Table 3.

Net change of extracellular acetoin and 2,3-butanediol concentrations in challenged batch cultures of C. acetobutylicum transformants harboring pMTL500E, pWUR459, or pWUR460, after 72 h of fermentation, compared to their inoculation levels

Plasmid	Challengea	Net change in extracellular concnb (mM) of:
		Acetoin	d-2,3-BDf	meso-2,3-BD	Acetoin + 2,3-BD
pMTL500E (control)	No challenge	15 ± 1	0	2 ± 0.4	17 ± 1
	Acetoinc	13 ± 2	0	4 ± 0.2	17 ± 2
	d-2,3-BDd	12 ± 2	−0.7 ± 1	1 ± 0.05	13 ± 2
	meso-2,3-BDe	15 ± 0.2	0.3 ± 0.6	2 ± 1	18 ± 1
pWUR459 (Padc-Cb-acr)	No challenge	0.2 ± 0.4	15 ± 1	1 ± 0.2	16 ± 1
	Acetoin	−20 ± 0.4g	24 ± 1	11 ± 0.5	15 ± 1
	d-2,3-BD	0.1 ± 0.2	12 ± 1	0.9 ± 0.1	13 ± 1
	meso-2,3-BD	0.2 ± 0.4	17 ± 2	−0.5 ± 0.2	17 ± 2
pWUR460 (Pthl-Cb-acr)	No challenge	0 ± 0.3	17 ± 1	0.8 ± 0.1	18 ± 1
	Acetoin	−21 ± 0.7	29 ± 4	11 ± 0.2	20 ± 4
	d-2,3-BD	0.2 ± 0.3	20 ± 3	0.9 ± 0.1	21 ± 3
	meso-2,3-BD	0.4 ± 0.4	21 ± 0.5	0.1 ± 1	22 ± 1

^aRacemic acetoin, d-2,3-BD, or meso-2,3-BD was added to the medium before inoculation at a concentration of 20 mM.

^bData are given as the means ± standard deviations for three replicate fermentations and calculated by subtracting the concentration after 72 h from the concentration at the time of inoculation. For example, for the d-2,3-BD challenge of C. acetobutylicum carrying pWUR460, the initial d-2,3-BD concentration of 20 mM was subtracted from the final concentration of 40 mM, resulting in a net production of 20 mM d-2,3-BD.

^cThe medium was supplemented with racemic acetoin.

^dThe medium was supplemented with d-(2R,3R)-2,3-BD.

^eThe medium was supplemented with meso-2,3-BD, which also contained approximately 10% racemic d/l-2,3-BD.

^fAnalysis was done by nonchiral HPLC, so no separation of enantiomers was possible; however, on the basis of previous results, the d-enantiomer is expected to have been formed.

^gNegative values indicate consumption of acetoin relative to inoculation conditions.