The Production of Industrially Important Bacterial Polysaccharides

Abstract

Until the 1950s plant tissues were the only practical commercial sources of polysaccharide gums and mucilages, but the development of the exopolysaccharides dextran from Leuconostoc mesenteroides in the early 1950s and xanthan gum from Xanthomonas campestris in the following two decades demonstrated the potential of fermentation in the production of industrially important gums. For a number of reasons (Whistler, 1973, pp. 51 3-542) dextran usage has been limited to high-value small-volume applications such as extension of blood plasma and the manufacture of chromatographic support media, and this has restricted the world production to a large extent. However, in the case of xanthan the situation is different, and world production is already several thousand tons. The unique physical properties shown by this gum (Whistler, 1973, pp. 473497) have produced an increasing demand, and industry throughout the world has been stimulated to investigate the possibilities of exploiting other micro-organisms to produce further similarly valuable polysaccharides. Examples of gums in development at the present time are the glucan pullulan from Aureobasidium pullulans (Yuen, 1974) and a heteropolysaccharide containing glucose, galactose, uronic acid and fucose from Erwinia carotovora (Kang & Kovacs, 1974). We became interested in microbial polysaccharides through reports that certain bacteria produce extracellular polysaccharides similar in many respects to the polyuronide alginic acid, which was previously known only from the brown algae. Alginic acid is a commercially important gum with gelling and viscoelastic properties of use in the food, textile, pharmaceutical and paper industries (Percival & McDowell, 1967). The bacteria concerned were Pseudomonas aeruginosa (Linker & Jones, 1966; Carlson & Mathews, 1966) and Azotobacter vinelandii (Gorin & Spencer, 1966). The former was rejected for study because of its association with pathogenic conditions in man, and A. vinelandii was selected for examination. Studies based on small-scale fermentation experiments suggested that the Arotobacter polysaccharide would be sufficiently valuable for the process to be feasible if high yields could be obtained. Arotobacter vinelandii (N.C.I.B. 9068) cells were grown initially in shake flasks under the fermentation conditions described by Gorin & Spencer (1966), and a 5% yield of polysaccharide was obtained. Analysis of the monosaccharide components both by hydrolysis and by reduction followed by hydrolysisconfirmed that the material contained a high proportion of mannuronic acid and guluronic acid. No other sugars were detected in the purified polymer, but the polysaccharide was partly acetylated. The position of linkage of both monosaccharide units was supported as being 1 +4 both by methylation and by periodate oxidation studies, and the acetyl groups were found to be present to a low and variable degree. Block structure analysis by the method of Penman & Sanderson (1972) indicated that the features typical of algal alginate were present in the bacterial polysaccharide. The behaviour of aqueous solutions of the gum toward multivalent metal cations was also very similar to that shown by algal material (Whistler, 1973, pp.