H-054. Nutrient Availability Regulates Swarmer Cell Differentiation in Caulobacter crescentus

J. C. England1, B. S. Perchuk2, M. T. Laub2, J. W. Gober1;
1Univ. of California, Los Angeles, CA, 2Massachusetts Inst. of Technology, Cambridge, MA.

Caulobacter crescentus is an oligotrophic bacterium, dwelling in nutrient-poor natural environments. The obligatory asymmetric cell division exhibited by these cells reflects a clever strategy to cope with a scarcity of critical nutrients in the environment. Every cell division generates a daughter motile swarmer cell that cannot differentiate into a sessile, reproductive stalked cell for a defined period of time under typical culture conditions. The motile swarmer cells are chemotactically competent and can be viewed as a cell-type committed to foraging for nutrients. In order to test the idea that limiting nutrients may influence the differentiation from swarmer to stalked cell type, we examined the growth kinetics of cells grown in continuous culture under either carbon or nitrogen limitation. In cultures grown under carbon limitation, a relatively constant cell density was maintained over a wide range of culture dilution rates. When the cells were cultured under nitrogen limitation, a distinctly different relationship between biomass and dilution rate was observed. At imposed dilution rates that were very low (doubling time > 13 hrs) the biomass remained relatively constant, but exhibited a rapid decrease when the dilution rate was increased. Enumeration of the different cell types present under different flow rates indicated that the observed decrease in biomass under the higher flow rates is likely attributable to the loss of non-reproducing swarmer cells, suggesting that under conditions of low available nitrogen, swarmer cell differentiation is repressed. Consistent with this, microarray and qPCR analysis demonstrated an increase in the abundance of several swarmer cell-associated mRNAs (e.g. flagellin) in cells grown at low flow rates under nitrogen limitation. Microarray analysis revealed an increase in the expression of nitrogen acquisition genes under these growth conditions. However, a deletion in ntrY, a critical nitrogen-regulatory gene, had no affect on swarmer cell development. These experiments indicate that nitrogen availability is a key environmental factor in governing swarmer cell differentiation.