Search

Growth kinetics and physicochemical surface properties were compared for three Listeria strains with differing degrees of virulence: L. monocytogenes LO28; its isogenic, nonhemolytic mutant L. monocytogenes Bof415; and a nonvirulent species, L. innocua (strain Lin9). The influences of growth stage (mid-exponential phase, early stationary phase, and mid-stationary phase) and culture temperature (20 and 37 degrees C) were assessed by determining the electrical properties and the hydrophobic-hydrophilic and Lewis acid-base characteristics of the three strains. L. innocua, although taxonomically very similar to L. monocytogenes, exhibited physicochemical surface properties that differed significantly from those of L. monocytogenes LO28 and L. monocytogenes Bof415. Indeed, under our experimental conditions, L. innocua cells presented a more marked electronegative character (particularly when cultured at 20 degrees C), as well as greater variability in their Lewis acid-base characteristics as a function of temperature and growth stage. Furthermore, the growth kinetics of the three strains revealed the onset of a decay phase after 16 h of culture at 37 degrees C for the L monocytogenes Bof415 mutant. All of these results demonstrate that under our experimental conditions, the growth and/or physicochemical characteristics of the slightly pathogenic or nonpathogenic Listeria strains (Bof415 and Lin9) differed from those of the virulent strain (L. monocytogenes LO28). Consequently, the use of Listeria strains recognized as nonvirulent appeared to provide a model that was not fully suitable for simulating the bioadhesive behavior of the pathogenic strains involved in foodborne diseases.

The aim of this study was to examine the physicochemical surface properties and the ability to adhere to stainless steel of three strains of Listeria monocytogenes after different cultivation procedures. To this end, bacteria were cultivated at 37 degrees C after storage at two frequently used temperatures (4 degrees C or -80 degrees C) and were then transferred into the liquid medium (trypticase soy broth supplemented with 6 g liter(-1) of yeast extract, pH 7.3) between one and four times. In addition, the influence of supplementing the growth medium with lactic acid was explored, this organic acid being representative of both the dairy and cured meat industries. The hydrophobic/hydrophilic and electron-acceptor/electron-donor characteristics of the strains were evaluated by the microbial adhesion to solvents method. Using this technique, we recorded an increase in the hydrophobic properties of one strain stored at 4 degrees C, with an increasing number of transfers in the media (P < 0.05). Another plant-isolated strain appeared more hydrophobic and stuck better to stainless steel when cells were stored at 4 degrees C rather than at -80 degrees C. Preculturing L. monocytogenes in a lactic acid-supplemented medium increased the affinity of microbial cells to solvents and the bacterial attachment to stainless steel (P < 0.05).

The attachment of Salmonella choleraesuis subsp. choleraesuis ATCC 15790 to beef muscle and adipose tissues was investigated. S. choleraesuis was found to adhere in higher numbers to muscle than to fat. The charge and the hydrophobicity of the surface of S. choleraesuis were evaluated by measurement of electrophoretic mobility, the contact angle with water, adhesion to hexadecane, and hydrophobic interaction chromatography. The overall negative charge of S. choleraesuis was masked by the high electrolyte concentration in the attachment medium (0.15 M phosphate-buffered saline). This bacterium was shown to possess a hydrophilic surface. Electrostatic interactions do not affect the attachment of S. choleraesuis to both lean and fat tissue, and there was no evidence for a role of hydrophobic interactions. However, the attachment of S. choleraesuis was reduced by 90% after mechanical removal of the flagella or after treatment of the bacteria with specific antiflagella serum. This reduction was attributed to a loss of bacterial mobility leading to a reduction in the number of cells reaching the tissue during the period of contact. Treatment of the tissue with a concentrated suspension of flagella or treatment of the bacteria with antisomatic serum (OMD) did not reduce the attachment of S. choleraesuis to tissues, indicating an absence of specific attachment sites for flagella or antigen O on the beef tissue surface.