3.2.2. Antibacterial Test

The antibacterial activity of each sample was determined by measurements of inhibition halo against using two representative bacteria, *S. aureus* and *E. coli* (Table 5). The samples grafted by both methods without AgNPs did not show bacterial inhibition, whereas the samples with AgNPs showed inhibition against the two types of bacteria. By the photochemical method, it was evident that the inhibition was slightly stronger against *E. coli,* which is Gram-negative bacteria, whereas by the gamma radiation method, the samples showed inhibition against *E. coli* in all cases and against *S. aureus* in most of them; the inhibition halo was clearly larger against *E. coli*. Previous studies have suggested that antimicrobial effects of AgNPs may be associated with characteristics of certain bacterial species.


**Table 5.** Antibacterial results of modified PET fabrics.

Due to the structural difference in the composition of the membrane of Gram-positive and Gram-negative bacteria, AgNPs have significantly less effect on the growth of Gram-positive bacteria [47]. In our case, the comparison between both types of grafted fabrics (obtained by UV method or by direct gamma irradiation) demonstrated that there was a clear difference in the effectiveness against both types of bacteria when the gamma irradiation method was used for grafting. This may result from the fact that the thicker coating of hydrogel contains a larger amount of AgNPs and, therefore, a higher content on Ag+ ions are presumably slowly released from the AgNPs surface. In this investigation, no tests at different contact times were carried out to corroborate the mentioned above; only tests at 24 h were carried out because the biofilm reaches its maturation at this time [48]. The AgNPs size within the samples is another important parameter to be considered in a future work. Besides, according to Taglietti et al., the antimicrobial effect could also be due to a direct contact between bacteria and the AgNPs [39,49].
