7 The genus Leuconostoc F. DELLAGLIO, L.M.T. DICKS and S. TORRIANI 7.1 Introduction The genus Leuconostoc is phenotypically related to Lactobacillus and Pediococcus (Stackebrandt et al., 1983; Stackebrandt and Teuber, 1988) and share many features with the heterofermentative lactobacilli. In a recent comparative study of the 16S rRNA sequences (Yang and Woese, 1989), it was shown that the leuconostocs form a natural phylogenetic group with Lb. confusus, Lb. halotolerans, Lb. kandleri, Lb. minor, and Lb. viridescens. The leuconostocs have complex nutritional requirements (Garvie, 1967b, 1986) and are found in plants, dairy products, meat and various fermented food products (Holzapfel and Schillinger, 1992). In silage fermentation Leuc. mesenteroides subsp. mesenteroides, Leuc. mesenter- oides subsp. dextranicum, and Leuc. paramesenteroides are well represented (Dellaglio et al., 1984; Woolford, 1984; Dellaglio and Torriani, 1986; Daeschel et al., 1987). Leuconostoc mesenteroides subsp. mesenteroides produces an excess of exopolysaccharides and is the major contaminant in sugar milling plants (Sharpe et al., 1972; Tilbury, 1975; Pivnick, 1980). Leuconostoc mesenteroides subsp. mesenteroides and Leuc. lactis are the dominant leuconostocs in milk and fermented milk products (Daly, 1983; Keller et al., 1987; Marshall, 1987). Leuconostoc mesenteroides subsp. cremoris and Leuc. paramesenteroides are less well represented in milk (El- Gendy et at., 1983), probably due to their slow growth under psychrotrophic conditions. Recently, a new species (Leuc. argentinum) has been isolated from Argentinian raw milk (Dicks et al., 1993). Leuconostoc mesenteroides subsp. mesenteroides, Leuc. amelibiosum, Leuc. carnosum, Leuc. gelidum and Leuc. citreum have been isolated from various meat products (Savell et at., 1981; Holzapfel and Gerber, 1986; Korkeala et at., 1988; Farrow et at., 1989; Schillinger et al., 1989; Shaw and Harding, 1989). Leuconostoc oenos plays a key role in the malolactic fermentation of wines (reviewed by Van Vuuren and Dicks, 1993). In spite of the wide distribution and practical importance of leuconostocs, these bacteria have not been studied to the same extent as the genus Lactobacillus and the other lactic acid bacteria. Scientists only recently became more aware of the practical importance of leuconostocs, especially B. J. B. Wood et al. (eds.), The Genera of Lactic Acid Bacteria © Chapman & Hall 1995