iv 
 

 

Preface 
 

You have to dream before your dreams can come true. 

~ Dr. A.P.J. Abdul Kalam 

 

My love and interest in science budded at a very young age, back when I was in 

elementary school. To realize the role and importance of science in everyday life, we were 

assigned to enlist every aspect of daily routines where science was at play. The endless 

list only got me more intrigued to observe and learn more. Residing in a society where 

numerous superstitions played a significant role and shaped the outlook of the people, my 

parents delved into inculcating a sense of judgment through reasoning and scientific basis. 

This inclination was further extended by my teacher, Mr. Nabarun Mondal, who made 

learning fun and interesting and helped me build a strong base, advancing beyond the 

syllabus to satiate my hunger for knowledge. He taught me to think big and aim at the 

sky, and accept every opportunity to follow my dreams. Since then, I have been 

determined to contribute to the field of science, as small as it may be, with the then 

President of India, Dr. A.P.J. Abdul Kalam, as my role model. 

 Though the journey has not been a bed of roses and some people around me 

derided me for dreaming big of being a scientist, affecting my confidence for a while, in 

the long run only to act as a pullback to soar higher and help me build a strong resolute 

to make my dream a reality, as Walt Disney had quoted “All our dreams can come true, 

if we have the courage to pursue them”. And years after, in 2018, I set out to commence 

my new journey as a Ph.D. scholar in the Molecular and Tissue Culture Laboratory in the 

Department of Tea Science, University of North Bengal, under the supervision of Dr. 

Malay Bhattacharya. It was an overwhelming experience as it was a big leap towards the 

fulfillment of my as well as my parents’ dream. Words are simply not enough to express 

my gratitude to my supervisor, Dr. Malay Bhattacharya. He believed in and encouraged 

me throughout these years of my learning and put forth positive challenges in my path to 

help shape me as a successful researcher and individual. His role in this journey has been 



v 
 

pivotal in inculcating deep thinking and analyzation quality, and never stopping me from 

feeding my boundless curiosities. His unwavering faith in me over the last few years has 

pushed me further to invest my best in every phase of my research life. I also extend my 

heartfelt gratitude to my thesis research committee members, Dr. Sonali Ray (Assistant 

professor), Dr. Chandra Ghosh (Associate professor) and Dr. Biswajit Sinha, for their 

support and valuable feedback. I am also thankful to Dr. Tanmayee Mishra for her kind 

words and support. I would also like to thank Mr. Mainak Mukherjee (Soil analyst) who 

helped me learn a few techniques in soil analysis. My appreciation also goes out to our 

department's non-teaching staff members, Mr. Subhash Singha, Mrs. Kalpana Subba and 

Mrs. Doli Mahato, for their cooperation.  

 I am especially indebted to Sukanya Acharyya, my fellow lab mate, roommate, 

and my closest friend from my post-graduation journey. She has been my rock and helped 

me learn and grow. She has been a very important figure in all my highs and lows during 

my tenure, with whom I’ve shared all my successes and failures, laughs and tears. She 

has been the greatest critic, but always had my back through any adversities. The list 

might go on. I greatly value your companionship. I am also profoundly grateful to my 

fellow lab mates and seniors, Arindam Ghosh, Soumya Majumder, Sahadeb Sarkar, Dr. 

Sourav Chakraborty, Dr. Reha Labar and Dr. Gargi Sen, and my juniors Preeti Subba and 

Sudeshna Nandi. They have been a source of constant motivation in every phase of this 

journey. Starting from our sample collection journeys to attending conferences and 

endless discussions in the laboratory regarding work and others, it has always been 

enlightening and fun. They had so much to offer and I consider myself lucky to get to 

know them all and learn so many things. I also extend my humbleness to the other scholars 

of this department, who have been a constant source of support in this endeavour. 

 I take this opportunity to offer special thanks to my close friends Sudipta 

Bhattacharjee, Baishakhi Ghosh, Priya Bhuiyya, Debrupa Sarkar, Moumita Mukherjee, 

Subhashree Mandal, and Suchaita Ghatak, who have been all ears to the quibbles and 

bants and yet understood and encouraged me and helped bring a smile on my face in my 

lows. I acknowledge their constant and unconditional support in these days when true 

friendships are rare. Also, a huge thanks goes to my energy source in the early mornings 

and late nights, coffee, and everyone who provided me with this fuel.  



vi 
 

 Last but not least, this whole journey and learning experience would not have seen 

the face of the day without the immense support of my parents. They are the roots of the 

flourishing career choices that I made way back. They are the source of my dreams and 

the most important figures to allow me to keep dreaming and taking every necessary step 

to put life to it. No acknowledging words are enough to express gratitude for the sacrifices 

they made all their life by giving me the space and time to be a self-dependent individual. 

Ma and Bapi, I promise to make you proud one day. 

  



xiii 
 

LIST OF FIGURES 
 

 Figures Page No. 

Figure 1 Role of phosphate solubilizing microorganisms 9 

Figure 2 Processes involved in the solubilization of phosphates by phosphate 

solubilizing microorganisms 

12 

Figure 3 Contribution of phosphate solubilizing microorganism in plant 

growth 

20 

Figure 4 Genetic engineering of phosphate solubilizing microorganisms 23 

Figure 5 Soil collection site map in the Darjeeling hills 48 

Figure 6 Plates exhibiting pesticide tolerance of the isolated consortia against 

four different concentrations 

56 

Figure 7 Graphical representation of the acquired pesticide tolerance by the 

phosphate solubilizing microorganism consortia isolated from 

inorganic vs organic tea plantations 

65 

Figure 8 Plates exhibiting heavy metal tolerance of the isolated phosphate 

solubilizing microorganism consortia 

69 

Figure 9 Heavy metal tolerance of the consortia isolated from organic tea 

plantations 

71-72 

Figure 10 Heavy metal tolerance of the consortia isolated from inorganic tea 

plantations 

75-76 

Figure 11 Plates exhibiting antibiotic tolerance of isolated phosphate 

solubilizing consortia 

79 

Figure 12 Graphical representation of antibiotic tolerance of phosphate 

solubilizing microorganism consortia isolated from organic tea 

plantations 

80 

Figure 13 Graphical representation of antibiotic tolerance of phosphate 

solubilizing microorganism consortia isolated from inorganic tea 

plantations 

81 

Figure 14 Plates exhibiting antifungal tolerance of the phosphate solubilizing 

microbial consortia 

82 

Figure 15 Graphical representation of antifungal tolerance of phosphate 

solubilizing microorganism consortia isolated from organic tea 

plantations 

83 



xiv 
 

Figure 16 Graphical representation of antifungal tolerance pattern of phosphate 

solubilizing microorganism consortia isolated from inorganic tea 

plantations 

84 

Figure 17 Isolation of pure culture phosphate solubilizing bacteria from the 

consortium 

85 

Figure 18 Biochemical characterization tests 93 

Figure 19 Growth study of the phosphate solubilizing bacterial isolates 95-96 

Figure 20 Dendrogram based on the biochemical characters of the phosphate-

solubilizing bacterial isolates 

98 

Figure 21 100% query cover of samples PSMR 6.4.3 and PSMR 5.3 100 

Figure 22 NCBI Accession for the partial 16S rDNA sequence 101-102 

Figure 23 Phylogeny analysis of the isolated phosphate-solubilizing bacteria 103 

Figure 24 Phylogeny analysis of the Staphylococcus hominis strains 104 

Figure 25 Plates displaying pesticide tolerance by the phosphate solubilizing 

bacteria at different concentrations 

105 

Figure 26 Average inhibition zones exhibited by the phosphate solubilizing 

bacterial isolates at all the four working concentrations (25, 50, 100 

and 200 µl/10 ml) of pesticides used 

105 

Figure 27 Average of the summation of inhibition zones exhibited by the 

isolates against the nine studied pesticides 

106 

Figure 28 A.Isolates inoculated in undiluted pure pesticide B. Viability of the 

isolates after fifteen days 

109 

Figure 29 Graphical representation of heavy metal tolerance of the phosphate 

solubilizing bacterial isolates at concentrations 2.5 mg/ml and 5 

mg/ml 

112-113 

Figure 30 Dendrogram based on heavy metal resistance of the phosphate-

solubilizing bacterial isolates at (a) 2.5 mg/ml, and (b)5 mg/ml 

116 

Figure 31 Plates displaying antibiotic tolerance assay of phosphate-solubilizing 

bacterial isolates 

119 

Figure 32 Graphical display of effectivity of the antibiotics used against the 

phosphate solubilizing bacterial isolates 

124 

Figure 33 Dendrogram depicting relatedness of the phosphate-solubilizing 

bacterial isolates based on their antibiotic susceptibility 

125 

Figure 34 In vitro plant growth promoting tests A. Indole acetic acid production 

B. Ammonia production C. Siderophore production D. Seed 

germination 

130 



xv 
 

Figure 35 Graphical representation of the pot experiment demonstrating the 

growth pattern of phosphate solubilizing bacterial isolate treated and 

non-treated Phaseolus vulgaris 

131 

Figure 36 Degradation and utilization of diesel by the phosphate solubilizing 

bacterial isolates 

132 

Figure 37 Degradation and utilization of petrol by the phosphate solubilizing 

bacterial isolates 

134 

Figure 38 Representative chromatogram of the sample PSMR 5.7 137 

Figure 39 Representation of the bioactive plant growth-promoting (PGP) 

compounds among all the metabolites obtained through area 

percentage coverage 

137 

Figure 40 Distribution of types of different metabolites (a) Area percentage of 

each chemical class of plant growth-promoting metabolites (b) 

Percentage of plant growth-promoting metabolite produced by each 

isolate based on their chemical class 

138 

Figure 41 Biosynthesis of hydroxamic acid (siderophore) 142 

Figure 42 Schematic representation of the proposed biosynthesis pathway of 

the metabolites as detected through GC-MS analysis following 

fundamental bacterial metabolism 

148 

Figure 43 Probable biosynthesis of ammonia through hydrolysis of detected 

metabolites - benzamide and butyramide by the enzyme amidase 

148 

Figure 44 Formation of bacterial alginate beads 152 

 

  



xvi 
 

LIST OF TABLES 
 

 Tables Page No. 

Table 1 List of working pesticides with their types and mode of action 38 

Table 2 List of antibiotics used in the experiment 40 

Table 3 Details of the soil collection sites 48 

Table 4 Moisture content of the collected soil samples 49 

Table 5 pH values of the soil samples 50 

Table 6 Electrical conductivity of the soil samples 51 

Table 7 Percentage of organic carbon and organic matter in the soil samples 53 

Table 8 Total percentage of nitrogen content in the soil samples 53 

Table 9 Quantification of available phosphorus in the soil samples 55 

Table 10 Inhibition zones of all 24 phosphate solubilizing microorganism 

consortia at four different concentrations against nine pesticides 

59 

Table 11 Colour-coded representation of the heavy metal tolerance of 

phosphate solubilizing microorganism consortia at concentrations 

2.5 mg/ml and 5 mg/ml 

67 

Table 12 Colour-coded representation of the antibiotic tolerance of phosphate 

solubilizing microorganism consortia 

79 

Table 13 Colour-coded representation of the antifungal tolerance of phosphate 

solubilizing microorganism consortia 

82 

Table 14 Solubilization index of the phosphate solubilizing bacterial isolates 86 

Table 15 Gram stain classification of the phosphate solubilizing bacteria 87 

Table 16 Motility and sulphide and indole producing ability of the isolated 

phosphate solubilizing bacteria 

87 

Table 17 Catalase test of the phosphate solubilizing bacteria 88 

Table 18 Coagulase test of the phosphate solubilizing bacteria 89 

Table 19 Citrate utilization test of the phosphate solubilizing bacteria 89 

Table 20 Urease test of the phosphate solubilizing bacteria 90 

Table 21 Starch hydrolysis ability of the phosphate solubilizing bacteria 90 

Table 22 Gelatin hydrolysis test of the phosphate solubilizing bacteria 91 

Table 23 Triple sugar iron test results of the phosphate solubilizing bacteria 92 

Table 24 Methyl red Voges-Proskauer test of the phosphate solubilizing 

bacteria 

93 

Table 25 Nitrate reduction test of phosphate solubilizing bacteria 94 

Table 26 Blood agar test of the phosphate solubilizing bacteria 95 



xvii 
 

Table 27 Identification of the phosphate solubilizing bacteria through 16S 

rRNA sequencing, their percentage query cover and identity score 

100 

Table 28 Accession numbers of the identified bacteria in GenBank 101 

Table 29 Colour-coded representation of pesticide tolerance of the phosphate 

solubilizing bacterial isolates against four different concentrations of 

pesticide 

104 

Table 30 Colour-coded map of minimum inhibition concentration (MIC) of 

the phosphate solubilizing bacterial isolates against six pesticides 

108 

Table 31 Maximum tolerance concentration (MTC) of the phosphate 

solubilizing bacterial isolates against mercury 

118 

Table 32 Maximum tolerance concentration (MTC) of the phosphate 

solubilizing bacterial isolates against cadmium 

118 

Table 33 Colour-coded map of antibiotic susceptibility assay of the eight 

phosphate solubilizing bacterial isolates 

119 

Table 34 Quantification of indole acetic acid production by the isolates 126 

Table 35 Ammonia production by the phosphate solubilizing bacterial isolates 127 

Table 36 Hydrogen cyanide production by the isolates 127 

Table 37 Production of siderophores by the isolates 128 

Table 38 In vitro seed germination study 129 

Table 39 Dry weight of the PSB treated and non-treated Phaseolus vulgaris 131 

Table 40 Incubation period of the isolates for the extraction of secondary 

metabolites produced 

135 

Table 41 Active plant growth promoting compounds obtained through GC-MS 

of the eight phosphate solubilizing bacterial isolates with their 

solubility profile 

139 

Table 42 Compounds having PGP activities, their chemical classification and 

biosynthesis pathways 

145 

Table 43 Pearson correlation among eight phosphate-solubilizing bacterial 

isolates 

150 

Table 44 Quantification of available phosphate induced by the action of the 

isolated phosphate solubilizing bacteria in sterilized untreated soil 

153