## 2 Materials and Methods  
### Natural Compounds and Strains  
The EOs _Citrus limon_ (_C.I._), _Mentha piperta_ (_M.p._), _Salvia officinalis_ (_S.o._) and _Thymus vulgaris_ (_T.v._), obtained by hydrodistillation, and purchased from a local herbalist's shop in Modena, Italy, have been studied and characterized in previous investigations by GC-MS and GC-FID for qual- and semi-quantitative analysis [42]. The bacteriocin bacLP17 is produced by _Enterococcus mundtii_ LP17, a strain previously isolated from red mullet and endowed with strong activity toward the pathogen [43]. The edible coating, provided by the SSICA research foundation (Parma, Italy), was obtained from pea proteins coming from processing residues [44]. The frozen shrimps, purchased from a supermarket in Modena, were brought to the laboratory using dry ice and the defrosting process was performed in a refrigerator (4 \({}^{\circ}\)C). _Listeria monocytogenes_ NCTC 10888 was used as a microorganism test.  
Minimal Inhibitory Concentration (MIC) and Fractional Inhibitory Concentration Index (FIC-Index) Determination  
The MIC values of _C. lemon_, _M. piperta_, _S. officinalis_ and _T. vulgaris_ EOs were determined against _L. monocytogenes_ NCTC 10888 by using the broth microdilution method in 96-well microplates, according to the Clinical Laboratory Standards Institute (CLSI) guidelines 2012 [45]. The test was performed in sterile 96-well microplates, with a well working volume of up to 200 mL of solution, by dispensing into each well 95 mL of Trypic Soy Broth (Oxoid S.p.A, Milan, Italy) and 5 mL of bacterial suspensions, to a final inoculum concentration of 10\({}^{4}\) CFU/mL. Then, 100 \(\mu\)L of EOs serial dilutions were added to obtain concentrations ranging from 512 to 0.125 \(\mu\)L/mL. Negative control wells consisted of bacteria in TSB without EOs. The plates were incubated at 37 \({}^{\circ}\)C for 24 h, mixed on a plate shaker at 300 rpm for 20 s, and the MIC was defined by the growth of the bacteria on the TSA plate. The checkerboard method was carried out in TSB by using the microdilution method to obtain the FIC-Index for the following six EO/EO associations, chosen based on the best MICs results obtained by the single compounds: _C. lemon_/_M. piperta_, _C. lemon_/_S. officinalis_, _C. lemon_/_T. vulgaris_, _M. piperta_/_S. officinalis_, _M. piperta_/_T. vulgaris_ and _S. officinalis_/_T. vulgaris_[46]. The same associations were also used in the experiments carried out with the addition of the bacteriocin bacLP17 (Tables 1-4).  
\begin{table}
\begin{tabular}{c c c c c} \hline _C. lemon_ & _M. piperta_ & _S. officinalis_ & _T. vulgaris_ & **bacLP17** \\ \hline
32 \(\mu\)L/mL & 32 \(\mu\)L/mL & 128 \(\mu\)L/mL & 8 \(\mu\)L/mL & 16 \(\mu\)L/mL \\ \hline \end{tabular}
\end{table}
Table 1: MICs of EOs and bacLP17 used alone against _Listeria monocytogenes_ NCTC 10888.  
\begin{table}
\begin{tabular}{c c c c c c} \hline _C. lemon_ & _C. lemon_ & _C. lemon_ & _M. piperta_ & _M. piperta_ & _S. officinalis_ \\ _M. piperta_ & _S. officinalis_ & _T. vulgaris_ & _S. officinalis_ & _T. vulgaris_ & _T. vulgaris_ \\ \hline
3 \(\mu\)L/mL & 1.5 \(\mu\)L/mL & 3 \(\mu\)L/mL & 1.5 \(\mu\)L/mL & 3 \(\mu\)L/mL & 6 \(\mu\)L/mL \\ +3 \(\mu\)L/mL & +6 \(\mu\)L/mL & +0.8 \(\mu\)L/mL & +6 \(\mu\)L/mL & +0.8 \(\mu\)L/mL & +0.8 \(\mu\)L/mL \\ \hline \end{tabular}
\end{table}
Table 2: MICs of the most advantageous EO/EO associations (chosen based on the FIC-Index) added to the coatings.  
\begin{table}
\begin{tabular}{c c c c c} \hline _C. lemon_ & _M. piperta_ & _S. officinalis_ & _T. vulgaris_ \\
**bacLP17** & **bacLP17** & **bacLP17** & **bacLP17** \\ \hline
3 \(\mu\)L/mL & 1.5 \(\mu\)L/mL & 12.5 \(\mu\)L/mL & 1.5 \(\mu\)L/mL \\ +0.8 \(\mu\)L/mL & +1.5 \(\mu\)L/mL & +0.4 \(\mu\)L/mL & +0.8 \(\mu\)L/mL \\ \hline \end{tabular}
\end{table}
Table 3: MICs of the most advantageous EO/bacLP17 combinations (chosen based on the FIC-Index) added to the coatings.