Methylobacterium rhodesianum cells tend to double the DNA content under growth limitations and accumulate PHB

The investigation of microbial population dynamics gains more and more importance for biotechnological processes insofar as people may assume that the individual cells of a population contribute differently to the overall productivity. Flow cytometry is known to be suitable to get information on specific features of single cells of a population. In the paper presented, the distributions of the DNA and PHB over the whole population of Methylobacterium rhodesianum MB126 were determined. Three different kinds of limitation, namely that of nitrogen, phosphate and carbon, were investigated and compared with an unlimited growth process. Some differences in the population dynamics were observed, obviously caused by the remaining chances of continuing metabolism under restricted growth conditions. Most impressive was the appearance of two subpopulations due to phosphate limitation, characterized, in addition to their DNA content, by their cellular PHB content. On the other hand, nitrogen and carbon limitations produced homogeneous populations with a high or without a PHB content, respectively. It was found that under growth-limiting conditions the individuals first unwind the program to ensure the genetic information by doubling the chromosome content, thus the organisms maintain the chance to restart the multiplication as the forward strategy of survival if ‘better’ conditions arise. Then they lay in an energy reserve in the form of PHB. An hypothesis about the transitions between different physiological states characterized by the cellular DNA content and the cell size depending on process conditions is formulated and demonstrated by a formal scheme.