## 3 Results
### PGP Features  
In general, bacteria produce IAA for metabolizing the L-tryptophan [30]. There are several methods for studying the production of IAA in bacteria, many of which do not include the addition of purified tryptophan, exploiting that contained in the peptones of the medium for growth. For this reason, we conducted our analysis with or without the addition of the amino acid. We observed a strong increase in IAA correlated to the addition of tryptophan at the culture medium (Tables 3 and 4). This was more evident for QH23 and QH24, where a relevant increase (2.0-3.5 fold) in the IAA production was highlighted. The IAA levels were also dependent on the NaCl concentration. QH23 and M21 showed their greatest IAA production in the concentration range of 0-75 mM NaCl, in either the presence or absence of tryptophan. At 600 mM, compared to the values obtained at 0 mM, in the presence of tryptophan, the production of IAA was reduced in QH23 and M21 by 38% and 62%, respectively. In the absence of tryptophan, at 600 mM NaCl, the reductions in QH23 and M21 were 30% and 50%, respectively. Moreover, in both conditions, M23 and QH24 exhibited the greatest IAA levels at 75 and 600 mM NaCl, respectively. A minor amount of IAA was present at 600 mM NaCl for M23 (reduction of 33%). It was undetectable for QH24 in the absence of added salt (Tables 3 and 4). \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c c c c c c} \hline \hline \multicolumn{1}{c}{} & \multicolumn{5}{c}{**Colony Characteristics**} & \multicolumn{5}{c}{**Microscopy Characteristics**} & \multicolumn{5}{c}{**Temperature**} & \multicolumn{5}{c}{**NaCl Concentration**} \\ \hline
**Strain** & **Shape** & **Margin** & **Elevation** & **Texture** & **Pigment** & **Shape** & **Arrangements** & **Gram** & **28** & **37** & **45** & **50** & **55** & **1.5** & **2.0** & **2.5** & **3.0** & **3.5** \\ \hline QH23 & Circular & Entire & Flat & Mucci & White- & Road & Single or in & - & + & + & - & - & - & + & + & + & - & - \\ QH24 & Circular & Entire & Flat & Mucci & White- & Road & Single or in & - & + & + & - & - & - & + & + & + & - & - \\ M21 & Circular & Irregular & Raised & Leathery & White- & Road & Single or in & + & + & + & + & + & - & + & + & - & - & - \\ M23 & Circular & Irregular & Raised & Leathery & White- & Road & Single or in & + & + & + & + & + & - & + & + & - & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Morphological and physiological characterizations of bacterial strains. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c c c c c c c c c} \hline \hline
**Strain** & **NO\({}_{\text{S}}\)** & **TRP** & **GLU** & **ADH** & **URE** & **ESC** & **GEL** & **PNPG** & **IGLU** & **IARA** & **I** & **MNE** & **IAM** & **I** & **MAN** & **i** & **NAG** & **i** & **MAL** & **i** & **GNT** & **i** & **CAP** & **i** & **ADI** & **i** & **MLT** & **i** & **CT** & **i** & **PAC** & **OX** & **CAT** \\ \hline Q  
Siderophore production capability of the selected isolates was evaluated at different concentrations of NaCl after 1 week of incubation. The data were shown as SPI (Siderophores Production Index) measured as the ratio between the diameter of siderophores diffusion (orange halo) and that of the growth spot (Table 5). In the experimental conditions, the bacterial growth was clearly evident only at high NaCl concentrations (600 mM), except for M21, which was unable to grow at any NaCl concentration. QH24 and M23 (Figure 2B,C) exhibited peak siderophore production at 600 mM NaCl (SPI = 3.1 \(\pm\) 0.2 and 3.3 \(\pm\) 0.5, respectively), whereas QH23 did not produce any siderophores (Figure 2A). Moreover, we evaluated the production of ammonium, the phosphate solubilization capability, and the ACC deaminase activity. As for IAA and siderophores, the ammonium production was dependent on the concentration of NaCl.