Screening of UV mutants of Listeria monocytogenes NCIM 5260 and growth optimization for production of listeriolysin O

Listeria monocytogenes is a foodborne pathogen and the causative agent of the life-threatening disease listeriosis. To address the severity and health risk caused by Listeria, a potential and safe vaccine for protecting humans is the need of the day. In the present work, mutants of L. monocytogenes were isolated by exposing the L. monocytogenes NCIM 5260 to UV rays for different time periods. Of the isolated ultraviolet (UV) mutants, the mutant producing maximum toxin listeriolysin O (LLO) was selected for improving the production of LLO (g/L) further by optimizing the medium composition using statistical methodology. Carbon and nitrogen sources were selected using two independent Plackett Burmann designs. For optimizing the final medium composition, sucrose, peptone, pH of the medium, temperature and rpm were selected at five different levels using central composite rotatable design by Response surface methodology. Optimized medium composition of sucrose (1 g/L), peptone (10 g/L), pH (7.6), temperature (38°C), speed of the shaker (200 RPM) yielded 0.64 g/L of LLO toxin. A 3.5-fold increase in the LLO toxin (g/L) titer was observed after medium optimization. LLO toxin thus produced can be inactivated and used as a vaccine candidate either as LLO toxoid and in combination with other toxoids as bivalent vaccine.