Salmonella is an important foodborne pathogen, whose ability to resist stress and survive can vary among strains. This variability is normally not taken into account when predictions are made about survival in foods with negative consequences. Therefore, we examined the contribution of variable phenotypic properties to survival under stress in 10 Salmonella serovars. One strain (Typhimurium 10) was intentionally RpoS-negative; however, another strain (Heidelberg) showed an rpoS mutation, rendering it inactive. We assessed an array of characteristics (motility, biofilm formation, bile resistance, acid resistance, and colony morphology) that show major variability among strains associated with a 10- to 19-fold difference between the highest and the lowest strain for most characteristics. The RpoS status of isolates did not affect variability in the characteristics, with the exception of resistance to NaCl, acetic acid, lactic acid, and the combination of acetic acid and salt, where the variability between the highest and the lowest strain was reduced to 3.1-fold, 1.7-fold, 2-fold, and 1.7-fold, respectively, showing that variability was significant among RpoS-positive strains. Furthermore, we also found a good correlation between acid resistance and lysine decarboxylase activity, showing its importance for acid resistance, and demonstrated a possible role of RpoS in the lysine decarboxylase activity in Salmonella.

ABSTRACT Survival rates of Escherichia coli and Staphylococcus aureus after high-pressure treatment in buffers that had large or small reaction volumes (Δ V °), and which therefore underwent large or small changes in pH under pressure, were compared. At a low buffer concentration of 0.005 M, survival was, as expected, better in MOPS (morpholinepropanesulfonic acid), HEPES, and Tris, whose Δ V ° values are approximately 5.0 to 7.0 cm 3 mol −1 , than in phosphate or dimethyl glutarate (DMG), whose Δ V ° values are about −25 cm 3 mol −1 . However, at a concentration of 0.1 M, survival was unexpectedly better in phosphate and DMG than in MOPS, HEPES, or Tris. This was because the baroprotective effect of phosphate and DMG increased much more rapidly with increasing concentration than it did with MOPS, HEPES, or Tris. Further comparisons of survival in solutions of salts expected to cause large electrostriction effects (Na 2 SO 4 and CaCl 2 ) and those causing lower electrostriction (NaCl and KCl) were made. The salts with divalent ions were protective at much lower concentrations than salts with monovalent ions. Buffers and salts both protected against transient membrane disruption in E. coli , but the molar concentrations necessary for membrane protection were much lower for phosphate and Na 2 SO 4 than for HEPES and NaCl. Possible protective mechanisms discussed include effects of electrolytes on water compressibility and kosmotropic and specific ion effects. The results of this systematic study will be of considerable practical significance in studies of pressure inactivation of microbes under defined conditions but also raise important fundamental questions regarding the mechanisms of baroprotection by ionic solutes.

The aim of this work was to study the effects of drying methods and conditions (i.e., ambient drying, hot air drying at 40°C, vacuum drying and low-pressure superheated steam drying within the temperature range of 70–90°C at an absolute pressure of 10kPa) as well as the concentration of galangal extract on the antimicrobial activity of edible chitosan films against Staphylococcus aureus. Galangal extract was added to the film forming solution as a natural antimicrobial agent in the concentration range of 0.3–0.9g/100g. Fourier transform infrared (FTIR) spectra and swelling of the films were also evaluated to investigate interaction between chitosan and the galangal extract. The antimicrobial activity of the films was evaluated by the disc diffusion and viable cell count method, while the morphology of bacteria treated with the antimicrobial films was observed via transmission electron microscopy (TEM). The antimicrobial activity, swelling and functional group interaction of the antimicrobial films were found to be affected by the drying methods and conditions as well as the concentration of the galangal extract. The electron microscopic observations revealed that cell wall and cell membrane of S. aureus treated by the antimicrobial films were significantly damaged.

The aim of this work was to examine a possible association between resistance of two Escherichia coli strains to high hydrostatic pressure and the susceptibility of their cell membranes to pressure-induced damage. Cells were exposed to pressures between 100 and 700 MPa at room temperature (approximately 20 degrees C) in phosphate-buffered-saline. In the more pressure-sensitive strain E. coli 8164, loss of viability occurred at pressures between 100 MPa and 300 MPa and coincided with irreversible loss of membrane integrity as indicated by uptake of propidium iodide (PI) and leakage of protein of molecular mass between 9 and 78 kDa from the cells. Protein release increased to a maximum at 400 MPa then decreased, possibly due to intracellular aggregation at the higher pressures. In the pressure-resistant strain E. coli J1, PI was taken up during pressure treatment but not after decompression indicating that cells were able to reseal their membranes. Loss of viability in strain J1 coincided with the transient loss of membrane integrity between approximately 200 MPa and 600 MPa. In E. coli J1 leakage of protein occurred before loss of viability and the released protein was of low molecular mass, between 8 and 11 kDa and may have been of periplasmic origin. In these two strains differences in pressure resistance appeared to be related to differences in the ability of their membranes to withstand disruption by pressure. However it appears that transient loss of membrane integrity during pressure can lead to cell death irrespective of whether cells can reseal their membranes afterwards.

Outbreaks of mass mortality in postlarval abalone, Haliotis diversicolor supertexta (L.), have swept across south China since 2002 and in turn have resulted in many abalone farms closing. Twenty-five representative bacterial isolates were isolated from a sample of five diseased postlarval abalone, taken 15 d postfertilization during an outbreak of postlarval disease in Sanya, Hainan Province, China in October 2004. A dominant isolate, referred to as Strain 6, was found to be highly virulent to postlarvae in an experimental challenge test, with a 50% lethal dose (LD50) value of 3.2 x 10(4) colony forming units (CFU)/mL, while six of the other isolates were weakly virulent with LD50 values ranging from 1 x 10(6) to 1 x 10(7) CFU/mL, and the remaining 18 isolates were classified as avirulent with LD50 values greater than 1 x 10(8) CFU/mL. Using both an API 20E kit and 16S recombinant DNA sequence analysis, Strain 6 was shown to be Vibrio parahaemolyticus. It was sensitive to 4 and intermediately sensitive to 5 of the 16 antibiotics used when screening the antibiotic sensitivities of the bacterium. Extracellular products (ECPs) prepared from the bacterium were lethal to postlarvae when used in a toxicity test at a concentration of 3.77 mg protein/mL, and complete liquefaction of postlarvae tissues occurred within 24 h postexposure. Results from this study implicate V. parahaemolyticus as the pathogen involved in the disease outbreaks in postlarval abalone in Sanya and show that the ECPs may be involved in the pathogenesis of the disease.

ABSTRACT A new primary model based on a thermodynamically consistent first-order kinetic approach was constructed to describe non-log-linear inactivation kinetics of pressure-treated bacteria. The model assumes a first-order process in which the specific inactivation rate changes inversely with the square root of time. The model gave reasonable fits to experimental data over six to seven orders of magnitude. It was also tested on 138 published data sets and provided good fits in about 70% of cases in which the shape of the curve followed the typical convex upward form. In the remainder of published examples, curves contained additional shoulder regions or extended tail regions. Curves with shoulders could be accommodated by including an additional time delay parameter and curves with tails shoulders could be accommodated by omitting points in the tail beyond the point at which survival levels remained more or less constant. The model parameters varied regularly with pressure, which may reflect a genuine mechanistic basis for the model. This property also allowed the calculation of (a) parameters analogous to the decimal reduction time D and z , the temperature increase needed to change the D value by a factor of 10, in thermal processing, and hence the processing conditions needed to attain a desired level of inactivation; and (b) the apparent thermodynamic volumes of activation associated with the lethal events. The hypothesis that inactivation rates changed as a function of the square root of time would be consistent with a diffusion-limited process.

The effect of inoculum size on population lag times of Listeria monocytogenes was investigated using the Bioscreen automated microtitre plate incubator and reader. Under optimum conditions, lag times were little affected by inoculum size and there was little variation between replicate inocula even at very low cell numbers. However, in media containing inhibitory concentrations of NaCl, both the mean lag time and variation between replicate inocula increased as the inoculum size became smaller. The variation in lag time of cells within a population was investigated in more detail by measuring the distribution of detection times from 64 replicate inocula containing only one or two cells capable of initiating growth. The variance of the lag time distribution increased with increasing salt concentration and was greater in exponential than in stationary phase inocula. The number of cells required to initiate growth increased from one cell under optimum conditions to 10(5) cells in medium with 1.8 M NaCl. The addition of spent medium from a stationary phase culture reduced the variance and decreased lag times. The ability to initiate growth under severe salt stress appears to depend on the presence of a resistant sub-fraction of the population, although high cell densities assist adaptation of those resistant cells to the unfavourable growth conditions by some unspecified medium conditioning effect. These results are relevant to the prediction of lag times and probability of growth from low numbers of stressed cells in food.

Strains of Escherichia coli O157 isolated from patients with clinical cases of food-borne illness and other sources exhibited wide differences in resistance to high hydrostatic pressure. The most pressure-resistant strains were also more resistant to mild heat than other strains, Strain C9490, a representative pressure-resistant strain, was also more resistant to acid, oxidative, and osmotic stresses than the pressure-sensitive strain NCTC 12079, Most of these differences in resistance were observed only in stationary-phase cells, the only exception being acid resistance, where differences were also apparent in the exponential phase. Membrane damage in pressure-treated cells was revealed by increased uptake of the fluorescent dyes ethidium bromide and propidium iodide, When strains were exposed to the same pressure for different lengths of time, the pressure-sensitive strains took up stain sooner than the more resistant strain, which suggested that the differences in resistance may be related to susceptibility to membrane damage. Our results emphasize the importance of including stress-resistant strains of E. coli O157 when the efficacy of a novel or mild food preservation treatment is tested.

The duration of lag in Listeria monocytogenes was examined in relation to the physico–chemical properties of the growth environment. It was supposed that lag would be determined by two hypothetical quantities, the amount of work that a cell has to perform to adapt to new conditions and the rate at which it can perform that work. If the rate at which the cell can perform the necessary work is a function of the maximum specific growth rate in the new environment, the hypothesis predicts that lag time should be related in some way to growth rate, provided cells are initially in approximately the same physiological state. Literature data suggest this is true for many organisms when temperature is the sole growth limiting factor. However, lag times of L. monocytogenes displayed an unusual response to temperature in which lag times of cells precultured at 37°C were shorter at 15°C than at 20°C or 25°C. Analysis of data from the Food Micromodel in which growth of L. monocytogenes was controlled by combinations of pH, NaCl concentration and temperature, showed that there was a linear relationship between lag time and mean generation time although there was much scatter in the data. When the effects of pH, solute type and concentration were investigated individually in this work the correlation between lag time and mean generation time was often poor. It would thus appear that the relationship between growth environment and lag time is more complex than the corresponding relationship between growth environment and maximum specific growth rate.

The thermal degradation of plasmid pUC18 held at temperatures between 100 and 135 degrees C was examined by measuring the ability of heat-treated plasmid preparations to transform Escherichia coli to ampicillin resistance using electroporation. Substantial protection against loss of transforming ability during heating was provided by concentrations of NaCl between 0.25 and 2.0 mol l-1. For example, the addition of 1.0 mol l-1 NaCl to samples heated at 100 degrees C for 15 min increased transformation frequency about 200-fold compared with samples heated without NaCl. In the presence of 0.5-2.0 mol l-1 NaCl, transforming capacity was not destroyed even by heating at 121 degrees C for 15 min, i.e. after a typical sterilization treatment. These findings may have implications for the safe disposal of genetically modified micro-organisms and recombinant DNA preparations.