Background: Biofilms are aggregates of bacteria residing in a self-assembled matrix, which protects these sessile cells against external stress, including antibiotic therapies. In light of emerging multidrug-resistant bacteria alternative strategies not based on traditional antibiotics are emerging. The present study evaluated the activity of colloidal silver nanoparticles (AgNPs) of different shapes against biofilms formed by Staphylococcus aureus (SA), methicillin-resistant SA (MRSA) and Pseudomonas aeruginosa (PA).
Methods: Colloidal AgNP spheres, cubes and stars were synthesised and their cytotoxicity on macrophages (THP-1) and bronchial epithelial cells (Nuli-1) was analysed by the lactate dehydrogenase assay. The antibiofilm activity was assessed in vitro by the resazurin assay and in an in vivo infection model in Caenorhabditis elegans.
Results: AgNP cubes and stars induced cytotoxicity, while AgNP spheres were not toxic. Spherical AgNPs showed substantial antibiofilm activity in vitro with 99%, 93% and 99% biofilm killing of SA, MRSA and PA, respectively, while significantly reducing mortality of infected nematodes. The in vivo antibiofilm activity was linked to the accumulation of AgNPs in the intestinal tract of C. elegans as observed by 3D X-ray tomography. AgNP spheres were physically stable in suspension for over 6 months with no observed loss in antibiofilm activity.
Conclusions: While toxicity and stability limited the utilisation of cubic and star-shaped AgNPs, spheres could be synthesised in a short time, were stable and non-toxic, and showed substantial in vitro and in vivo activity against clinically relevant biofilms. AgNP spheres hold potential as pharmacotherapy, e.g. as topical treatment for biofilm-related infections.