Bacterial Endophytes of the Medicinal Herb Hygrophila spinosa T. Anders and Their Antimicrobial Activity

Aims: The ethnobotanical herb Hygrophila spinosa T. Anders (Acanthaceae) is native to India and used in traditional ayurvedic medicines for its pharmacologically important phytochemicals. This study aims to isolate and characterize the culturable bacterial endophytes of H. spinosa and evaluate their antimicrobial properties.
Place and Duration of Study: The experiments were performed in the Department of Botany, Serampore College, Serampore as well as in the Microbiology Laboratory, Department of Botany, University of Calcutta, Kolkata during 2011 to 2012.
Methodology: Bacterial endophytes were isolated from healthy plant tissues following surface sterilization and plating on nutrient agar, glycerol asparagine agar and tryptic soy agar. They were characterized physio-biochemically following standard microbiological and biochemical methods. The endophytes were screened for production of antimicrobial compounds following cross-streak assay against test strains Bacillus subtilis, B. cereus, Escherichia coli, Pseudomonas cepacia, Klebsiella pneumoniae and Staphylococcus aureus on nutrient agar plates.
Results: Eleven phenotypically distinguishable bacterial endophytes were isolated from surface sterilized leaf, stem and root tissues and Shannon Weaver diversity index clearly revealed more diverse (0.83) types of endophytes in leaves than in stem (0.48) and root (0.41) tissues. Physio-biochemical features of the isolates clearly indicated distinct variation in their sugar fermentation profiles along with NaCl tolerance. The endophytes produced important enzymes like catalase, amylase, gelatinase, nitrate reductase and lipase. The bacterial isolates belonged to the genera Bacillus, Paenibacillus, Pseudomonas, Ralstonia, Staphylococcus, Micrococcus and Acidomonas. Antibiotic sensitivity profile, however, have indicated that the isolates were mostly resistant to amoxycillin and bacitracin, while they were highly susceptible to tetracycline followed by neomycin and streptomycin. Interestingly, the bacterial endophytes of H. spinosa give a definite stamp on their antimicrobial activity against E. coli and K. pneumoniae followed by S. aureus. Two isolates, Paenibacillus HGS 202 and Acidomonas HGR 302 obtained from stem and root segments respectively showed antimicrobial activity against B. subtilis, B. cereus, E. coli, K. pneumoniae and S. aureus.
Conclusion: This study identified 11 bacterial endophytes harbored by the leaves, stem and root of H. spinosa which demonstrated antibacterial activity against Gram-positive as well as Gram-negative bacterial strains. Moreover these endophytic bacterial isolates could be exploited as sources of antibacterial substances.