## 1 Introduction  
The development of low consumption energy technologies is of growing relevance in sustainable agriculture to allow sustainable food production while supporting an increasing world population. One possible low energy and environmentally friendly technology is the utilization of plant growth-promoting bacteria (PGPB), which are microorganismsthat live in association with plants, covering practically all their surfaces and colonizing their interior, the xylem vessels in particular [1]. Among PGPB strains inoculated to increase agricultural productions, _Azospirillum_ is one of the most well-known. _Azospirillum brasilense_, together with _Gluconacetobacter diazotrophicus_[2], _Herbospirillum seropedicae_[3], and _Burkholderia ambifaria_[4] strains live in association with crop plants. Carrots host a specific microbial biomass, composed of Gram positive and negative bacteria, actinomycetes, and fungi, that change according to agronomic management [5]. Furthermore, as reported by [PERSON] and [PERSON], cocci are the predominant endophytic bacterial community compared to other salad vegetables, namely cucumber, onion, and tomato. This crop is particularly suitable for Fucino plateau, the main agricultural productive area of the Abruzzo region where the greatest carrots production is obtained [6]. This consortium has already demonstrated to have positive effects on the growth of _Allium cepa_ L. [7], _Artemisia eriantha_ Ten [8], _Cannabis sativa_ L. [9], _Lycopersion esculent_ L. [10], and ancient _Triticum_[11]. Based on these positive effects, we hypothesized that a _A. brasilense_, _G. diazotrophicus_, _H. seropedicae_, and _B. ambifaria_ consortium could be a valid biostimulant tool for _Daucus carota_ L.  
Strains belonging to PGPB can promote plant growth and development through different mechanisms. Among them, the production of auxins plays an important role in plant growth stimulation [12]. The best-known molecule belonging to this class is indole-3-acetic acid (IAA). Soil bacteria differ in their IAA synthesizing capability depending on soil fertility status and organic matter content [13]. Bacteria use this phytohormone to interact with plants as part of their colonization strategy. Furthermore, this IAA can also be a signalling molecule in bacterial communication [14]. Plant growth and development promotion ascribed to PGPB is also due to their ability to solubilize soil nutrients, such as phosphorus [15]. The plant growth-promoting (PGP) strains with this capability are known as phosphate-solubilizing bacteria (PSB) and can convert inorganic and inaccessible forms for the absorption of phosphate (PO\({}_{4}\)\({}^{3-}\)) into available ones (e.g., HPO\({}_{4}\)\({}^{-2}\), H\({}_{4}\)PO\({}^{4-}\)) [16]. This conversion is usually mediated by the organic acids produced by PSB, which induce the release of phosphates from insoluble complexes by using various approaches (i.e., lowering soil pH, chelation, and mineralization) [17]. The bacteria belonging to PGPB also help plants to counteract stress-related to biotic and abiotic factors that play an essential role in diminishing crop productivity [18; 19]. This stress affects plant-water relation at cellular and whole plant level, causing specific and unspecific reactions and damage [18]. However, PGPB possess different mechanisms counteracting them. The decrease of the deleterious synthesis of \"stress ethylene\", through 1-aminocyclopropane-1-carboxylate (ACC) deaminase enzyme, is one of these principal mechanisms [18]. The direct precursor of ethylene in the biosynthetic pathway is ACC [20]. The enzyme ACC deaminase catalyzes the cleavage of ACC to ammonia and \(\alpha\)-ketobutyrate, decreasing the synthesis of stress ethylene [21]. The presence and activity of this enzyme have been detected in several PGPB species [22; 23]. Several techniques can be applied to investigate changes in the microbial community.