The application of 13C and 31P Nuclear Magnetic Resonance (NMR) spec­troscopy can provide information on both metabolic and energy status dur­ing cell growth through determination of the levels of various phosphorylated intermediates and energy rich compounds as shown in earlier studies on wild-type strains of Z. mobilis [48,52-55].

More recent research with 31P NMR has identified a less energized state of ZM4 (pZB5) when grown on xylose media [56,57]. 31P NMR studies have established that levels of nucleoside tri-phosphates (mostly ATP) and sugar phosphates were lower for growth on xylose compared to that on glu­cose, with this energy limitation resulting in a potential growth restriction. The presence of by-products identified as xylitol, acetate, lactate, acetoin and dihydroxyacetone by 13C NMR spectroscopy and high-performance li­quid chromatography may also result in some inhibition of growth. Further 31P NMR studies [58] have shown that the addition of inhibitory concentra­tions of sodium acetate caused decreased levels of nucleotide tri-phosphates and sugar phosphates, together with increased cytoplasm acidification.

2.4