Nanomolar levels of dimethylsulfoniopropionate, dimethylsulfonioacetate, and glycine betaine are sufficient to confer osmoprotection to Escherichia coli

We combined the use of low inoculation titers (300 +/- 100 CFU/ml) and enum eration of culturable cells to measure the osmoprotective potentialities of dimethylsulfoniopropionate (DMSP), dimethylsulfonioacetate (DMSA), and gly cine betaine (GB) for salt-stressed cultures of Escherichia coli. Dilute ba cterial cultures were grown with osmoprotectant concentrations that encompa ssed the nanomolar levels of GB and DMSP found in nature and the millimolar levels of osmoprotectants used in standard laboratory osmoprotection bioas says. Nanomolar concentrations of DMSA, DMSP, and GB were sufficient to enh ance the salinity tolerance of E. coli cells expressing only the ProU high- affinity general osmoporter. In contrast, nanomolar levels of osmoprotectan ts were ineffective with a mutant strain (GM50) that expressed only the low -affinity ProP osmoporter. Transport studies showed that DR ISA and DMSP, l ike GB, were taken up via both ProU and Prop, Moreover, ProU displayed high er affinities for the three osmoprotectants than ProP displayed, and Prop, like ProU, displayed much higher affinities for GB and DMSA than for DMSP. Interestingly, ProP did not operate at substrate concentrations of 200 nM o r less, whereas ProU operated at concentrations ranging from 1 nM to millim olar levels. Consequently, proU(+) strains of E. coli, but not the proP(+) strain GM50, could also scavenge nanomolar levels of GB, DMSA, and DMSP fro m oligotrophic seawater. The physiological and ecological implications of t hese observations are discussed.