A comparative study on cellular networks and differential gene expressions involved in swarming motility and biofilm formation in hyper-swarming and normal-swarming Bacillus cereus strains under various stress conditions

Bhadani, Nirbhay Kumar (2022) A comparative study on cellular networks and differential gene expressions involved in swarming motility and biofilm formation in hyper-swarming and normal-swarming Bacillus cereus strains under various stress conditions. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Nirbhay Kumar Bhadani (11RS032)) 11RS032.pdf - Submitted Version Restricted to Repository staff only Download (7MB)

Abstract

Bacteria evolved with the abilities to survive under different environmental conditions. Swarming and biofilm formation on different surfaces are two of the most important survival strategies observed in the bacterial world. The present study was involved in the comparison of swarming and biofilm formation between the natural hyperswarmer MHS and the normal swarmer MSM-S1 strains of Bacillus cereus. It was observed that the swarming motility and hence colony growth was reduced with imposed harsh conditions and the effect was more prominent for the natural hyperswarmer strain MHS as compared to the normal swarmer MSM-S1 and the decreased swarming in MHS was found to be associated with decreased flagellation. Interestingly, a considerable number of bacterial nanotubes, connecting other cells and substratum, were observed in the growing colonies of MHS and the abundance of nanotubes decreased with the decreased swarming imposed by harsh conditions. For MSM-S1 strain very few nanotubes or flagella were observed irrespective of the growth conditions. Both the strains of Bacillus cereus were found to be poor biofilm formers and increased salinity in growth media could not induce any significant alteration in biofilm load after 12 hours of growth. An inverse correlation between the swarming and biofilm formation was observed for the hyperswarmer strain MHS for all concentrations of NaCl used in this study. No correlation was observed between the swarming and biofilm formation for the normal swarmer MSM-S1 strain in the presence of 1% NaCl in growth media. However, in the presence of 2% and 3% NaCl, inverse correlations were observed for MSM-S1. Expression profiles of genes involved in flagellation (bfla) and nanotube formation (ymdb) supported the observed salinity driven alteration in swarming ability in MHS. Interestingly, the expression of plcr, the master regulator of quorum sensing, was found to be reduced with salinity induced decreased swarming indicating the involvement of quorum sensing in the hyperswarming motility in MHS strain. However, the expression of nprb gene (neutral protease associated with quorum sensing) increased with the decrease in swarming. The present study thus helps understanding the hyperswarming motility in Bacillus cereus strain MHS and its association with flagellation, nanotube formation, and quorum sensing.

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