The increasing prevalence of resistant bacteria has emerged as a critical public health concern due to their ability to resist multiple antibiotics. This study aimed to investigate whether repeated treatments with silver nanoparticles (AgNPs) could gradually decrease bacterial resistance to antibiotics. The methodology involved three consecutive applications of AgNPs on six bacterial strains, followed by assessing their susceptibility to 38 different antibiotics. To our knowledge, the following three phenomena were observed for the first time. (1) During three consecutive AgNP applications, it was revealed that all the studied bacteria gradually became more susceptible to 38 antibiotics; by the end of the treatments, susceptibility had doubled for five bacteria and tripled for Klebsiella pneumoniae compared to the susceptibility before the first AgNP application. (2) Three consecutive AgNP treatments led to 27-47% restoration of bacterial susceptibility to antibiotics, which had already completely lost their activity before the initial AgNP application. (3) Unlike previous studies, we discovered a novel effect: the repeated AgNP applications increased the susceptibility of Salmonella enteritidis and Staphylococcus aureus to AgNPs themselves. Obtained results suggest that AgNP treatments may offer a new promising strategy to combat antibiotic resistance.

The increase in bacterial resistance to antibiotics is a global problem for public health. In our previous works, it was shown that the application of AgNPs in cow mastitis treatment increased S. aureus and S. dysgalactiae susceptibility to 31 antibiotics due to a decrease in the bacterial efflux effect. The aim of the present work was to shed light on whether the change in adhesive and anti-lysozyme activities caused by AgNPs also contribute to the restoration of bacterial susceptibility to antibiotics. In vivo sampling was performed before and after cow mastitis treatments with antibiotics or AgNPs. The isolates were identified, and the adhesive and anti-lysozyme activities were assessed. These data were compared with the results obtained for in vitro pre-treatment of reference bacteria with AgNPs or antibiotics. The present study revealed that bacterial treatments in vitro and in vivo with AgNPs: (1) decrease the bacterial ability to adhere to cells to start an infection and (2) decrease bacterial anti-lysozyme activity, thereby enhancing the activity of lysozyme, a natural “antibiotic” present in living organisms. The obtained data contribute to the perspective of the future application of AgNPs for recovering the activity of antibiotics rapidly disappearing from the market.