activity, the peptide targets the membrane as a whole and is non-selective; it does not discriminate and disrupts bacterial and mammalian cells with equal ability. Thus, it is only used topically.

“Unsurprisingly, there has been ongoing interest in understanding this antimicrobial peptide and developing derivatives that are less toxic and that mainly target pathogens."

Pfukwa points out that the peptide's activity against a bacterial cell membrane depends on its concentration in the cell or on the membrane because a specific concentration is required to trigger inter-peptide interaction, self-association, into an active form to cause the formation of pores on the membrane.

“Unfortunately, there aren't many studies on self-association of the peptide in solution environments that mimics the environment in the cell membrane environment. Understanding the self-association is important as it leads towards a better understanding of how the peptide behaves."

Pfukwa says scientists working on the design and synthesis of new antimicrobial peptides and in drug development will benefit from her research.

• ​Photo: Dr Cathrine Pfukwa at the graduation ceremony. Photographer: Stefan Els

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