ESKAPE pathogens- comprising Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, Enterobacter species along with fungal pathogens are among the primary causes of hospital acquired infections/nosocomial infections including surgical site infection, pneumonia, urinary tract infection, bloodstream infection, etc. [1]. These pathogens are particularly concerning due to their ability to develop resistance to multiple conventional antimicrobial strategies including antibiotics which has been associated with high morbidity and mortality rates worldwide [2]. Conventional antimicrobial techniques involve use of high temperatures, toxic chemicals and sophisticated processes etc. to achieve complete elimination of pathogens. In recent times, Nanotechnology has excelled tremendously well in designing and developing various antimicrobial techniques [3]. Among various nanotechnology-based approaches, plasmonic photothermal is an emerging technique [[3], [4], [5]]. The photo-absorbing agent i.e., plasmonic nanoparticles upon external electromagnetic irradiation results in electron excitations at the surface of nanoparticles that further relax by electron-electron scattering to maintain equilibrium. Furthermore, to sustain thermal equilibrium, electron-phonon (lattice phonon) coupling occurs for energy exchange followed by phonon-phonon coupling that lead to the lattice cooling by heat dissipation to the surroundings. This leads to temperature elevation in the surroundings of the nanoparticles and is known as plasmonic photothermal phenomenon [4,6,7]. This physical phenomenon to kill/inactivate microbes via dissipated heat is being actively explored [[8], [9], [10], [11], [12], [13], [14]]. Recently a study [10] demonstrated antibacterial efficiency of plasmonic photothermal technique for complete destruction of considerably high loads of bacteria (Bacillus subtilis, ∼2.5 × 107 to 5 × 108 CFU/mL) in ∼5 to 10 min. Such high loadings are found in pus, soft tissue, and wound infections [15]. There is further need to specifically address the inactivation kinetics for plasmonic photothermal based eradication of different pathogens and their combinations.

So far, most of the reported inactivation kinetics are based on isothermal and non-isothermal approaches wherein the required temperature is attained via water baths, oil baths, or thermoresistometer, etc. For example, a study [16] reported non-isothermal inactivation of Escherichia coli (∼1 × 109 CFU/mL, 100 μL) using circulating water bath at varying heating rates from 0.15 to 1.64 °C/min within temperature range of 30 °C to 55 °C. It was revealed that ∼5 Log reduction of E. coli was achieved in ∼80–100 min and ∼ 250–300 min at 1.64 °C/min and 0.15 °C/min heating rate, respectively. Furthermore, among various studies that targeted destruction of Staphylococcus aureus, a study stated from equivalence calculations that ∼1 Log reduction of the pathogen can be achieved in ∼24 s at ∼70–72 °C in water bath [17]. Likewise, for Salmonella the study determines ∼1 Log reduction in ∼1.4 min at 60 °C in water bath. Also, from the equivalence calculation same reduction can be obtained in ∼0.18 min at 65 °C reported in the study [18]. One another study [19] demonstrated thermal mitigation of biofilms of Pseudomonas aeruginosa which is a common infecting agent of medical devices, wherein about 6 orders of biofilm reduction with initial load of ∼1.7 × 109 CFU/mL is reported in water bath at 80 °C for 30 min.

All the above discussed studies report the isothermal and non-isothermal based inactivation patterns mostly via water bath treatment of a particular pathogen and not for combination of pathogens. To the best of our knowledge, such pathogen inactivation kinetics (temperature-time kinetics) are not reported for plasmonic photothermal based antimicrobial activity and these can lead to paradigm shift towards various antimicrobial applications of plasmonic photothermal technique such as sterilization etc.

Therefore, the current study reports the detailed temperature time kinetics for inactivation of clinically relevant pathogens viz., ESKAPE panel and Candida albicans (individual and their combinations) that are cause of nosocomial infections [1] and possess global mortality [2] subjected to plasmonic photothermal technique considering varying temperatures, irradiation durations (time) in the presence of triangular silver nanoplates. Corresponding mechanism of complete eradication of such broad spectrum of pathogens via plasmonic photothermal technique is also examined via cytoplasmic DNA efflux that gives the bigger picture about the pathogen membrane breakage and permeability. Further, ROS generation is enumerated for its synergistic effect on pathogen elimination.