Fructophilic characteristics of Fructobacillus spp. may be due to the absence of an alcohol/acetaldehyde dehydrogenase gene (adhE).

Fructophilic strains of Leuconostoc spp. have recently been reclassified to a new genus, i.e., Fructobacillus. Members of the genus are differentiated from Leuconostoc spp. by their preference for fructose on growth, requirement of an electron acceptor for glucose metabolism, and the inability to produce ethanol from the fermentation of glucose. In the present study, enzyme activities and genes involved in ethanol production were studied, since this is the key pathway for NAD(+)/NADH cycling in heterofermentative lactic acid bacteria. Fructobacillus spp. has a weak alcohol dehydrogenase activity and has no acetaldehyde dehydrogenase activity, whereas both enzymes are active in Leuconostoc mesenteroides. The bifunctional alcohol/acetaldehyde dehydrogenase gene, adhE, was described in Leuconostoc spp., but not in Fructobacillus spp. These results suggested that, due to the deficiency of the adhE gene, the normal pathway for ethanol production is absent in Fructobacillus spp. This leads to a shortage of NAD(+), and the requirement for an electron acceptor in glucose metabolism. Fructophilic characteristics, as observed for Fructobacillus spp., are thus due to the absence of the adhE gene, and a phenotype that most likely evolved as a result of regressive evolution.