Quantitative analysis of growth stimulation by glycine betaine in Salmonella typhimurium

The accumulation of compatible solutes, such as glycine betaine, is known to stimulate growth under conditions of osmotic stress. In Salmonella typhimurium the accumulation of glycine betaine is mediated by two osmotically activated transport systems, ProP and ProU. This study was undertaken to determine the quantitative relationship between glycine betaine accumulation from the environment and growth stimulation, and also the relative roles of the high affinity (ProU) and low affinity (ProP) transport systems. Our data show that relatively low concentrations of glycine betaine (> 10 microM) are sufficient to stimulate growth and that under these conditions ProP and ProU transport systems are equivalent. At external concentrations of glycine betaine below 1 microM, cells able to express the ProU transport system possess a significant advantage over cells that only possess ProP. At high osmolarity the correlation between growth stimulation and cytoplasmic glycine betaine concentration is limited. At low glycine betaine concentrations further accumulation of the compatible solute stimulated growth. However, once the cells had accumulated 100 nmol glycine betaine per OD650 unit biomass no greater growth stimulation was observed in cells with higher levels of the compatible solute. The implications of these data for growth and pathogenicity of bacteria in natural ecosystems, such as foods, are discussed.