Differential protein expression by Pseudomonas fragi submitted to various stresses

We have analyzed the impact of various stressing conditions on the physiolo gical and molecular responses or the main psychrotrophic spoilage bacterium of refrigerated meat and meat products, Pseudomonas fragi. A survival stud y using conventional plating was first performed to select the stressing ag ents and parameters. Some of these mimicked cleaning-disinfection processes but with less drastic conditions in order to keep alive enough bacterial c ells for the protein expression characterization. Cultures of P. fragi, at the beginning of the stationary phase of growth, were submitted to individu al pH (5.4, 10.5), osmotic (8% Na2SO4, pH 7.0), biocide (fatty amine) shock s or combined treatments (8% Na2SO4, pH 10.5; 8% Na2SO4, pH 10.5 + biocide; pH 5.4 + pH 10.5 and pH 10.5 + pH 5.4) and the molecular responses were in vestigated by comparing autoradiograms of two-dimensional gel electrophores is (2-DE) patterns of proteins radiolabeled with L-[S-35]methionine. The ob servation of qualitative and relative quantitative variations in protein ex pression, determined with Melanie II image analysis software (Bio-Rad), rev ealed the overexpression of a total of 91 proteins for the eight challenges by comparison with the nonshocked controls. Some proteins appeared to be m ore or less general stress proteins whereas others were specific for one ch emical treatment. The appraisal of the type of molecular response according to the type of treatment was analyzed statistically.