Journal of Advances in Microbiology

Phosphorus availability is a major factor influencing plant growth, while beneficial bacteria associated with plant tissues and the rhizosphere can contribute to nutrient mobilisation. Gluconacetobacter and phosphate-solubilising bacteria are of particular interest because of their plant-associated characteristics. However, information on the phenotypic and biochemical characteristics of beetroot-associated Gluconacetobacter and rhizospheric phosphate-solubilising bacteria remains limited. The present study aimed to isolate and characterise Gluconacetobacter associated with beetroot and phosphate-solubilising bacteria (PSB) from the beetroot rhizosphere. Nineteen root and rhizospheric soil samples were collected from beetroot-growing locations in Hatkanangale, Karveer, Kagal and Shirol tehsils of Kolhapur district. Gluconacetobacter and PSB were isolated using serial dilution and pour-plate techniques on LGIP and Pikovskaya's media, respectively. Five Gluconacetobacter isolates, designated Ga-I to Ga-V, and three PSB isolates, designated PSB-I to PSB-III, were obtained and characterised using morphological, Gram-staining and biochemical tests. All Gluconacetobacter isolates were Gram-negative, rod-shaped and circular in colony form, with colony colours ranging from yellowish orange to deep orange. The PSB isolates were also Gram-negative and rod-shaped, produced white colonies, and showed circular or irregular colony morphology. Biochemical testing demonstrated variation among isolates in methyl red reaction, H2S production, oxidase activity and other evaluated characteristics. Phosphate-solubilisation ability of the PSB isolates was assessed on Pikovskaya's medium containing tricalcium phosphate. All three isolates produced clear zones indicating phosphate-solubilising activity. PSB-I recorded the highest solubilisation index of 2.83, followed by PSB-II at 1.92 and PSB-III at 1.70. These observations demonstrate phenotypic and biochemical variability among the bacterial isolates recovered from beetroot-associated samples and identify PSB-I as the isolate showing the highest phosphate-solubilisation index under the conditions evaluated.