Physiological responses of a methylotrophic bacterium after sudden shifts from C-limited chemostat to C-excess batch growth conditions

The response of steady-state continuous cultures of Methylobacterium sp. RXM to the addition of methanol pulses was studied. The increase of methanol concentration in the medium did not result in cell death under any of the conditions tested. When the growth rate of the steady-state cultures was low (D = 0-046 h -1 ), the specific growth rate increased. When the concentration of methanol in the pulse was increased from 36 mmol l -1 to 280 mmol l -1 , uncoupled growth occurred and the molar cell yield decreased. Conversely, steady-state cultures at high growth rate (D = 0.2 h -1 ) showed a decrease in both specific growth rate and molar cell yield after the addition of the methanol pulses (32 and 164 mmol l -1 ). For all conditions, formaldehyde and formate were excreted into the medium but the levels did not exceed 1.13 mmol l -1 . Slow-growing cultures were characterized by cells with high derepressed specific activities of methanol dehydrogenase and low specific activities of formaldehyde and formate dehydrogenases, fast-growing cells had lower specific activity for methanol dehydrogenase and higher activities of formaldehyde and formate dehydrogenases, resulting in the excretion of lower concentrations of formaldehyde and formate. It is concluded that slow-growing cultures are more stable than fast-growing cultures for low methanol concentration fluctuations, and it is expected that maximum growth yields throughout the fermentation time are better achieved under the former conditions. However, for large fluctuations in the substrate concentration, the bacterial metabolic responses were identical both for slow-growing and fast-growing cultures.