## 4 Discussion  
Microorganisms may significantly increase plants' resistance to abiotic and biotic stresses if their unique tolerance to extreme environmental conditions, ubiquity, genetic diversity, and interactions with crops are properly taken advantage of, and if methods of their successful application in agricultural production are developed. The use of individual species of microorganisms or microbial consortia as biopreparations or biofertilisers may influence the yield of crops and thus provide new methods for the application of microorganisms in agriculture, including the cultivation of legumes. A key factor influencing the beneficial effects of bacterial consortia is the interaction between their members to guarantee a stable long-term co-existence [20]. The communication relies on the production, detection, and response to extracellular signalling molecules that regulate and shape the bacterial population in the consortium, where only compatible microbes are involved in altering the plant defence response affecting overall plant health and growth [21]. Consortium communication is highly dependent on molecular signals; among them, quorum sensing plays a significant role in bacterial compatibility in consortium formulations [22]. Quorum sensing allows bacteria to switch between two distinct gene expression programmes: (i) one at low cell density for individual and asocial behaviour, and (ii) another at high cell density for social and group behaviours, which are preferential for consortia [23]. The results of the in vitro investigations revealed some interactions which may occur between the endophytic bacteria of the _Bacillus_ and _Pseudomonas_ genera and the symbiotic bacteria of the _Bradyrhizobium_ sp. genus, which were selected for the research. Reference  
publications provide a lot of information about the properties of bacteria belonging to the PGPR group and their influence on plants. However, there have been few studies analysing the direct interactions between endophytic bacteria and rhizobia. [PERSON] and [PERSON] [24] conducted research on interactions between the strains of _Sinorhizobium meliloti_ (rhizobia) and _Herbospirillum frisgense_ (endophytic bacteria). The authors observed that _H. frisgense_ stimulated the growth of _S. meliloti_ in all variants of the experiment (48 h culture, sediment, supernatant). When the same bacterial strains were used for inoculation in the reverse order, _S. meliloti_ had a positive effect on _H. frisgense_ in the sediment obtained after centrifugation, but there were antagonistic relations observed in the other cases. The negative effects were explained by the fact that during incubation, secondary metabolites produced by _H. frisgense_, which were toxic to _S. meliloti_, had accumulated in the medium. The analysis of the influence of the endophytic strains on the symbiotic bacteria used in our experiment showed the mutualistic effect of the endophytic bacteria selected for coinoculation. This was manifested by increased microbial growth around the wells. It is impossible to definitely determine what stimulated the growth of the endophytic bacteria. However, it is very likely that this effect of _Bacillus subtilis_ and _Pseudomonas fluorescens_ on the strains of rhizobia was caused by the production of phytohormones, such as indoleacetic acid (IAA), gibberellins, and cytokins [12; 25], or by the production of vitamins by bacteria of the _Bacillus_ genus [26]. By contrast, the bacteria of the _Bradyrhizobium_ genus used in the experiment were neutral to the endophytic bacteria. Such an ambivalent interaction between bacterial strains towards each other was also observed by [PERSON] et al. [27].