Strategies to potentiate antimicrobial photoinactivation by overcoming resistant phenotypes

Abstract

Conventional antimicrobial strategies have become increasingly ineffective due to the emergence of multidrug resistance among pathogenic microorganisms. The need to overcome these deficiencies has triggered the exploration of alternative treatments and unconventional approaches towards controlling microbial infections. Photodynamic therapy (PDT) was originally established as an anticancer modality and is currently used in the treatment of age-related macular degeneration. The concept of photodynamic inactivation requires cell exposure to light energy, typically wavelengths in the visible region that causes the excitation of photosensitizer molecules either exogenous or endogenous, which results in the production of reactive oxygen species (ROS). ROS produce cell inactivation and death through modification of intracellular components. The versatile characteristics of PDT prompted its investigation as an anti-infective discovery platform. Advances in understanding of microbial physiology have shed light on a series of pathways, and phenotypes that serve as putative targets for antimicrobial drug discovery. Investigations of these phenotypic elements in concert with PDT have been reported focused on multidrug efflux systems, biofilms, virulence and pathogenesis determinants. In many instances the results are promising but only preliminary and require further investigation. This review discusses the different antimicrobial PDT strategies and highlights the need for highly informative and comprehensive discovery approaches.

Figure 1

Schematic illustration of photodynamic acion. The PS initially absorbs a photon that excites it to the first excited singlet state and this can relax to the more long lived triplet state. This triplet PS can interact with molecular oxygen in two pathways, type I and type II, leading to the formation of reactive oxygen species (ROS) and singlet oxygen respectively. In the absence of oxygen the PS may interact with a substrate (R) in a pathway known as Type III.

Figure 2

Left: superimposed Van der Waals (VdW) models of the lowest energy docked structures of berberine [75] TBO (yellow) and MB (magenta) in apex of the inverted "V" depicted by the transmembrane α-helices 4 and 6 as they separate from top to bottom (in this orientation the extracellular side at the top; cartoon representation of the protein colored according to the sequence, from red to blue). Right: MB showed a clear specificity for the site overlapping with berberine, the calculated binding energies being practically indistinguishable. The hydrophobic pocket of negative electrostatic potential is rich in hydrophobic and aromatic residues, mainly form helices 4, 5 and 6. The structure of the dyes were optimized with the method for fundamental vibrational frequencies B3LYP/6-311+G(d,p level of theory using the gaussian03 [129] package in a model polar solvent (PCM acetonitrile) [130]. The structures were docked onto the whole chamber formed by the twelve Transmembrane Domains (TMs) of the protein at the height to which they span the cell membrane in order to locate their binding sites, using a Lamarckian genetic algorithm implemented in Autodock 4.2.3. Afterwards, the whole protocol was applied in a smaller region comprising their main binding site. Their highly delocalized charge distribution was modeled from the QM results instead of the default program charges [131].

Figure 3

Extension of the hybrid antimicrobial approach [72, 74] to compounds for use in antimicrobial PDT