Density, Diversity, and Growth-Promoting Potential of Cowpea-Associated Bacteria Isolated From Soils with Cassava Wastewater

Abstract The objective of this study was to isolate and select bacteria from soils with previous cassava wastewater deposition, aiming to identify strains capable of promoting vegetative growth of Vigna unguiculata L. Walp. Eight soil samples were collected near cassava flour processing facilities in Vitória da Conquista – Bahia and cultured on Burks and FAM media. Fifty-five bacterial isolates were obtained and evaluated for morphological characteristics, population density, calcium phosphate solubilization, sodium chloride tolerance, and ability to promote cowpea growth. Bacterial densities ranged from 2.31 × 10¹ to 2.15 × 10⁶ cells g⁻¹ soil in Burks medium and from 2.31 × 10¹ to 1.49 × 10⁵ cells g⁻¹ soil in FAM medium. Among the 55 isolates obtained, four were able to nodulate cowpea, and 27 tolerated NaCl concentrations up to 30 g L⁻¹. High bacterial densities were observed in both media, with most isolates showing rapid growth and low to moderate phosphate solubilization. Isolates tolerant to up to 30 g L⁻¹ NaCl were detected. Strains from the genera Bacillus and Paraburkholderia both widely reported as plant growth-promoting rhizobacteria (PGPR) were identified. Some non-nodulating isolates enhanced root length and dry weight, while nodulating strains UFRB BA72C2-1, UFRB FA34C2-2, UFRB FA51B1, and UFLA 03-164 T were effective in nodulation and vegetative growth promotion.