Photosensibilization and photocatalysis: comparative analysis of antimicrobial efficacy, mechanism and germinated seeds microbiological control

The microbiological contamination of germinated seeds is a serious problem worldwide. The aim of this work was to compare the two innovative non-thermal microbiological control methods (chlorophyllin based photosensitization and photocatalysis with ZnO nanoparticles), evaluating antimicrobial efficacy, the mechanism of action and possibilities to apply these methods to increase microbiological safety of germinated seeds. Both methods have been shown to be effective against Fusarium oxysporum (growth delay up to 52.2% depending on concentration), whereas photocatalysis (10-3 M, 30 J/cm2; 7 log CFU/ml inactivation) is much more effective than photosensitization (1.5×10-5 M, 30 J/cm2; 1.35 log CFU/ml inactivation) in inactivating Escherichia coli. The mechanism of antimicrobial action is also different: chlorophyllin with bacteria interacts only externally, violating the cell wall, and photoactivated ZnO nanoparticles not only affect the outer membrane of bacteria, but possibly penetrate inside, internalize cells and damage intracellular structures. Nonetheless, for both methods, reactive oxygen species are the main instruments to destroy biomolecules. The analysis of SERS spectra showed that in both cases, E. coli is affected by lipids, carbohydrates and proteins, but the damage is different. After photocatalysis (10-3 M ZnO nanoparticles, 18 J/cm2), the mesophilic bacteria on the seeds surface decreased by 2.74 log CFU/g, E. coli - 2.28 log CFU/g, micromycetes - 2.09 log CFU/g. After photosensitization (5×10-4 M Chl; 18 J/cm2), the number of these microorganisms decreased by 2.66 log CFU/g, 1.22 log CFU/g and 1.23 log CFU/g, respectively. In addition, both methods have improved seeds germination (14.5-16.2% compared to control), but photocatalysis has led to more pronounced growth of roots and shoots (compared to control with 83.5% and 62.5%) than photosensitization (51% and 20.7%). Despite the fact that this work is the first attempt to apply photosensitization and photocatalysis for microbiological control of germinated seeds and a lot of research must be done in the future, the results are promising. May be, these methods will be able to compete with existing microbiological methods for germinated cereals in the future