Isolation, characterisation and evaluation of sulfur-oxidising bacteria from rhizospheric soil samples of diverse agronomic crops

Abstract

Sulfur is a crucial macronutrient that influences plant growth and development. It is considered the sixth major macronutrient after nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg). Among diverse agronomic crops, pulses and oilseed are more dependent on sulfur than cereals. However, sulfur deficiency is becoming a significant constraint on crop productivity and quality because of the excessive use of fertilisers and continuous cropping. Soil naturally contains sulfur in forms such as elemental sulfur and thiosulfate, which are not readily available to plants. Soil microorganisms, especially bacteria, convert these substances into sulfate, after oxidation, making them accessible to plants. In the present study, effective sulfur-oxidising bacteria (SOBs) were isolated and characterised from rhizospheric soil samples of paddy, black gram and maize fields. The SOB were isolated using modified thiosulfate agar medium and assessed for pH reduction, sulfate ion production, extracellular enzyme production and plant growth-promoting activities (PGPA) using standard methods. A total of 93 SOB isolates were obtained during primary screening. An effective SOB isolate, designated SOB6, exhibited the highest sulfur-oxidising activity, reducing pH to 4.47 and producing 103.7 mg/L sulfate. It was identified and characterised through morphological, biochemical and molecular methods. Molecular analysis based on 16S rRNA gene sequencing identified the isolate SOB6 as Priestia flexa, confirming its sulfur-oxidising potential and expanding the known functional diversity of this species. In conclusion, P. flexa SOB6 demonstrated efficient sulfur-oxidising activity under laboratory conditions, indicating its potential role in sulfur transformation processes and warranting further investigation for agricultural applications.