INTRODUCTION Heavy metal pollution of soil has become global environmental problem due to intensively increasing industrialization and agricultural activities (Mcgrath et al., 1995). In Pakistan, there is an unchecked disposal of raw sewage from domestic and industrial areas on cropped soils. Concentrations of heavy metals (Pb, Cd and Cr) in vegetable fields irrigated with untreated sewage are advancing towards phytotoxic levels (Hussain, 2000; Qadir et al., 2000). Accumulation of heavy metals in crop plants through soil-root interface is a critical hazard (Sharma and Agrawal, 2006; Yoon et al., 2006; Maimon et al., 2009). Thus, under this scenario, there is a need to reclaim soil contaminated with heavy metals. Phytoremediation, an emerging green technology is receiving much more attention for rehabilitation of soils polluted with heavy metals because it is non-intrusive and inexpensive means of remediating contaminated soils (Alkorta and Garbisu, 2001; Prasad and Freitas, 2003; Wei et al., 2004; Ali et al., 2013). Phytoremediation can take place in many ways, but phytoextraction is an in-situ promising environment friendly technique for the reclamation of polluted soil (Nascimento and Xing, 2006). But, the success of phytoextraction is hindered by less solubility of heavy metals in soil solution and low bioavailability to plants (Turgut et al., 2004). The use of elemental sulfur in soil has been suggested to decrease pH and increase solubility and bioavailability of heavy metals in soil (Kayser et al., 2000). In Pakistan, the soils are mostly alkaline (pH>7) and calcareous (CaCO3>3%) in nature (Sharif et al., 2000; Jafar et al., 2012) with 15% soils are poor in sulfur (<10 mg/kg SO4 -2 ), and 30% (11-30 mg/kg SO4 -2 ), 33% (31-99 mg/kg SO4 -2 ) and 22% (mg/kg SO4 -2 ) soils fall in acceptable, adequate and high range, respectively (NFDC, 1992; Sutaria, 2016). Sulfur is the most important plant nutrient after nitrogen, phosphorus and potassium (Jamal et al., 2010) and is a natural acidifying element. But due to greater fixation, low availability of plant nutrients is an immense issue. Plant roots take sulfur mainly in the form of sulfate (SO4 -2 ). Sulfur oxidizing bacteria (SOB) oxidize elemental sulfur into sulfate in alkaline calcareous soils and produce sulfuric acid which decreases the soil pH and increases the solubility and plant availability of nutrients (Hitsuda et al., 2005; Abdou, 2006; El-Tarabily et al., 2006) as well as heavy metals and thus improve the condition of alkali and heavy metal polluted soils. Although, majority of the reported SOB are associated with genera Thiobacillus, Thiothrix, Thiomicrospira, Achromatium and Desulfuromonas, but heterotrophic bacteria Pak. J. Agri. Sci., Vol. 55(2), 349-359; 2018 ISSN (Print) 0552-9034, ISSN (Online) 2076-0906 DOI: 10.21162/PAKJAS/18.6319 http://www.pakjas.com.pk ISOLATION, SCREENING AND IDENTIFICATION OF LEAD AND CADMIUM RESISTANT SULFUR OXIDIZING BACTERIA Sana Ashraf 1,* , Zahir Ahmad Zahir 1 , Hafiz Naeem Asghar 1 and Muhammad Asghar 2 1 Institute of Soil & Environmental Sciences, University of Agriculture, Faisalabad, Pakistan; 2 Department of Biochemistry, University of Agriculture, Faisalabad, Pakistan * Corresponding author’s e-mail: sanaashrafenv@gmail.com Sulfur oxidizing bacteria (SOB) oxidize elemental sulfur (S ° ) or reduce sulfur compounds to generate sulfuric acid which reduces the pH of media. In this study, lead and cadmium tolerant SOB were isolated by using thiosulphate medium from samples collected from different ecologies and screened on account of pH reduction, sulfate ions production, phosphorus solubilization index and phosphorus bioleaching tests. Results indicated that sulfate ions production (800 mg/L by Bacillus sp. strain SS-16) and phosphorus solubilization (713 mg/L by Bacillus sp. strain SS-16) from rock phosphate were significantly increased as compared to control due to pH reduction (net decrease of 4.52 points). The tolerance of isolated SOB by minimum inhibitory concentration (MIC) and minimum bacterial concentration (MBC) was evaluated at different concentrations of lead and cadmium and Bacillus sp. strain SS-16 showed its ability to tolerate lead and cadmium up to 1000 and 180 mg/L, respectively. Organic acids production ability of three efficient isolated SOB (SS-4, SS-8 and Bacillus sp. SS-16) was evaluated under normal and stressed (lead and cadmium polluted) conditions. Results indicated that short chain organic acids amount increased under Pb and Cd stressed conditions as compared to normal medium. The isolated SOB were further studied for their morphological and biochemical properties. The most efficient bacterium belonged to sewage sludge ecology and was identified as Bacillus sp. SS-16. Keywords: Sulfur oxidizing bacteria, phosphorous solubilization, MIC, MBC, heavy metal resistant bacteria, organic acid profiling