ABSTRACT
C. glutamicum is naturally able to utilize a broad spectrum of carbon sources for its growth and energy supply. Among these are sugars such as the monosaccharides glucose, fructose, mannose, and ribose or disaccharides such as sucrose and maltose (Blombach and Seibold, 2010). Also alcohols such as ethanol (Arndt et al., 2008) or inositol (Krings et al., 2006) and organic acids such as L-lactic acid (Stansen et al., 2005), D-lactic acid (Kato et al., 2010), acetic acid (Gerstmeir et al., 2003), propionic acid (Claes et al., 2002), pyruvic acid (Cocaign et al., 1993) and gluconic acid (Lee et al., 1998) can be utilized by C. glutamicum. Furthermore, it utilizes some amino acids such as L-glutamate and L-glutamine (Kramer et al., 1990; Schultz et al., 2007) and sugar alcohols such as arabitol and mannitol (Laslo et al., 2012).Interestingly, C. glutamicum wild type can grow on different aromatic compounds, which can be found, e.g., as constituents of lignocellulosic hydrolysates and which affect the performance of most other production hosts (Rumbold et al., 2010). Among these are benzoate, phenol (Shen et al., 2004), 3-hydrobenzoate, gentisate (Shen et al., 2005), protocatechuate, vanillate, 4-hydroxybenzoate, 4-cresol (Qi et al., 2007; Shen and Liu, 2005), resorcinol (Huang et al., 2006), benzyl alcohol, 2,4-dihydroxybenzoate, 3,5-dihydroxytoluene (Shen and Liu, 2005), naphthalene (Lee et al., 2010), vanillin and ferulic acid (Merkens et al., 2005).For the utilization of complex carbon sources such as hydrolysates of lignocellulosic material, silage or chitin, which consist of a mixture of sugars, it is of advantage to use a microorganism able to co-utilize different sugars. C. glutamicum is such an organism capable of the co-utilization of carbon sources (Gerstmeir et al., 2003; Wendisch et al., 2000), when supplemented as mixtures (Blombach and Seibold, 2010). This has been shown for monophasic
growth of C. glutamicum on glucose mixed with other sugars such as fructose (Dominguez et al., 1997) or organic acids such as lactate, pyruvate, propionate, and acetate (Claes et al., 2002; Gerstmeir et al., 2003; Wendisch et al., 2000). Diauxic growth and sequential utilization of carbon sources, as shown for E. coli (Kremling et al., 2009) or B. subtilis (Hueck and Hillen, 1995) occurs only rarely in C. glutamicum but has been described for preferential utilization of glucose in mixtures of glucose and either glutamate (Kronemeyer et al., 1995) or ethanol (Arndt and Eikmanns, 2007).Interestingly, even when additional carbon utilization pathways are introduced into C. glutamicum, these carbon sources are co-utilized, e.g., with glucose. After introducing pathways for catabolism of glycerol, xylose, arabinose and cellobiose co-utilization with glucose was observed (Kawaguchi et al., 2006; Gopinath et al., 2011; Rittmann et al., 2008; Sasaki et al., 2009). Therefore, C. glutamicum represents an excellent candidate for the development of processes based on complex carbon sources (Zahoor ul Hassan et al., 2012). 11.2.1 Starch, Molasses, Glucose, Fructose, and
SucroseTraditionally, amino acid production by C. glutamicum is based on starch hydrolysates or on molasses. 11.2.1.1 StarchStarch is one of the major substrates for industrial amino acid production; however, it has to be hydrolyzed to glucose before fermentation. To reduce costs direct utilization of raw starch is desirable (Kimura, 2005). Heterologous production and secretion of α-amylase from Streptomyces griseus in recombinant C. glutamicum (Fig. 11.1) enabled faster growth on starch than on glucose, given that sub-stoichiometric glucose concentrations are present during inoculation (Seibold et al., 2006). However, a complete utilization of starch was not observed as some starch degradation products still remained in the medium (Seibold et al., 2006). Similarly, surface display of α-amylase from Streptococcus bovis anchored via pgsA from Bacillus subtilis enabled these recombinants to utilize starch as sole source of carbon (Tateno et al., 2007b).
