## 4 Discussion  
Elastomers are manufactured or natural polymeric materials with elastic and viscous qualities that are widely used in the food and medical industries, in addition to industrial settings. Investigations on the antimicrobial activity of newly formulated compounds on microorganisms responsible for nosocomial or foodborne infections are relatively few. The present study aimed to evaluate the antimicrobial activity of 10 different elastomers (two treated rubbers and two controls, five treated silicones and one untreated control) against _L. pneumoniae_ and _S. enteritidis_, which are considered indicators to be of water quality in specific environmental/artificial conditions and fecal pollution in DWDSs or in food industries. The tested elastomers, in addition to the control composition, contain additives (already approved by the USFDA) with antimicrobial properties. They were tested at two different bacterial concentrations and compared to elastomers without additives used as controls. The elastomers with additives are patented and the specific composition was known only at the end of the experiments; their composition is reported in Tables 1 and 2. According to the manufacturer's information, the antimicrobial action is due to the contact of the microorganisms to the elastomer surface and not to the spread of the additives in the tested solution, as confirmed by the UNI EN 13130-1:2005 [17] test procedure for specific migration, with values of <0.0001 mg/kg. It is worth noting that _L. pneumoniae_ growth was better controlled by silicone VQM105, followed by VQM500L at both concentrations. The survival rate of _S. enteritidis_ was less clearly defined, sometimes showing better results at \(10^{4}\) CFU/mL and other times at \(10^{3}\) CFU/mL. Rubber elastomers revealed better antimicrobial efficacy of NBROM than NBRCA on _L. pneumoniae_ and reverse results on _S. enteritidis_. These unexpected results could depend on the low counts of the tested solution and/or on the mechanisms or method of exposure to the antimicrobial additives. _L. pneumoniae_ seemed to be more susceptible for reasons that cannot be conclusively demonstrated. There is not a specific literature on this topic or even experience on these elastomers reported from other research groups; the data presented herein are the first set of experiments on these materials and on these microorganisms. It could be speculated that the differences may depend on the different interactions with the substrate, on the greater adhesion properties or on the different communication capabilities through the quorum sensing of _L. pneumoniae_ compared to _S. enteritidis_. More experiments need to be carried out on these specific topics to confirm or deny these hypotheses. VQM105 is a complete silicone formulation containing both accelerators, zinc oxide and magnesium oxide, which are known to be additives that improve abrasion resistance and are crosslinkers for viscosity enhancement at the manufacturing stages. For these compounds, a small amount of antibacterial action is already recognized, as demonstrated by a recent study [18]. In the VQM105 formulation, all antimicrobial silver-based additives and zinc pyrithione, whose antibacterial and antifungal action is already demonstrated, were present. The other silicone formulations are still efficacious against _L. pneumoniae_ and _S. enteritidis_, although in different and sometimes contrasting ways. These different levels of antimicrobial activity could depend on the absence of one of the main components and/or one of the accelerators that could change the structure of the elastomers and therefore the exposure of the tested bacteria to the antimicrobial additives. Overlapping activity was observed in the case of the complete formulation NBROM, including the already mentioned accelerators and antimicrobial additives. It showed complete efficacy versus _L. pneumoniae_ but much less toward _S. enteritidis_ (a 0% vs. 18.5% survival rate).