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Research has been conducted on the reaction of microorganisms to the hyperosmotic stress. The use of the transport system by Lactococcus lactis for glycine betaine has been examined.

Growth of a glutamate transport-deficient mutant of Rhodobacter sphaeroides on glutamate as sole carbon and nitrogen source can be restored by the addition of millimolar amounts of [Na.sup.+]. Uptake of glutamate ([K.sub.t] of 0.2 [[micro]molar]) by the mutant strictly requires [Na.sup.+]([K.sub.m] of 25 mM) and is inhibited by ionophores that collapse the proton motive force (pmf). The activity is osmotic-shock-sensitive and can be restored in spheroplasts by the addition of osmotic shock fluid. Transport of glutamate is also observed in membrane vesicles when [Na.sup.+], a proton motive force, and purified glutamate binding protein are present. Both transport and binding is highly specific for glutamate. The [Na.sup.+]-dependent glutamate transporter of Rb. sphaeroides is an example of a secondary transport system that requires a periplasmic binding protein and may define a new family of bacterial transport proteins.