## 1 Introduction  
Curdlan is a neutral linear exopolysaccharideacharide that is composed exclusively of more than 12,000 subunits of glucose linked by \(\beta\)-(1,3)-glycosidic bonds, with an average molecular weight of 74,000 Da. The number of glucose subunits and molecular weight vary with the characteristics of the producing strain. Curdlan was initially biosynthesized in conditions of nitrogen limitation using _Alcaligenes faecalis_ variety myxogenes, now identified as _Agrobacterium biovar_ 1 [1,2]. Until now, only bacteria belonging to the _Rhizobium_, _Alcaligenes_, _Agrobacterium_, _Cellulomonas_ and _Bacillus_ species have been reported to produce these biopolymers [3, 4, 5, 6, 7]. The transcriptome profiling of _Agrobacterium_ sp. ATCC 31749 revealed a conserved regulatory mechanisms of curdlan biosynthesis. The analysis revealed around 100-fold upregulation of the curdlan synthesis operon upon transition to nitrogen exhaustion, thus setting the prominent role that transcriptional regulation exerts on exopolysaccharide synthesis. Along with genetic, biochemical and physiological analysis, the authors revealed a multifaceted network of regulation for curdlan biosynthesis, including acidocalcisomes, RpoN (Sigma factor 54 )-independent nitrogen regulation, c-di-GMP (bis-(3',5')-cyclic-dimeric-guanosine monophosphate), polyP (Polyphosphate) metabolism and the stringent response. The stringent response signal, pppGpp (Guanosine pentaphosphate), appeared to be essential for transcriptional activation of curdlan biosynthesis [8]. Curdlan production is of great interest due to its unique rheological and thermal gelling properties. When suspended in water, curdlan can form two different types of gels by temperature adjustment: a thermo-reversible, weak gel obtained by heating at 60 \({}^{\circ}\)C and subsequent cooling and a strong, thermo-irreversible gel obtained at temperatures greater than 80 \({}^{\circ}\)C [9]. The typical properties of curdlan have prompted its use in texture improvement, water retention and the thermal stability of diverse foods. Indeed, curdlan has frequently been reported as useful additive for several food products [10]. High potential for curdlan use has also been found in the pharmaceutical industry, because of its potent biological activities. The biopolymer acts as an immune stimulator against bacterial and viral infections, and it has been found to exhibit high anti-acquired immunodeficiency syndrome (AIDS) virus activity [11]. Curdlan sulfate exhibits strong inhibitory action on blood coagulation and can thus be used for the treatment of thrombosis [12]. Research also reported its application in the production of biodegradable plastics for medical applications [13] and its use as a drug delivery carrier for the sustained release of drugs [14]. The demand for curdlan applications is increasing and requires a highly-efficient production process. Mutant strains with increased curdlan production capacity have been identified [15]. The effect of pH on curdlan production has been evaluated, and two-stage pH control with increased production has been carried out [16,17]. The supplementation of uracil was also found to improve the curdlan fermentation yield [18]. Other decisive aspects involved in curdlan biosynthesis have been investigated, including temperature [3], carbon source [19,20], carbon concentration [3,21], nitrogen supply [5], nitrogen concentration [3,21], phosphate concentration [2,7] and oxygen supply [3,17,22,23]. Immobilized cell systems have been applied in the production of secondary metabolites and polymers. These systems possess a number of advantages, such as the repeated and prolonged use of cells, easy separation of the cells from the medium, reduced contamination risk, better operational stability and increased productivity [24; 25; 26; 27; 28; 29].