Evidence for proliferation of Enterobacter cloacae on carbohydrates in cucumber and pea spermosphere

Abstract

Enterobacter cloacae EcCT501 grew on a variety of mono- and oligo-saccharides in vitro but did not grow on any of the polysaccharides tested. Carbohydrates found in seed exudates supported growth as did major monosaccharide constituents of seed storage carbohydrates and various seed-associated oligosaccharides. In addition, a rifampicin-resistant mutant of E. cloacae EcCT501, strain 501R3, grew on several of these mono- and oligo-saccharides in a natural gravely loamy sand soil. Glycosidase activities were detected in E. cloacae EcCT501 cultures that have the potential to degrade carbohydrate seed exudates and the oligosaccharide components of seed storage carbohydrates. Aqueous extracts from corn and pea seed induced activities of α-galactosidase 15- and 18-fold, respectively. Activities of α-glucosidase, β-glucosidase, and β-xylosidase were slightly induced by corn and pea seed extracts. Induction of these glycosidase activities by these extracts is consistent with their involvement in spermosphere proliferation. No carbohydrate depolymerase activities were detected. Strain A-11, which was isolated after mutation of strain 501R3 with transposon mini-Tn5 Km, lost the ability to grow on or had a reduced growth rate on several carbohydrates. Strain A-11 was similar to strain 501R3 in other nutritional tests. In cucumber spermosphere proliferation assays populations of strains 501R3 and A-11 were significantly different (P = 0.001) at 20 and 45 h. Strain A-11 did not proliferate in cucumber spermosphere, while strain 501R3 increased in number 21-fold after 45 h. In pea spermosphere, populations of strains A-11 and 501R3 increased 345- and 690-fold, respectively, after 45 h (significantly different at P = 0.1). These data are consistent with the hypothesis that spermosphere mono- and oligo-saccharides have roles as nutrients during spermosphere proliferation by E. cloacae. Key words: carbohydrate nutrition, spermosphere proliferation, glycosidase.