Isolation of luminescent bacteria from Bay of Bengal and their molecular characterization

Luminescence is the emission of light by an object. Living organisms including certain bacteria are capable of luminescence. Bacteria are the most abundant luminescent organisms in nature. Bacterial luminescence has been studied most extensively in several marine bacteria. Bacterial luminescence is due to the action of the enzyme called luciferase. The luminescent bacteria exist in nature either as free living bacteria or in symbiotic association ship with certain marine organisms. Research on luminescent bacteria has always been a fascinating one. In the present study, twenty free living luminescent bacteria were isolated from Bay of Bengal, India using soft agar overlay method in sea water complex agar (SWCA). All the 20 strains were characterized for certain biochemical tests and they were tentatively identified that they are all Photobacterium spp. The effect of salinity, pH glycerol concentration and heavy metals on the growth and luminescence of these 20 strains was also studied. In this part of experiment, visual scoring was done to categorize the luminescence. In case of salinity, it has been found that up to 6% of NaCl the intense of luminescence was good and thereafter it declined. Further, in some strains it was completely ceased beyond 9% of salinity. Luminescence was not greatly affected by pH in liquid medium however; the same was affected in solid medium. The intensity of luminescence has increased with increasing concentrations of glycerol ranging from 0.3 to 1.2%. All the 20 luminescent bacteria were characterized for their tolerance to heavy metals and antibiotics. Copper and zinc at 1 mg/ml concentration have inhibited the growth and luminescence of the all strains. Surprisingly, mercury at the same concentration has inhibited only two strains (AMET1913 and AMET1920). However, at 2 mg/ml concentration mercury has inhibited the growth and luminescence of all the 20 strains. Selected six luminescent bacterial strains were also characterized for their antibiotic susceptibility against six different antibiotics. It has been found that most of the strains were sensitive to all the six antibiotics tested. Since, the bioluminescence is regulated by quorum sensing, the effect of culture filtrate extracted with dichloromethane was also tested for its effect on luminescence. These DCM extracts haven‟t influenced the luminescence much