# Evaluating Soil Bacteria for the Development of New Biopreparations with Agricultural Applications
## 4 Discussion
Our study is in the first to investigate the cytotoxicity of soil bacterial metabolites towards the Sf-9 insect cell model. Based on the obtained results, we recommend the following uses of the isolated bacterial strains: _B. licheniformis_, _B. subtilis_, and _B. velezensis_ as biocontrol agents; _B. subtilis_, _B. licheniformis_, _B. velezensis_, _P. amylolytica_, and _P. megaterium_ as plant growth biostimulants; _B. subtilis_, _B. licheniformis_, _B. velezensis_, _P. amylolytica_, and _P. megaterium_ as crop residue decomposition stimulants; _B. licheniformis_ and _B. subtilis_ as bioinsecticides. These microorganisms can be used individually or in a mixture of several strains (to produce a biopreparation with a broad spectrum of action), provided that the cultivation conditions are optimized to obtain the highest possible yield of spores. In the current study, samples were collected exclusively from fields after maize cultivation in central Poland (warm temperate zone of the transitional type). It should be noted that they may not be microorganisms characteristic of other types of crops and climatic conditions in the world, which may affect the adaptability of the tested bacteria in the conditions of target use as soil biopreparations. Future studies incorporating diverse soil types, climate regions, and more refined experimental designs are necessary to generalize the findings and fully evaluate bacterial adaptability for broader agricultural applications. Additionally, the experimental conditions, such as temperature range and nutritional assessments, are currently under further investigation to provide a more comprehensive understanding of bacterial growth characteristics. Future work should also focus on assessing the efficacy of using the isolated strains under conditions simulating the target use--e.g., in plant microcosms and greenhouse crops and assessment of the impact of applying these strains on the abundance and biodiversity of native soil microbiota. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app15126400/s1](https://www.mdpi.com/article/10.3390/app15126400/s1)]([https://www.mdpi.com/article/10.3390/app15126400/s1](https://www.mdpi.com/article/10.3390/app15126400/s1)). Supplementary materials: Table S1. Description of the tested soil samples, Table S2. Characteristic of harvest residues samples, Table S3. Media and staining solutions used in research, Table S4. The optimal growth conditions of isolated bacterial strains after 7 days, Figure S1. Hydrolytic activities of selected WK strains on screening media supplemented with (1) skimmed milk: (a) strain WK-2 after 5 days at 30\({}^{\circ}\)C (EAI = 1.59); (b) strain WK-20 after 5 days at 30 \({}^{\circ}\)C (EAI = 1.14) and (c) strain WK-7 after 5 days at 30 \({}^{\circ}\)C (no proteolytic activity); (2) soluble starch: (d) strain WK-7 after 5 days at 30 \({}^{\circ}\)C (without iodine staining); (e) strain WK-7 after 5 days at 30 \({}^{\circ}\)C (with iodine staining; no amylolytic activity) and (f) strain WK-6 after 5 days at 30 \({}^{\circ}\)C (EAI = 0.33); (3) carboxymethylcellulose: (g) strain WK-7 after 4 days at 30 \({}^{\circ}\)C (without Congo Red staining); (h) strain WK-13 after 5 days at 30\({}^{\circ}\)C (EAI = 0.30) and (i) strain WK-14 after 5 days at 30 \({}^{\circ}\)C (no cellulolytic activity). Conceptualization, J.S. and P.R. ; Methodology, J.S., P.R., M.S. ([PERSON]), R.J., A.M.B., B.G., and A.N. ; Formal analysis, P.R., M.S. ([PERSON]), and A.N. ; Investigation, J.S., P.R., M.S. ([PERSON]), M.S. ([PERSON]), R.J., and A.N. ; Resources, J.S., A.M.B., R.J., and B.G. ; Data curation, J.S. ; Writing--original draft preparation, J.S., P.R., M.S. ([PERSON]), A.N., A.M.B., B.G., and M.S. ([PERSON]); Writing--review and editing, J.S., P.R., and M.S. ([PERSON]); Visualization, J.S., P.R., M.S. ([PERSON]), R.J., and A.N. ; Supervision, J.S. ; Project administration, J.S. ; Funding acquisition, J.S. All authors have read and agreed to the published version of the manuscript.