The cryoprotective effects of glycine betaine on bacteria

Abstract

It is well known that in response to a decrease in environmental temperature, some plants and animals accumulate several cryoprotective substances inside the cell. However, the role of glycine betaine as a bacterial cryoprotectant is only beginning to be understood.The growth-enhancing effect of glycine betaine on the food-borne pathogen Listeria monocytogenes at low temperature was first reported by a laboratory at the University of California, USA. This study also revealed that the uptake of glycine betaine into the bacterial cell was stimulated at low temperature. Recently, the same laboratory demonstrated a role for an ATP-dependent transport system in the cold-induced uptake of glycine betaine. Additionally, this transporter was shown to be inhibited by the glycine betaine analogs carnitine, dimethylglycine and γ-butyrobetaine [1xCharacterization of glycine betaine porter I from Listeria monocytogenes and its roles in salt and chill tolerance. Mendum, M.L. and Smith, L.T. Appl. Environ. Microbiol. 2002; 68: 813–819Crossref | Scopus (45)See all References][1].The growth-enhancing effect of glycine betaine on L. monocytogenes at low temperature was attributed to its ability to prevent cold-induced aggregation of cellular proteins and its role in maintaining membrane fluidity at low temperature. It has been shown that following inductive synthesis of some heat shock proteins (HSPs), which are believed to protect the cell from thermal stress, the frequency of survival of an Escherichia coli strain under frozen storage conditions was remarkably increased; this was thought to result from a chaperoning effect of the HSPs preventing denaturation of the cellular proteins at low temperature. Hence, it seems possible that the cryoprotective properties of glycine betaine result from a similar effect. The fact that this osmoprotectant is already known to act as a chemical chaperone during thermal stress strengthens this suggestion.An additional role of glycine betaine in regulating membrane fluidity cannot be ruled out, as its ability to increase membrane fluidity was demonstrated earlier using bilayers of small unilamellar vesicles. It has also been observed that when two strains of L. monocytogenes were grown in the presence of glycine betaine at low temperature, the amount of a branched-chain fatty acid that was essential for survival of the strains in cold (anteiso-C 15:0) was slightly enhanced. Subsequently, a cold-sensitive mutant deficient in this branched-chain fatty acid was found to have decreased membrane fluidity [2xCorrelation of long-range membrane order with temperature-dependent growth characteristics of parent and a cold-sensitive, branched-chain-fatty-acid-deficient mutant of Listeria monocytogenes. Jones, S.L. et al. Arch. Microbiol. 2002; 177: 217–222Crossref | Scopus (26)See all References][2]. It therefore appears that glycine betaine helps to maintain membrane fluidity at low temperature by promoting the synthesis of specific fatty acids. Recent investigations on isolated thylakoid membrane demonstrate the mutifaceted nature of its cryoprotective effects. Similar studies involving liposomes made of total lipids extracted from some cold-adapted bacteria promise further insights into the mechanism responsible for the cryoprotective effects of glycine betaine.