Sporulation, a Pitfall in the Path of PHB Production

The concept of bioplastic is fascinating to our world, because of its potentiality to deal with one of the major global problems like plastic pollution (Kalia et al. J Sci Ind Res 59:433–445, 2000; Kalia et al. Nat Biotechnol 21:845–846, 2003). Polyhydroxybutyrates (PHB) are the best example for the polymers by plant or microorganisms from a wide range of habitats (Reddy et al. Bioresour Technol 87:137–146, 2003; Porwal et al. Bioresour Technol 99:5444–5451, 2008; Singh. Environ Microbiol 17:854–864, 2015). PHB refers to the polyesters of 3-hydroxybutyrate and can be extracted from various species like Ralstonia, Bacillus, Streptomyces, Pseudomonas, etc., which are extensively discussed in published reviews (Singh et al. Microb Cell Fact 8:38, 2009; Jendrossek and Pfeiffer. Environ Microbiol 16:2357–2373, 2014). Bioplastic has the distinct feature of being biodegradable. Further, the use of biowaste as substratum for bioplastic-producing organisms presents an interesting concept to deal with another global problem of waste management (Kumar et al. J Appl Microbiol 106:2017–2023, 2009; Kumar et al. Indian J Microbiol 55:1–7, 2015; Kumar et al. Int J Biol Macromol, 2015; Patel et al. Biomas Bioenerg 36:218–225, 2012; Patel et al. Bioresour Technol 176:136–141, 2015). Both Gram-positive and Gram-negative bacteria are reported to produce polyhydroxyalkanoates (PHA); among them Gram-negative bacteria, Ralstonia eutropha is the most extensively studied organism (Brigham et al. Appl Environ Microbiol 78:8033–8044, 2012). One major rationale to investigate the Gram-positive bacteria for their ability to produce PHB is the absence of immunogenic lipopolysaccharide which co-purifies with the PHB when Gram-negative organisms are employed, making PHB non-appealing for the use in medical purposes like various human tissue grafts (Valappil et al. Antonie Van Leeuwenhoek 91:1–17, 2007; Singh et al. Microb Cell Fact 8:38, 2009). Additional appeal for using Gram-positive bacteria, specifically Bacillus spp., is its ability to produce copolymers which are superior to their counterparts, that is, homopolymers given their enhanced characteristics like elasticity, etc. (Patel et al. Indian J Microbiol 51:418–423, 2011; Kumar et al. Indian J Microbiol 54:151–157, 2014; Kumar et al. Int J Biol Macromol, 2015). Gram-positive bacteria exist in two alternative phases in its life cycle, that is, vegetative cells and sporulation. The adverse environmental conditions drive the Bacillus vegetative cells into their transition to spores. Sporulation is an intrinsic characteristic of the Bacillus species and is mainly regulated by a master regulator of sporulation, namely, Spo0A (Slepecky and Law. J Bacteriol 82:37–42, 1961; Singh et al. Indian J Microbiol 55:234, 2015).