Influence of ions on growth and production of exopolysaccharides by Lactobacillus delbrueckii subsp. bulgaricus NCFB 2772.

Several lactic acid bacteria produce exopolysaccharides (EPS), either attached to the cell wall or excreted into the environment as slime material. EPS produced by Lactobacillus delbrueckii subsp. bulgaricus (Lb. bulgaricus) and Streptococcus thermophilus play an important role in improving the texture and stability of yogurt and preventing syneresis (Cerning, 1990; Nakajima et al. 1990). The amount and composition of the EPS produced by lactic acid bacteria are dependent on a number of factors, such as temperature, initial pH, carbon source and the availability of minerals, vitamins and other medium components. In previous work it was shown that the production and sugar composition of the EPS fromLb. bulgaricusNCFB2772 are affected by the carbohydrate source (Grobben et al. 1995, 1996). In a simplified defined medium, from which several vitamins and trace elements were omitted, EPS production by Lb. bulgaricus significantly increased, although growth of the strain was reduced (Grobben et al. 1998). EPS may contribute to the uptake of nutrients and metal ions when these are present in very low concentrations (Cerning, 1990), and it has been suggested that limiting amounts of nutrients in the growth medium in combination with a carbon excess may stimulate the formation of EPS (Sutherland, 1972). On the other hand, some ions may be essential to the functions of enzymes involved in EPS production. However, most investigations on the role of ions in the production of EPS have revealed that increasing ion concentrations stimulate the formation of EPS. Mozzi et al. (1995a) found that growth and EPS production by Lb. casei grown in APTgl broth was stimulated by the addition of MnSO % or triammonium citrate, but the addition of Mg#+ or K+ only stimulated growth and not the specific production of EPS. With Lactobacillus sp. KPB-167B, the addition of 5 m-CaCl # to MRSL medium resulted in an enhancement of the polysaccharide kefiran (Yokoi & Watanabe, 1992). Racine et al. (1991) found that increasing phosphate levels stimulated EPS production by Propionibacterium acidi-propionici. In the present study, we have investigated the