STUDIES ON PLASMID STABILITY AND LTB PRODUCTION BY RECOMBINANT VIBRIO-CHOLERAE IN BATCH AND CHEMOSTAT CULTURES - A LESSON FOR OPTIMIZING CONDITIONS FOR CHEMICAL INDUCTION

Plasmid content, its stability and the expression of B-subunit of Esch erichia coli heat-labile enterotoxin (LTB) in Vibrio cholerae/r-pMMB68 system have been studied in batch as well as in chemostat cultures. U pon induction with isopropyl-beta-D-thiogalactopyranoside (IPTG), cult ures secreted LTB into the extracellular milieu. Highest specific LTB production rate of 7.3 mg mg(-1) h(-1) was achieved in batch culture i nduced at the late exponential growth phase. The plasmid pMMB68 was fa irly stable up to 20 generations, even in the absence of selection pre ssure. Instability of the plasmid was accelerated in the presence of I PTG and at higher dilution rates. Maximum productivity of 2.1 mg l(-1) h(-1) was achieved in continuous culture, which remained constant at a range of dilution rates from 0.20 to 0.35 h(-1).