Megacell phenotype and its relation to metabolic alterations in transketolase deficient strain of Bacillus pumilus.

Fermentation with transketolase (tkt) deficient strain of Bacillus is the only reported industrially viable process for production of D-ribose, a commercially important pentose sugar. In addition to direct effects of tkt deficiency, the mutation in non-oxidative part of pentose phosphate pathway (PPP) is known to display several unexpected physiological characteristics such as decreased ability to utilize D-glucose, altered carbon catabolite repression, lack of motility, etc. Here we demonstrate the morphological plasticity of tkt deficient strain of Bacillus pumilus ATCC 21951 and its possible relation with D-ribose productivity, a measure of carbon flux through PPP. The bacilli divide normally in nutrient rich media such as Luria-Bertani (LB) broth while showing cell elongation of up to 20-fold without a visible septum accompanied by moderate to high extracellular D-ribose accumulation in glucose-rich media. The cells stained with DAPI (4'-6-diamidino-2-phenylindole) and anti FtsZ antibody showed nucleoid separation and Z-ring formation in LB broth but not in glucose-rich media. FtsZ protein is known to localize at the future division site forming a ring, called Z-ring, at an early stage in cytokinesis. The strain experiences inhibition or delay in Z-ring formation resulting in cell elongation, possibly due to its altered cell membrane composition resulting from tkt deficiency. We hypothesize that the lack of PPP intermediates may have two effects on the strain: (i) altered the cell membrane leading to delay in Z-ring formation and cell elongation; and (ii) induction of genes of the oxidative part of PPP resulting in D-ribose accumulation. Nutrient rich media such as LB broth may alleviate these metabolite deficiencies thereby restoring normal cell division and inhibiting excessive D-ribose accumulation. The D-ribose productivity and cell elongation may therefore be co-morbid. The results have implications in designing optimal media and monitoring strategy based on morphological analysis.