Photothermal Inactivation of Microorganisms under Relaxation of Highly Excited States of Sensitizers
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BIOPHYSICS
Photothermal Inactivation of Microorganisms under Relaxation of Highly Excited States of Sensitizers S. N. Letutaa, *, S. N. Pashkevicha, A. T. Ishemgulova, and A. N. Nikiyana a
Orenburg State University, Orenburg, 460018 Russia *e-mail: [email protected]
Received November 8, 2019; revised November 8, 2019; accepted May 22, 2020
Abstract—This study demonstrates the susceptibility of the Escherichia coli and Bacillus subtilis planktonic bacteria to inactivation by shock acoustic waves that arise from the rapid formation and collapse of vapor bubbles in a medium locally heated to its boiling point. Local heating of the medium occurred due to heat release through the relaxation of highly excited electronic states of exogenous molecules of organic dyes. Dye molecules were excited by nanosecond laser pulses. Highly excited electronic states were formed as a result of stepwise absorption of two quanta of laser radiation. The dependence of the efficiency of microorganism inactivation on the dye concentration, excitation power density, and the distance from the shock wave source was studied. Keywords: exogenous thermosensitizers, acoustic shock waves, inactivation of microorganisms DOI: 10.1134/S0006350920040089
Alternative strategies for the control of pathogenic microorganisms are in high demand due to the increase in the number of strains that are resistant to traditional antimicrobial agents [1]. Photodynamic processing, i.e. the joint destructive effect of light and photosensitizers (PS) on cells to which microbes cannot develop immunity is a promising method of nondrug action on bacteria and fungi. The mechanism of photodynamic therapy is based on the selective oxidation of substrates by reactive oxygen species generated by photosensitizers [2–5]. Photosensitizers are usually molecules of organic dyes with high triplet quantum yields [6, 7]. The major limitations of photodynamic therapy are associated with the low penetration depth of light and photosensitizers in biological media. Some, e.g., biofilms, contain almost no oxygen [8, 9] and the photodynamic effect is exhibited only in a thin peripheral layer. Other physical processes have to be “switched on” to inactivate bacteria in deeper layers. Here, we present the results of the studies of damage to planktonic bacteria caused by acoustic shock waves. Such waves appear in the medium due to the rapid formation and collapse of vapor bubbles in locally heated areas. Rapid local heating of the medium occurs under the conditions of nonradiative relaxation of short-lived highly exited electronic states (HEES) of sensitizer molecules. Abbreviations: HEES, highly excited electronic states.
The HEES of dyes are effectively populated under the conditions of irradiation of molecules with nanosecond laser pulses at a power density P ≥ 5 MW/cm2 as a result of stepwise absorption of two quanta of exciting radiation [10, 11]. The relaxation of HEES molecules is mostly nonradiative and the absorbed energy is very rapidly converted to heat. If a thermal sensitizer is
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