## 2 Results and Discussion
### The Antimicrobial Activity of P13#1  
The Minimum Inhibitory Concentration (MIC) values of P13#1 were measured on a wide panel of Gram(+) and Gram(\(-\)) strains, including several strains belonging to the so-called ESKAPE pathogens, i.e., a relatively small group of pathogens which causes frequent hospital infections. Data are shown in Table 1.  
Concerning _P. aeruginosa_, we included laboratory, highly virulent and clinical strains: PAO1 is a well-known and widely used reference strain and PA14 is a highly virulent strain [49], whereas AA2, KK27 and RP73 are clinical strains isolated from cystic fibrosis (CF) patients [50; 51]. Among these, _P. aeruginosa_ RP73 is a \"late\" strain which shows specific mutations related to its ability to cause chronic infections due to a long adaptation to the peculiar environment provided by the lung of a CF patient [51; 52; 53]. Regarding _S. aureus_, we included a laboratory strain (ATCC 6538P) and two clinical strains, WKZ-2, a methicillin-resistant strain [54], and Newman, a highly virulent strain [55]. As positive controls, we included three well-known antimicrobial peptides, namely, (P)GKY20, polymyxin B and vancomycin. The reference peptide (P)GKY20 is the recombinant form of GKY20, a cryptic CAMP derived from the C-terminus of human thrombin [56; 57]. It is a cathelicidin-like peptide with wide-spectrum antimicrobial activity [56; 57]. Polymyxin B is a cationic lipoprotein, very similar to colisin (polymyxin E), which specifically kills Gram(\(-\)) strains \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{2}{c}{**MIC (MBC) 1**} & \multicolumn{2}{c}{**MIC**} \\  & \multicolumn{2}{c}{**(\#M)**} & \multicolumn{2}{c}{**(\#M)**} \\ \hline
**Bacterial Strain** & **P13\#1** & **(P)GKY20** & **Vanc 2** & **Pol B 2** \\ \hline
**Gram(\(-\))** & & & \\ \hline _Pseudomonas aeruginosa_ PAO1 & 1.56 (1.56) & 3.12 (3.12) & 0.18 \\ \hline _Pseudomonas aeruginosa_ KK27 & 1.56 (1.56) & nd 3  & 0.36 \\ \hline _Pseudomonas aeruginosa_ RP73 & 1.56 (1.56) & 1.56 (1.56) & 0.09 \\ \hline _Pseudomonas aeruginosa_ AA2 & 1.56 (1.56) & nd & 0.36 \\ \hline _Pseudomonas aeruginosa_ PA14 & 1.56 (1.56) & 6.25 (6.25) & 0.18 \\ \hline _Klebsiella pneumoniae_ ATCC 700603 & 1.56 (1.56) & 6.25 (6.25) & 0.36 \\ \hline _Acinetobacter baumannii_ ATCC 17878 & 1.56 (1.56) & 3.12 (3.12) & 0.18 \\ \hline _Burkholderia cencopacac_ LMG 18863 & \textgreater{}50 & \textgreater{}50 & \textgreater{}46.2 \\ \hline _Escherichia coli_ ATCC 25922 & 1.56 (1.56) & 6.25 (6.25) & 0.72 \\ \hline _Salmonella typhimurium_ ATCC 14028 & 1.56 (1.56) & 3.12 (3.12) & 0.36 \\ \hline _Salmonella enteritidis_ 706 RIVM & 1.56 (1.56) & 6.25 (6.25) & 0.72 \\ \hline
**Gram(+)** & & & \\ \hline _Staphylococcus aureus_ ATCC 6538P & 0.78 (0.78) & 1.56 (1.56) & 0.34 \\ \hline _Staphylococcus aureus_ WKZ2 (MRSA) & 1.56 (1.56) & nd & 0.34 \\ \hline _Staphylococcus aureus_ Newman & 3.12 (3.12) & 6.25 (6.25) & 0.17 \\ \hline _Enterococcus faecalis_ ATCC 29212 & 1.56 (1.56) & 1.56 (1.56) & 0.34 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Antimicrobial activity. by binding to LPSs and inducing the permeabilization of their outer membranes [22]. Vancomycin is a cyclic glycopeptide that selectively inhibits Gram(+) strains by interfering with cell-wall synthesis. As expected for a cathelicidin mimic, P13#1 showed wide-spectrum antimicrobial activity, with MIC values generally below 2 uM. In the panel of investigated strains, the sole resistant strain was _Burkholderia encopaceae_ LMG 18863. It is worth noting that this strain was also resistant to (P)GKY20 and polymorphxin B. This finding was not unexpected; in fact, it is well-known that _Burkholderia_ strains are naturally resistant to membrane-targeting antimicrobial peptides due to the biosynthesis of an unusual LPS which bears positively charged groups that hinder the insertion of cationic peptides into the outer membrane, thus preventing its damage and crossing [58; 59]. We also determined the Minimum Bactericidal Concentration (MBC) values of P13#1 and (P)GKY20.