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 AgNO3 (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 AgNO3 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 AgNO3 and cross-resistance to 14 conventional antibiotics upon continuous exposure (up to 5 weeks) to sub-inhibitory concentrations (EC20) 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.