Lysozyme increases bactericidal activity of ceragenin CSA-13 against Bacillus subtilis

Introduction
Bacillus subtilis and other opportunistic bacilli are responsible for infrequent but serious infections such as post-surgery and post-traumatic endophthalmitis. Lysozyme is a natural protein found in various body fluids, exerting direct antibacterial activity and involved in modulation of the immune response in the site of inflammation. Ceragenins (CSAs) are cationic lipids based on cholic acid structure. CSA-13, the best-characterised molecule of its family, is distinct due to its broad-spectrum of antibacterial activity against aerobic and anaerobic Gram-positive and Gram-negative bacteria, as well as multidrug-resistant strains of fungi, parasites, and some viruses.

Aim of the research
To assess whether a combination of lysozyme (the antibacterial enzyme present in various human body fluids) and CSA-13 (a new synthetic mimic of natural antimicrobial peptides) will display higher bactericidal activity against B. subtilis in comparison to their activity alone.

Material and methods
The antimicrobial activities of lysozyme, CSA-13, and their combination were determined using a killing assay, and changes in bacterial cell morphology upon exposure to these antimicrobials were visualised by atomic force microscopy (AFM). In addition, interactions between the tested compounds were analysed using reductions in bacterial counts and determination of synergistic effects.

Conclusions
The effects of combined treatment involving lysozyme and CSA-13 against B. subtilis indicate synergistic antibacterial activity that might be used in the development of new methods to combat infections caused by this genus of bacteria.