Heat and osmotic stress responses of probiotic Lactobacillus rhamnosus HN001 (DR20) in relation to viability after drying

The viability of lactic acid bacteria in frozen, freeze-dried, and air-dried forms is of significant commercial interest to both the dairy and food industries. In this study we observed that when prestressed with either heat (50degreesC) or salt (0.6 M NaCl), Lactobacillus rhamnosus HN001 (also known as DR20) showed significant (P < 0.05) improvement in viability compared with the nonstressed control culture after storage at 30degreesC in the dried form. To investigate the mechanisms underlying this stress-related viability improvement in L. rhamnosus HN001, we analyzed protein synthesis in cultures subjected to different growth stages and stress conditions, using two-dimensional gel electrophoresis and N-terminal sequencing. Several proteins were up- or down-regulated after either heat or osmotic shock treatments. Eleven proteins were positively identified, including the classical heat shock proteins GroEL and DnaK and the glycolytic enzymes glyceraldehyde-3-phosphate dehydrogenase, lactate dehydrogenase, enolase, phosphoglycerate kinase, and triose phosphate isomerase, as well as tagatose 1,6-diphosphate aldolase of the tagatose pathway. The phosphocarrier protein HPr (histidine-containing proteins) was up-regulated in cultures after the log phase irrespective of the stress treatments used. The relative synthesis of an ABC transport-related protein was also up-regulated after shock treatments. Carbohydrate analysis of cytoplasmic contents showed higher levels (20 +/- 3 mug/mg of protein) in cell extracts (CFEs) derived from osmotically stressed cells than in the unstressed control (15 :+/- 3 mug/mg of protein). Liquid chromatography of these crude carbohydrate extracts showed significantly different profiles. Electrospray mass spectrometry analysis of CFEs revealed, in addition to normal mono-, di-, tri-, and tetrasaccharides, the presence of saccharides modified with glycerol.