Limits of antibacterial activity of triangular silver nanoplates and photothermal enhancement thereof for Bacillus subtilis.

Currently, nanoparticles are being actively explored for antimicrobial applications involving variety of pathogens. Bacillus subtilis is a major concern considering its sporulation and biofilm formation capability which involves high bacteria loadings. Also, there is natural ability of B subtilis to adapt and develop resistance to the silver nanoparticles alone. So, this study reports the limits of antibacterial activity of triangular silver nanoplates (∆AgNPs) and further photothermal enhancement for B. subtilis ATCC 6051 for considerably high bacterial load of 2.5 × 107 to 5 × 108 CFU/ml. Triangular silver nanoplates were synthesized using one pot synthesis method and showed significant photothermal response i.e., ∼36 °C temperature rise on near infrared irradiation as well as photothermal stability. Triangular silver nanoplates alone showed absolute destruction for 2.5 × 107 CFU/ml initial B. subtilis load in 5 min. Whereas, for further higher bacterial loads, the antibacterial efficacy of ∆AgNPs is observed to be insignificant. For higher initial bacterial loads of 5 × 107 CFU/ml and 5 × 108 CFU/ml, photothermally enhanced triangular silver nanoplates resulted in complete destruction of bacteria in about 5 and 10 min, respectively. Antibacterial efficacy and mechanism of the destruction assessed via scanning electron microscopy and LIVE/DEAD assay confirmed morphological deformities. Further the generation of higher levels of reactive oxygen species is also confirmed due to photothermal activation of ∆AgNPs. The study concludes that ∆AgNPs alone are effective only up to bacterial load of 2.5 × 107 CFU/ml. Whereas, for higher bacterial loads of B. subtilis, photothermally activated ∆AgNPs lead to irreversible damage due to multiple targeting mechanisms leading to absolute elimination in short span of 5-10 min for the chosen irradiation conditions. Ultimately, this study demonstrates photothermally enhanced silver nanoplates as a potential antimicrobial agent for considerably high bacterial loads of B. subtilis. Overall, the broader window of considered high bacterial loadings and its irradiation by this technique shows the full-proof nature of photothermal applications for scenarios involving high cell density such as biofilms and wound infections etc. Further, the concept may be useful for sterilization or decontamination of samples, devices, etc. because B. subtilis and its spores are the challenges during sterilization.