Pseudofructophilic Leuconostoc citreum Strain F192-5, Isolated from Satsuma Mandarin Peel

Abstract

Fructophilic lactic acid bacteria (FLAB), composed of Fructobacillus spp., Lactobacillus kunkeei, and Lactobacillus apinorum, are unique in that they prefer d-fructose over d-glucose as a carbon source. Strain F192-5, isolated from the peel of a satsuma mandarin and identified as Leuconostoc citreum, grows well on d-fructose but poorly on d-glucose and produces mainly lactate and acetate, with trace amounts of ethanol, from the metabolism of d-glucose. These characteristics are identical to those of obligate FLAB. However, strain F192-5 ferments a greater variety of carbohydrates than known FLAB. Comparative analyses of the genomes of strain F192-5 and reference strains of L. citreum revealed no signs of specific gene reductions, especially genes involved in carbohydrate transport and metabolism, in the genome of F192-5. The bifunctional alcohol/acetaldehyde dehydrogenase gene (adhE) is conserved in strain F192-5 but is not transcribed. This is most likely due to a deletion in the promoter region upstream of the adhE gene. Strain F192-5 did, however, ferment d-glucose when transformed with a plasmid containing the allochthonous adhE gene. L. citreum F192-5 is an example of a pseudo-FLAB strain with a deficiency in d-glucose metabolism. This unique phenotypic characteristic appears to be strain specific within the species L. citreum This might be one of the strategies lactic acid bacteria use to adapt to diverse environmental conditions.IMPORTANCE Obligate fructophilic lactic acid bacteria (FLAB) lack the metabolic pathways used in the metabolism of most carbohydrates and differ from other lactic acid bacteria in that they prefer to ferment d-fructose instead of d-glucose. These characteristics are well conserved at the genus or species level. Leuconostoc citreum F192-5 shows similar growth characteristics. However, the strain is metabolically and genomically different from obligate FLAB. This is an example of a strain that evolved a pseudo-FLAB phenotype to adapt to a fructose-rich environment.

Keywords: Leuconostoc citreum; adhE; comparative genomics; fructophilic lactic acid bacteria; pseudofructophilic.