Pyrazole-pyridinium porphyrins and chlorins as powerful photosensitizers for photoinactivation of planktonic and biofilm forms of E. coli

In nature, microorganisms can form highly structured complexes designed as biofilms, that cause severe and chronic infectious in humans. The formation of biofilms provides microbes many advantages, resulting in a higher tolerance to the conventional treatments. Antimicrobial photodynamic therapy (aPDT) is an efficient therapeutic alternative to eradicate microbial cells, which combines a photosensitizer (PS), like porphyrins (Pors) or chlorins (Chls), oxygen and light to induce the formation of reactive oxygen species (ROS) that can lead to cell damage and even death. For this purpose, cationic Pors and Chls (1a and 2a, respectively) bearing 4-(1H-pyrazol-3-yl)pyridinium groups were synthesized and characterized, and their aPDT efficiency investigated against planktonic and biofilm forms of Escherichia coli, a Gram-negative bacterium. The obtained results demonstrate that Por 1a and Chl 2a exhibit high aPDT efficacy towards planktonic and biofilm forms of E. coli. For the planktonic cells, both PSs at 2.0 μM caused a bacterial photoinactivation till the detection limit of the method, after 30 min of red light irradiation at an irradiance of 14 mW cm−2 (25.2 J cm−2). However, under white light (25 mW cm−2), the aPDT efficiency of Por 1a was improved and completely inactivated E. coli after 10 min (15 J cm−2) of treatment. On the other hand, the photodynamic efficiency of Chl 2a at 2.0 μM has similar behaviour under the different light conditions. For the biofilm's assays, a complete photoinactivation was obtained by increasing the concentration of 1a and 2a to 10 and 5.0 μM, respectively, after 60 min under white light at an irradiance of 100 mW cm−2. The compound 1a exhibited a higher antimicrobial efficiency against the planktonic cells of E. coli, while 2a derivative was more efficient towards biofilms. This photodynamic efficiency can be related with the higher levels of 1O2 produced by 1a, as well as with the higher absorption in the red region exhibited by 2a, that might allow a better penetration in the biofilm structure. The potential of theses PSs is magnified with the low PS concentration and low light doses required, when compared with other cationic PSs, to achieve the total inactivation for both planktonic and biofilm forms of E. coli, which is a clear advantage for a clinical application.