**Background:** Soil zinc deficiency is a global agricultural problem, particularly in developing countries like Egypt, where low plant-available Zn reduces crop yield and contributes to human Zn deficiency. Conventional Zn fertilizers are costly, environmentally harmful, and rapidly convert to insoluble forms. Plant growth-promoting rhizobacteria that solubilize insoluble zinc compounds offer a sustainable alternative. This study aimed to isolate and characterize zinc-solubilizing bacteria (ZSB) from Egyptian soils, formulate them in sodium alginate beads, and evaluate their effect on maize growth and zinc accumulation.
**Methods:** Five soil samples were collected from fields in El Monofia and Giza, Egypt. Twenty bacterial isolates were obtained via serial dilution on modified Bunt and Rovira medium containing 0.1% insoluble ZnO and ZnCO₃. Zinc solubilization was assessed by measuring hydrolysis capacity (HC = clear zone diameter/colony diameter) on solid media with ZnO, ZnCO₃, or both. Zinc tolerance was tested in nutrient broth with ZnSO₄ concentrations from 20 to 500 mg/kg. The three most potent and tolerant isolates (B3, B5, C6) were identified by 16S rRNA gene sequencing. For encapsulation, bacterial cultures were mixed 1:1 (v/v) with 6% sodium alginate, dropped into 0.1 M CaCl₂ to form beads, and stored at room temperature and 4°C. Viability was tested monthly for 3 months by rehydrating beads in 0.8% NaCl and plate counting. A pot experiment was conducted in a greenhouse (30–37°C, 60–65% relative humidity) using 18 cm pots with 2 kg sterile soil. Eight treatments were tested in duplicate: negative control (no Zn, no bacteria), positive control (ZnCO₃ only), and six treatments with ZnCO₃ plus free or encapsulated B3, B5, or C6. Each pot received 3 mL bacterial inoculum or 120 beads. Maize seeds were surface-sterilized and one seed planted per pot. After 60 days, plants were harvested, dried at 70°C for 72 h, and dry weight measured. Zinc content was analyzed by atomic absorption spectrometry.
**Key Results:** Among 20 isolates, eight showed high zinc solubilization. Maximum HC (7.00 ± 0.34) was observed for isolate B5 with combined ZnO and ZnCO₃, followed by B3 (HC: 6.17 ± 0.17). B3 showed the highest HC for ZnO alone (5.50 ± 0.17), and B5 for ZnCO₃ alone (4.60 ± 0.40). Only isolates B3, B5, and C6 tolerated 500 mg/kg ZnSO₄; most others tolerated up to 400 mg/kg. 16S rRNA sequencing identified B3 as Acinetobacter calcoaceticus (100% similarity), B5 as Bacillus proteolyticus (99.84%), and C6 as Stenotrophomonas pavanii (99.40%). Encapsulated bacteria maintained viability over 3 months: B3 remained at ~8.8 log₁₀ CFU/mL at room temperature and decreased slightly to 8.7 log₁₀ CFU/mL in fridge; B5 remained at ~9.2 log₁₀ CFU/mL; C6 ranged from 8.9 to 9.15 log₁₀ CFU/mL. In the pot experiment, all bacterial treatments increased growth over controls. The highest fresh weight was 35.50 g (free B3), followed by 32.25 g (B5 beads) and 31.95 g (C6 beads). The highest dry weight was 16.19 g (free B3), then 15.46 g (C6 beads). Free B3 had the tallest shoots (32.20 cm), and free B5 the longest roots (14.43 cm). Zinc content in negative control plants was 34.28 mg/kg; positive control was 109.24 mg/kg. Free B3 achieved the highest zinc content (370.20 mg/kg), followed by C6 beads (358.28 mg/kg), free C6 (299.13 mg/kg), B3 beads (212.61 mg/kg), free B5 (200.60 mg/kg), and B5 beads (162.12 mg/kg).
**Clinical Implications:** This study demonstrates that three bacterial strains—Acinetobacter calcoaceticus, Bacillus proteolyticus, and Stenotrophomonas pavanii—can effectively solubilize insoluble zinc and enhance zinc uptake in maize. Sodium alginate encapsulation preserved bacterial viability for at least 3 months at both room and refrigeration temperatures, offering a practical formulation for storage and application. The findings support the development of low-cost, eco-friendly biofertilizers for zinc biofortification of staple crops, which could help address widespread zinc deficiency in soils and human populations, particularly in developing countries. Further field trials are needed to confirm efficacy under diverse environmental conditions.