Do silver-chitosan nanocomposites promote bacterial resistance to silver or common antibiotics?

Silver-containing nanomaterials are versatile antimicrobials for slowing down the rapid increase of nosocomial infections caused by antibiotic-resistant bacteria. However, silver can also promote resistance in bacteria to both silver itself and conventional antibiotics. Despite this, the topic is still poorly studied.

Silver-chitosan nanocomposites (nAgCSs) with a primary size of ∼50 nm and weight ratios of 1:1 (nAgCS-1) and 1:3 (nAgCS-3) were synthesised and studied for potential antimicrobials, such as wound dressings, antimicrobial coatings. These nAgCSs exhibited antibacterial efficacy at level of 6.25–14.6 mg Ag/L level (minimal inhibitory concentration, MIC) against Escherichia coli and Staphylococcus aureus being comparable to the efficiency of AgNO ₃ (MIC 3.13–7.29 mg Ag/L) or benzalkonium chloride (MIC 0.91–6.25 mg/L). Low molecular weight chitosan also demonstrated antibacterial activity against S. aureus (MIC 25 mg/L), though it was less effective against E. coli (MIC 70 mg/L). Notably, nAgCS-3 was almost as efficient as AgNO ₃ against S. aureus (MIC 6.25–7.29 mg/L) and benzalkonium chloride against E. coli (MIC 6. mg/L). Confocal laser scanning microscopy revealed a noticeable aggregation of S. aureus cells caused by exposure to nAgCS-3, an effect that was less pronounced in E. coli .

The development of bacterial resistance to nAgCSs and AgNO ₃ and cross-resistance to 14 conventional antibiotics upon continuous exposure (up to 5 weeks) to sub-inhibitory concentrations (EC ₂₀ ) of the studied silver compounds was examined. No resistance development was observed for the studied silver compounds and antibiotics in either E. coli or S. aureus . Thus, silver-chitosan nanocomposites show promise as efficient and not AMR-inducing compounds for antimicrobial applications, such as wound dressings and surfaces.