## 4 Discussion
Both microbes and plants have a high iron requirement (about 1 \(\times\) 10\({}^{-5}\)-1 \(\times\) 10\({}^{-7}\) and 1 \(\times\) 10\({}^{-4}\)-1 \(\times\) 10\({}^{-9}\) M, respectively) and this need is even more accentuated in the rhizosphere, where the competition for this element among plant roots, bacteria and fungi is very intense [39]. Bacteria react to iron deficiency by siderophore synthesis [40] and are involved in both direct (stimulation of plant growth by improving iron nutrition) and indirect effects (stimulation of plant growth by phytopathogen inhibition) on plant growth. The positive effects of siderophores synthesized by bacteria on plant growth have been demonstrated by [PERSON] et al. [41]. In this work, _Arabidopsis thaliana_ plants were supplied with ferri-siderophores produced by _Pseudomonas fluorescens_ C7 as a sole source of iron, with the result that the bacterial Fe-pyoverdine complex was efficiently taken up by the plants and an increased iron content inside plant tissues associated with the improved plant growth was observed [41]. On the otherhand, siderophores produced by PGPB limit iron acquisition by phytopathogenic fungi, which are unable to produce iron chelators with same the high affinity for iron as the siderophores produced by PGPB. Plant pathogen suppression by bacterial siderophores has been reported in the biocontrol of _Macrophomina phaseolina_, _Rhizoctonia solani_, _Pythium_ spp., and _Fusarium_ spp. [40]. Additionally, bacterial siderophores may trigger induced systemic resistance (ISR) in plants, thereby enhancing plant resistance to phytopathogen infection [42] as observed in rice infected by _Magnaporthe oryzae_ and treated with a pseudobactin siderophore synthesized by _Pseudomonas fluorescens_ WCS374r. Plants inoculated with a mutant of this bacterial strain unable to produce pseudobactin were no longer resistant to growth inhibition by this pathogen [43]. Based on the taxonomic identification and the physiological traits involved in plant growth stimulation, eight endophytic bacterial strains (two isolated from cucumber and six from sorghum) were selected for further analysis. In view of a future possible application of these strains as biofertilizers in field conditions, it was considered essential to assess their antibiotic resistance profile. In fact, in the environment and especially in soil, antibiotic resistance genes may be acquired through horizontal gene transfer from innocuous bacterial species to species considered as potentially pathogenic [44]. Due to the extensive use of antibiotics in animal farming in recent decades, soil represents a sink of antibiotic resistant human pathogens [45; 46; 47; 48]. Antibiotic resistance in opportunistic pathogens is a serious concern for human and animal health and is strictly associated with increased hospitalization and mortality rates [49; 50]. Moreover, antibiotic resistant pathogenic bacteria, as well as their antibiotic resistance genes, can be transferred from organic fertilizers and manure to vegetables and fruits via irrigation [51; 52]. It has been recently reported [44] that antibiotics can be taken up by plant roots, and then translocated to the fruit and leaves, where a sub-lethal amount of antibiotics (such as tetracycline, streptomycin, and ciprofloxacin) has been detected. In addition, plant internal colonization by opportunistic human pathogens occurs in different crops [53; 54]. In this regard, none of the eight selected strains belonged to species reported to be a human opportunist pathogen and they all showed a low level of resistance to the antibiotics considered in this work. All eight strains were used in order to assess their effective plant internal colonization ability. Only five strains were detected after re-inoculation inside sorghum roots and/or leaves, with a density of around \(1\times 10^{3}\) CFU/g plant tissue.