Osmosensing and osmoregulatory compatible solute accumulation by bacteria

Bacteria inhabit natural and artificial environments with diverse and fluct uating osmolalities, salinities and temperatures. Many maintain cytoplasmic hydration, growth and survival most effectively by accumulating kosmotropi c organic solutes (compatible solutes) when medium osmolality is high or te mperature is low (above freezing). They release these solutes into their en vironment when the medium osmolality drops. Solutes accumulate either by sy nthesis or by transport from the extracellular medium. Responses to growth in high osmolality medium, including biosynthetic accumulation of trehalose , also protect Salmonella typhimurium from heat shock. Osmotically regulate d transporters and mechanosensitive channels modulate cytoplasmic solute le vels in Bacillus subtilis, Corynebacterium glutamicum, Escherichia coli, La ctobacillus plantarum, Lactococcus lactis, Listeria monocytogenes and Salmo nella typhimurium. Each organism harbours multiple osmoregulatory transport ers with overlapping substrate specificities. Membrane proteins that can ac t as both osmosensors and osmoregulatory transporters have been identified (secondary transporters ProP of E. coli and BetP of C. glutamicum as well a s ABC transporter OpuA of L. lactis). The molecular bases for the modulatio n of gene expression and transport activity by temperature and medium osmol ality are under intensive investigation with emphasis on the role of the me mbrane as an antenna for osmo- and/or thermosensors. (C) 2001 Elsevier Scie nce Inc. All rights reserved.