Photodynamic inactivation : a novel antimicrobial treatment to multi-drug resistant pathogens

Abstract

The worldwide increase in the incidence of multiple antibiotic-resistant bacterial pathogens has led to the investigation of photodynamic inactivation (PDI) as an alternative treatment for bacterial infections. PDI is the inactivation of bacterial cells by reactive species that are generated after specific visible light illuminates a non-toxic photosensitizer (PS) that has attached onto the cells. This study aimed to investigate the effect of different PSs and light on Staphylococcus aureus, methicillin-resistant S. aureus (MRSA), Escherichia coli and E. coli that produces extended-spectrum β-lactamase (ESBLs). In order to select ESBL-producing E. coli strains for the PDI study, 103 such strains isolated from patients from a local hospital were examined for the presence of the β-lactamase genes blaTEM, blaSHV and/or blaCTX-M. blaCTX-M was the most common β-lactamase gene detected among these isolates. One of the isolates that harboured both the blaTEM and blaCTX-M resistant gene was investigated for the antimicrobial efficacy of four PSs, poly-L-lysine chlorin (e6) conjugate (pL-ce6), polyethyleneimine chlorin (e6) conjugates (PEI-ce6), toluidine blue O (TBO), and hypericin (HY). The uptake of pL-ce6, PEI-ce6 and hypericin was in a concentration dependent manner, except in the case of hypericin in ESBL-producing E. coli. The results demonstrated that the four study strains demonstrated different susceptibility to the tested PSs and the PDI effect on the tested bacterial strains was dependent upon the drug concentration and the light dose. The MRSA and ESBL-producing E. coli exhibited equal susceptibility to the tested PS as compared their ATCC control strains. S. aureus was most susceptible to PEI-ce6, and least susceptible to TBO, whereas E. coli was most susceptible to pL-ce6 and least susceptible to hypercin. In addition, PDI results show that under the same drug and light dose conditions, the killing effect can be maintained among the bacteria of the same strain, even though they have different antibiotic resistant profiles. The interactions of three PSs (pL-ce6, PEI-ce6 and HY) with the bacterial membranes were also studied for S. aureus (ATCC 25923), an MRSA, E. coli (ATCC 25922) and an ESBL-producing E. coli which harboured both the blaTEM and blaCTX-M resistant gene. Surface morphological changes and membrane integrity of PS-treated cells evaluated using scanning electron microscopy and propidium iodide staining showed that the cytoplasmic membrane was the prime reaction site for pL-ce6 and PEI-ce6 on the four studied strains. It is possible that small amounts of pL-ce6 and PEI-ce6 bind to the outer membrane and then pass through the inner membrane to reach the cytoplasm. Hypericin, on the other hand, is solely effective on S. aureus and MRSA probably because hypericin cannot overcome the membrane barrier of E. coli. In conclusion, results indicated that the PDI effect of the tested PSs on the four pathogens were drug and light dose dependent. Their photokilling effect reflected their membrane damaging effect. PDI offers a new and effective alternative antimicrobial regimen, especially pL-ce6-mediated PDI and PEI-ce6-mediated PDI. Hypericin may be developed as an alternative anti-MRSA therapeutic option. The importance of this study lies in generating new insight using PDI in the treatment of local clinical antibiotic resistant infections.