To study PNP biodegradation by immobilized cells of Bacillus
spp., entrapment of cells within beads was carried out by slightly
modifying the method of Megharaj et al. (1992). Cells of Bacillus
spp., grown in MSM containing 180 mMPNP and 0.01% yeast extract,
were aseptically harvested by centrifugation at 6000 rpm for
20 min and suspended in 50 ml sterile distilled water. Sodium
alginate was added to another aliquot of 50 ml sterilized distilled
water and gently stirred for 1 h, boiled and cooled. Bacterial cell
suspension was then added to sodium alginate solution and stirred
for 30 min to ensure proper mixing. All these steps were done
under aseptic conditions in a laminar flow hood. The mixture, with
a final concentration of 3% (w/v) sodium alginate and bacterial cells
was run down, drop-wise, through a burette into 0.4 M CaCl2
solution for stabilization. The beads were washed twice with sterilized
distilledwater before use. The bacterial cell density in a single
bead was determined following the method described by