Na⁺ requirement for glutamate-dependent sugar transport byFusobacterium nucleatum ATCC 10953

Abstract

Resting cells ofFusobacterium nucleatum ATCC 10953, when provided with glutamic acid (Na⁺ salt) as fermentable energy source, rapidly accumulated [¹⁴C]glucose, from the medium. Sugar accumulation was not observed when Na⁺ glutamate was replaced by ammonium glutamate. However, addition of Na⁺ (chloride) to the latter system elicited uptake of [¹⁴C]glucose by the organism. Of other monovalent cations tested, only Li⁺ was found to be slightly stimulatory, but K⁺, Rb⁺, and Cs⁺ ions were ineffective. For determination of the role(s) of Na⁺ in sugar accumulation, the transport of [¹⁴C]glucose and [¹⁴C]glutamic acid by the cells was studied independently, with lysine as an alternate (and Na⁺-independent) energy source. In the presence of lysine, cells ofF. nucleatum 10953 accumulated [¹⁴C]glucose from a Na⁺-free medium, but, in contrast, uptake and fermentation of [¹⁴C]glutamic acid was Na⁺-dependent. The glucose transport system is Na⁺-independent. However, our data indicate dual role(s) for Na⁺ in the transport and intracellular metabolism of glutamic acid. The Na⁺-dependent glutamate fermentation pathway provides the necessary energy for active transport of glucose by the resting cell.