Development of Antimicrobial Phototreatment Tolerance: Why the Methodology Matters

Abstract

Due to rapidly growing antimicrobial resistance, there is an urgent need to develop alternative, non-antibiotic strategies. Recently, numerous light-based approaches, demonstrating killing efficacy regardless of microbial drug resistance, have gained wide attention and are considered some of the most promising antimicrobial modalities. These light-based therapies include five treatments for which high bactericidal activity was demonstrated using numerous in vitro and in vivo studies: antimicrobial blue light (aBL), antimicrobial photodynamic inactivation (aPDI), pulsed light (PL), cold atmospheric plasma (CAP), and ultraviolet (UV) light. Based on their multitarget activity leading to deleterious effects to numerous cell structures—i.e., cell envelopes, proteins, lipids, and genetic material—light-based treatments are considered to have a low risk for the development of tolerance and/or resistance. Nevertheless, the most recent studies indicate that repetitive sublethal phototreatment may provoke tolerance development, but there is no standard methodology for the proper evaluation of this phenomenon. The statement concerning the lack of development of resistance to these modalities seem to be justified; however, the most significant motivation for this review paper was to critically discuss existing dogma concerning the lack of tolerance development, indicating that its assessment is more complex and requires better terminology and methodology.

The Effect of Sub-Lethal Successive Applications of Photodynamic Therapy on Candida albicans Biofilm Depends on the Photosensitizer

“…The repeated application of antimicrobial treatment at low (sub-lethal) concentrations can make a microbial population resistant or tolerant [ 14 ]. The possible capacity of a microorganism to develop antimicrobial resistance mediated by aPDT has encouraged research in this area [ 7 , 15 , 18 , 27 ]. The production of ROS caused by sub-lethal applications of aPDT may not be sufficient to promote the inactivation of a microorganism; however, the accumulation of ROS in the cell can cause the changes responsible for selecting the least susceptible strains (or cell clones) or more tolerant ones that would survive the treatment [ 11 , 15 ].…”

“…The possible capacity of a microorganism to develop antimicrobial resistance mediated by aPDT has encouraged research in this area [ 7 , 15 , 18 , 27 ]. The production of ROS caused by sub-lethal applications of aPDT may not be sufficient to promote the inactivation of a microorganism; however, the accumulation of ROS in the cell can cause the changes responsible for selecting the least susceptible strains (or cell clones) or more tolerant ones that would survive the treatment [ 11 , 15 ]. Nevertheless, there is a lack of knowledge on the effect of successive applications of aPDT at sub-lethal doses on C. albicans biofilm.…”

“…The investigation of antimicrobial resistance and tolerance against conventional antimicrobials consists of successive exposure of the microorganism to sub-lethal (or sub-optimal) concentrations [ 14 , 15 ]. Sub-lethal doses of aPDT promote oxidative stress without causing irreversible changes in microbial cells, and, therefore, it is highly recommended for studies that evaluate antimicrobial resistance, tolerance, and persistence against aPDT applications [ 15 ]. In this context, the potential of sub-lethal doses of aPDT on some microorganisms has been investigated [ 7 , 16 , 17 , 18 , 19 ].…”

Genetic Factors Affect the Survival and Behaviors of Selected Bacteria during Antimicrobial Blue Light Treatment

“…More concerningly, low-fidelity polymerases expressed during the SOS-dependent stress response could result in the emergence of tolerant phenotypes during blue light treatments, as demonstrated in S. aureus [ 138 ]. Thus, further investigations should be conducted to unravel the mechanisms and frequencies of blue light tolerance, or resistance, in other bacteria [ 28 ]. In addition, alternative strategies can be developed using compounds capable of inhibiting different components of the SOS pathway.…”

“…Importantly, sensitivities to blue light vary across bacterial strains, i.e., light dosage required to inactivate different strains may vary considerably [ 25 , 26 , 27 ]. Thus, as with other antimicrobial treatments, antimicrobial blue light may be applied in an insufficient amount (i.e., sub-lethal), potentially resulting in the development of tolerance [ 28 ]. However, the underlying mechanism of this phenomenon remains elusive and this represents an important research gap that needs to be addressed to improve future designs of blue light-based antimicrobial treatments, particularly in deciding whether complementary treatments are necessary.…”