## 4 Discussion  
The results of the current study demonstrated that the amount of the ZnO nanoparticles should be increased (up to 0.06 g in 100 mL of the coating carrier) to maintain the bacteriolytic effect of the coating against selected Gram-negative and Gram-positive bacterial strains. The results of the previous work [12] demonstrated that a methyl-hydroxy-propyl-cellulose (MHPC) coating containing ZnO nanoparticles (0.082 g in 100 mL of the coating carrier) inhibited the growth of _S. aureus_, _E. coli_, _B. cereus_, and _P. aeruginosa_. It is worth mentioning that accelerated irradiation of this coating did not influence its antibacterial properties due to the shielding properties of ZnO nanoparticles [12; 13; 14; 15; 16; 17; 18; 19]. The decreased amount of the nanoparticles in MHPC coating (0.041 g in 100 mL of the coating carrier) led to obtaining the active layer that inhibited the growth of _S. aureus_ but only decreased the number of _E. coli_ and _P. syringae_ cells, confirming that its antibacterial activity was lower [11]. The previous work [11] also indicated that the coating containing a decreased amount of ZnO (0.041 g/100 mL of the coating carrier) and decreased amount of gernaiol was more active against _S. aureus_, _E. coli_, and _P. syringae_ strains than the coatings containing a decreased amount of gernaiol or decreased amount of nano ZnO, confirming the synergistic effect of these active agents. The analysis of the antibacterial properties of _U. tomentosa_ and _F. betulina_ showed that the activity of these two extracts was very low [9]. A lower than 1-log reduction in the number of _S. aureus_ cells was noticed for 5% of extracts of described plants. While analyzing the antimicrobial properties of _F. betulina_ extracts on _B. subtilis_ cells, it was demonstrated that they demonstrated lower than a 2-log reduction but higher than a 1-log reduction in the number of living cells. _U. tomentosa_ extract was found to be even less active. Unfortunately, both extracts were not effective against _E. coli_ and _P. syringae_ strains. The findings of the current work indicated that the coatings containing 5% of _U. tomentosa_ or _F. betulina_ extracts with the addition of a decreased amount of the nanoparticles of ZnO (0.03 g/100 mL of the coating carrier) demonstrated high antibacterial activity. The synergistic effect between active agents caused a complete inhibition of the  
Figure 9: (**a**) Sliced cooked ham before storage (sample “0”). (**b**) Sliced pieces of ham stored for 48 h in uncoated PP bags, separated by uncovered PP spacers. (**c**) Sliced pieces of ham stored for 96 h in uncoated PP bags, separated by uncovered PP spacers. (**d**) Sliced pieces of ham, stored for 48 h in PP bags covered with the Ut layer, separated by PP spacers coated with the Ut layer on each side. (**e**) Sliced pieces of ham, stored for 96 h in PP bags covered with the Ut layer, separated by PP spacers coated with the Ut layer, separated by PP spacers coated with the Ut layer on each side. (**f**) Sliced pieces of ham, stored for 48 h in PP bags covered with the Fb layer, separated by PP spacers coated with the Fb layer on each side. (**g**) Sliced pieces of ham, stored for 96 h in PP bags covered with the Fb layer, separated by PP spacers coated with the Fb layer on each side. growth of _S. aureus, B. atropaeus_, and _E. coli_ strains and a reduction in the number of _P. syringae_ cells. Additionally, SEM analysis confirmed that the PP film was homogenously and thoroughly covered with the active Ut and Fb layers. However, small pores were observed on the Ut coating, which might have led to a faster release of the active substances from this coating than from the Fb coating or which might have been caused by the release of the active compounds from the Ut layer. Comparing the coatings Ut and Fb, it may be suggested that the Ut active layers were able to display a greater influence on the growth of microorganisms than Fb coating, but for a shorter time.