Cold shock and its effect on ribosomes and thermal tolerance in Listeria monocytogenes

The ups and downs of ectoine: structural enzymology of a major microbial stress protectant and versatile nutrient

Ectoine and its derivative 5-hydroxyectoine are compatible solutes and chemical chaperones widely synthesized by Bacteria and some Archaea as cytoprotectants during osmotic stress and high- or low-growth temperature extremes. The function-preserving attributes of ectoines led to numerous biotechnological and biomedical applications and fostered the development of an industrial scale production process. Synthesis of ectoines requires the expenditure of considerable energetic and biosynthetic resources. Hence, microorganisms have developed ways to exploit ectoines as nutrients when they are no longer needed as stress protectants. Here, we summarize our current knowledge on the phylogenomic distribution of ectoine producing and consuming microorganisms. We emphasize the structural enzymology of the pathways underlying ectoine biosynthesis and consumption, an understanding that has been achieved only recently. The synthesis and degradation pathways critically differ in the isomeric form of the key metabolite N-acetyldiaminobutyric acid (ADABA). γ-ADABA serves as preferred substrate for the ectoine synthase, while the α-ADABA isomer is produced by the ectoine hydrolase as an intermediate in catabolism. It can serve as internal inducer for the genetic control of ectoine catabolic genes via the GabR/MocR-type regulator EnuR. Our review highlights the importance of structural enzymology to inspire the mechanistic understanding of metabolic networks at the biological scale.

Combined Methylome, Transcriptome and Proteome Analyses Document Rapid Acclimatization of a Bacterium to Environmental Changes

Polynucleobacter asymbioticus strain QLW-P1DMWA-1^T represents a group of highly successful heterotrophic ultramicrobacteria that is frequently very abundant (up to 70% of total bacterioplankton) in freshwater habitats across all seven continents. This strain was originally isolated from a shallow Alpine pond characterized by rapid changes in water temperature and elevated UV radiation due to its location at an altitude of 1300 m. To elucidate the strain’s adjustment to fluctuating environmental conditions, we recorded changes occurring in its transcriptomic and proteomic profiles under contrasting experimental conditions by simulating thermal conditions in winter and summer as well as high UV irradiation. To analyze the potential connection between gene expression and regulation via methyl group modification of the genome, we also analyzed its methylome. The methylation pattern differed between the three treatments, pointing to its potential role in differential gene expression. An adaptive process due to evolutionary pressure in the genus was deduced by calculating the ratios of non-synonymous to synonymous substitution rates for 20 Polynucleobacter spp. genomes obtained from geographically diverse isolates. The results indicate purifying selection.

Ribosomal background of the Bacillus cereus group thermotypes

In this study we reconstructed the architecture of Bacillus cereus sensu lato population based on ribosomal proteins, and identified a link between the ribosomal proteins’ variants and thermal groups (thermotypes) of the bacilli. The in silico phyloproteomic analysis of 55 ribosomal proteins (34 large and 21 small subunit r-proteins) of 421 strains, representing 14 well-established or plausible B. cereus sensu lato species, revealed several ribosomal clusters (r-clusters), which in general were well correlated with the strains’ affiliation to phylogenetic/thermal groups I–VII. However, a conformity and possibly a thermal characteristic of certain phylogenetic groups, e.g. the group IV, were not supported by a distribution of the corresponding r-clusters, and consequently neither by the analysis of cold-shock proteins (CSPs) nor by a content of heat shock proteins (HSPs). Furthermore, a preference for isoleucine and serine over valine and alanine in r-proteins along with a lack of HSP16.4 were recognized in non-mesophilic thermotypes. In conclusion, we suggest that the observed divergence in ribosomal proteins may be connected with an adaptation of B. cereus sensu lato members to various thermal niches.

Differential scanning calorimetry of whole Escherichia coli treated with the antimicrobial peptide MSI-78 indicate a multi-hit mechanism with ribosomes as a novel target

Differential Scanning Calorimetry (DSC) of intact Escherichia coli (E. coli) was used to identify non-lipidic targets of the antimicrobial peptide (AMP) MSI-78. The DSC thermograms revealed that, in addition to its known lytic properties, MSI-78 also has a striking effect on ribosomes. MSI-78’s effect on DSC scans of bacteria was similar to that of kanamycin, an antibiotic drug known to target the 30S small ribosomal subunit. An in vitro transcription/translation assay helped confirm MSI-78’s targeting of ribosomes. The scrambled version of MSI-78 also affected the ribosome peak of the DSC scans, but required greater amounts of peptide to cause a similar effect to the unscrambled peptide. Furthermore, the effect of the scrambled peptide was not specific to the ribosomes; other regions of the DSC thermogram were also affected. These results suggest that MSI-78’s effects on E. coli are at least somewhat dependent on its particular structural features, rather than a sole function of its overall charge and hydrophobicity. When considered along with earlier work detailing MSI-78’s membrane lytic properties, it appears that MSI-78 operates via a multi-hit mechanism with multiple targets.

Diversity of acid stress resistant variants of Listeria monocytogenes and the potential role of ribosomal protein S21 encoded by rpsU

The dynamic response of microorganisms to environmental conditions depends on the behavior of individual cells within the population. Adverse environments can select for stable stress resistant subpopulations. In this study, we aimed to get more insight in the diversity within Listeria monocytogenes LO28 populations, and the genetic basis for the increased resistance of stable resistant fractions isolated after acid exposure. Phenotypic cluster analysis of 23 variants resulted in three clusters and four individual variants and revealed multiple-stress resistance, with both unique and overlapping features related to stress resistance, growth, motility, biofilm formation, and virulence indicators. A higher glutamate decarboxylase activity correlated with increased acid resistance. Whole genome sequencing revealed mutations in rpsU, encoding ribosomal protein S21 in the largest phenotypic cluster, while mutations in ctsR, which were previously shown to be responsible for increased resistance of heat and high hydrostatic pressure resistant variants, were not found in the acid resistant variants. This underlined that large population diversity exists within one L. monocytogenes strain and that different adverse conditions drive selection for different variants. The finding that acid stress selects for rpsU variants provides potential insights in the mechanisms underlying population diversity of L. monocytogenes.

Quantifying variability on thermal resistance of Listeria monocytogenes

Knowledge of the impact of strain variability and growth history on thermal resistance is needed to provide a realistic prediction and an adequate design of thermal treatments. In the present study, apart from quantifying strain variability on thermal resistance of Listeria monocytogenes, also biological variability and experimental variability were determined to prioritize their importance. Experimental variability was defined as the repeatability of parallel experimental replicates and biological variability was defined as the reproducibility of biologically independent reproductions. Furthermore, the effect of growth history was quantified. The thermal inactivation curves of 20 L. monocytogenes strains were fitted using the modified Weibull model, resulting in total 360 D-value estimates. The D-value ranged from 9 to 30 min at 55 °C; from 0.6 to 4 min at 60 °C; and from 0.08 to 0.6 min at 65 °C. The estimated z-values of all strains ranged from 4.4 to 5.7 °C. The strain variability was ten times higher than the experimental variability and four times higher than the biological variability. Furthermore, the effect of growth history on thermal resistance variability was not significantly different from that of strain variability and was mainly determined by the growth phase.

Enhancing the stress responses of probiotics for a lifestyle from gut to product and back again

Before a probiotic bacterium can even begin to fulfill its biological role, it must survive a battery of environmental stresses imposed during food processing and passage through the gastrointestinal tract (GIT). Food processing stresses include extremes in temperature, as well as osmotic, oxidative and food matrix stresses. Passage through the GIT is a hazardous journey for any bacteria with deleterious lows in pH encountered in the stomach to the detergent-like properties of bile in the duodenum. However, bacteria are equipped with an array of defense mechanisms to counteract intracellular damage or to enhance the robustness of the cell to withstand lethal external environments. Understanding these mechanisms in probiotic bacteria and indeed other bacterial groups has resulted in the development of a molecular toolbox to augment the technological and gastrointestinal performance of probiotics. This has been greatly aided by studies which examine the global cellular responses to stress highlighting distinct regulatory networks and which also identify novel mechanisms used by cells to cope with hazardous environments. This review highlights the latest studies which have exploited the bacterial stress response with a view to producing next-generation probiotic cultures and highlights the significance of studies which view the global bacterial stress response from an integrative systems biology perspective.

Role of growth temperature in freeze-thaw tolerance of Listeria spp

The food-borne pathogen Listeria monocytogenes can grow in a wide range of temperatures, and several key virulence determinants of the organism are expressed at 37 degrees C but are strongly repressed below 30 degrees C. However, the impact of growth temperature on the ability of the bacteria to tolerate environmental stresses remains poorly understood. In other microorganisms, cold acclimation resulted in enhanced tolerance against freezing and thawing (cryotolerance). In this study, we investigated the impact of growth temperature (4, 25, and 37 degrees C) on the cryotolerance of 14 strains of L. monocytogenes from outbreaks and from food processing plant environments and four strains of nonpathogenic Listeria spp. (L. welshimeri and L. innocua). After growth at different temperatures, cells were frozen at -20 degrees C, and repeated freeze-thaw cycles were applied every 24 h. Pronounced cryotolerance was exhibited by cells grown at 37 degrees C, with a <1-log decrease after 18 cycles of freezing and thawing. In contrast, freeze-thaw tolerance was significantly reduced (P < 0.05) when bacteria were grown at either 4 or 25 degrees C, with log decreases after 18 freeze-thaw cycles ranging from 2 to >4, depending on the strain. These findings suggest that growth at 37 degrees C, a temperature required for expression of virulence determinants of L. monocytogenes, is also required for protection against freeze-thaw stress. The negative impact of growth at low temperature on freeze-thaw stress was unexpected and has not been reported before with this or other psychrotrophic microorganisms.

Comparison of methods for detection and isolation of cold- and freeze-stressed Escherichia coli O157:H7 in raw ground beef

A comparison was made of the relative efficiencies of three enrichment media, RapidChek Escherichia coli O157:H7 enrichment broth (REB), R&F broth (RFB), and modified E. coli broth containing novobiocin (mEC+n), and four selective plating media for detection of cold- and freeze-stressed E. coli O157:H7 in raw ground beef. Ground beef (25 g) was inoculated with E. coli O157:H7 at < or =0.5 and < or =2 CFU/g, and samples were then enriched immediately or were stored at 4 degrees C for 72 h or at -20 degrees C for 2 weeks and then enriched. After 8 or 20 h of enrichment, the cultures were plated onto R&F E. coli O157: H7 chromogenic plating medium, cefixime-tellurite sorbitol MacConkey agar, CHROMagar O157, and Rainbow agar O157 and tested using the RapidChek E. coli O157 lateral flow immunoassay and a multiplex PCR assay targeting the E. coli O157: H7 eae, stx1, and stx2 genes. Recovery of E. coli O157:H7 on the four agar media was 4.0 to 7.9 log CFU/ml with the REB enrichment, 1.4 to 7.4 log CFU/ml with RFB, 1.7 to 6.7 log CFU/ml with mEC+n incubated at 42 degrees C, and 1.3 to 3.3 log CFU/ml from mEC+n incubated at 35 degrees C. The percentages of positive ground beef samples containing nonstressed, cold-stressed, and freeze-stressed E. coli O157:H7 as obtained by plating, the immunoassay, and the PCR assay were 97, 88, and 97%, respectively, with REB, 92, 81, and 78%, respectively, with RFB, 97, 58, and 53%, respectively, with mEC+n incubated at 42 degrees C, and 22, 31, and 25%, respectively, with mEC+n incubated at 35 degrees C. Logistic regression analyses of the data indicated significant main effects of treatment, type of medium, enrichment time, inoculum concentration, and detection method. In particular, a positive result was 1.1 times more likely to occur after 20 h of enrichment than after 8 h, 25 times more likely with RFB and REB than with mEC+n at 35 degrees C, 3.7 times more likely with an initial inoculum of < or = 2.0 CFU/g than with < or = 0.5 CFU/g, 2.5 to 3 times more likely using freeze-stressed or nonstressed bacteria than with cold-stressed bacteria, and 2.5 times more likely by plating than by the immunoassay or the PCR assay. REB had better overall performance for enrichment of cold- and freeze-stressed E. coli O157:H7 present in ground beef than did the other media examined.

Effects of storage and the presence of a beef microflora on the thermal resistance of Salmonella Typhimurium DT104 in beef and broth systems

AIMS

To investigate the effects of storage and the presence of a beef microflora on the thermal resistance of Salmonella serotype Typhimurium DT104 on beef surfaces and in a broth system during subsequent heat treatments after extended low-temperature storage (4 degrees C for 14 days) or mild temperature abuse (10 degrees C for 7 days).

METHODS AND RESULTS

Surviving Salm. Typhimurium DT104 cells were estimated after heating in a water bath (55 degrees C) by plating beef and broth samples on tryptone soya agar and overlaying with xylose-lysine-deoxycholate agar. In beef and broth systems, D(55) values for Salm. Typhimurium DT104 stored at 4 degrees C or 10 degrees C in the presence or absence of a beef microflora were significantly lower (P < 0.01) than the D values for this organism heat-treated immediately after inoculation. In beef systems, the D(55) values were significantly lower (P < 0.05) in the presence of a beef microflora than the D(55) values obtained in 'pure' culture under all temperature/storage combinations. However, in broth systems, there was no significant difference between the D(55) values obtained in 'pure' culture and the D(55) values obtained from systems containing beef microflora.

CONCLUSIONS

Storage of Salm. Typhimurium DT104 significantly reduced the thermal resistance of the pathogen in beef and broth systems. In the presence of high numbers of a Gram-negative beef microflora, the heat sensitivity of the pathogen was further increased on beef surfaces but not in broth.

SIGNIFICANCE AND IMPACT OF THE STUDY

Studies investigating the survival of Salm. Typhimurium DT104 in different food systems will help define safe food preservation processes and will aid in the elimination this pathogen from the food production environments.

Cold stress tolerance of Listeria monocytogenes: A review of molecular adaptive mechanisms and food safety implications

The foodborne pathogen Listeria monocytogenes has many physiological adaptations that enable survival under a wide range of environmental conditions. The microbes overcome various types of stress, including the cold stress associated with low temperatures in food-production and storage environments. Cold stress adaptation mechanisms are therefore an important attribute of L. monocytogenes, enabling these food pathogens to survive and proliferate to reach minimal infectious levels on refrigerated foods. This phenomenon is a function of many molecular adaptation mechanisms. Therefore, an improved understanding of how cold stress is sensed and adaptation measures implemented by L. monocytogenes may facilitate the development of better ways of controlling these pathogens in food and related environments. Research over the past few years has highlighted some of the molecular aspects of cellular mechanisms behind cold stress adaptation in L. monocytogenes. This review provides an overview of the molecular and physiological constraints of cold stress and discusses the various cellular cold stress response mechanisms in L. monocytogenes, as well as their implications for food safety.

Molecular and physiological analysis of the role of osmolyte transporters BetL, Gbu, and OpuC in growth of Listeria monocytogenes at low temperatures

Listeria monocytogenes is a ubiquitous food-borne pathogen found widely distributed in nature as well as an undesirable contaminant in a variety of fresh and processed foods. This ubiquity can be at least partly explained by the ability of the organism to grow at high osmolarity and reduced temperatures, a consequence of its ability to accumulate osmo- and cryoprotective compounds termed osmolytes. Single and multiple deletions of the known osmolyte transporters BetL, Gbu, and OpuC significantly reduce growth at low temperatures. During growth in brain heart infusion broth at 7 degrees C, Gbu and OpuC had a more pronounced role in cryoprotection than did BetL. However, upon the addition of betaine to defined medium, the hierarchy of transporter importance shifted to Gbu > BetL > OpuC. Upon the addition of carnitine, only OpuC appeared to play a role in cryoprotection. Measurements of the accumulated osmolytes showed that betaine is preferred over carnitine, while in the absence of a functional Gbu, carnitine was accumulated to higher levels than betaine was at 7 degrees C. Transcriptional analysis of the genes encoding BetL, Gbu, and OpuC revealed that each transporter is induced to different degrees upon cold shock of L. monocytogenes LO28. Additionally, despite being transcriptionally up-regulated upon cold shock, a putative fourth osmolyte transporter, OpuB (identified by bioinformatic analysis and encoded by lmo1421 and lmo1422), showed no significant contribution to listerial chill tolerance. Growth of the quadruple mutant LO28deltaBCGB (deltabetL deltaopuC deltagbu deltaopuB) was comparable to the that of the triple mutant LO28deltaBCGsoe (deltabetL deltaopuC deltagbu) at low temperatures. Here, we conclude that betaine and carnitine transport upon low-temperature exposure is mediated via three osmolyte transporters, BetL, Gbu, and OpuC.

Characterization and localization of fluorescent Pseudomonas cold shock protein(s) by monospecific polyclonal antibodies

Cold shock protein (CSP) from Pseudomonas fluorescens MTCC 103 and cold resistant protein (CRP) from its mutant CRPF8 of 14 and 35 kd, respectively were purified to homogeneity by HPLC. Polyclonal antibodies were raised against these proteins and the expression level was checked at different temperatures, i.e., 4, 10, 20, 30 and 37 C. Furthermore, morphological changes in P. fluorescens MTCC 103 and its mutant (CRPF8) were analyzed by transmission electron microscopy (TEM). Localization of CSP and CRP documented with immunoelectron microscopy, using colloidal gold particles conjugated with secondary antibodies being the probe were used. Nevertheless, the results of cytosolic localization of CSP and CRP were evident. Furthermore, the expression of CSP and CRP increased with decrease in temperature and the cell wall thickness of the mutant exhibited 2-fold increase, thus facilitating low temperature survival.

Changes in heat resistance resulting from pH and nutritional shifts of acid-adapted and non-acid-adapted Listeria monocytogenes Scott A

Stationary-phase Listeria monocytogenes cells that were either pH dependent acid adapted or not acid adapted were heat challenged at 60 degrees C in a two-level full factorial design for three variables. The three variables and the levels consisted of tryptic soy broth (TSB) and sterile cell-free culture supernatant (sterile TSB), the presence and absence of 1% added glucose, and pH 4.8 and pH 7. Non-acid-adapted cells were most heat resistant when challenged in TSB (mean decimal reduction times at 60 degrees C: D60 = 1.16 min). In the absence of added glucose, non-acid-adapted cells had similar D60-values for inactivations at pH 4.8 and pH 7; however, the presence of glucose caused non-acid-adapted cells challenged at pH 4.8 to be more heat sensitive (D60 = 0.65 min) than those inactivated at pH 7 (D60 = 1.03 min), indicating an interaction between glucose and pH. Overall, the significantly decreased heat resistance of the acid-adapted cells was due to the presence of glucose (D60 = 0.78 min without glucose, D60 = 0.59 min with glucose). Acid-adapted cells heat challenged in TSB had similar D60-values for inactivations at pH 4.8 and pH 7; however, acid-adapted cells in sterile TSB challenged at pH 4.8 (D60 = 0.52 min) had significantly lower heat resistance than did cells challenged at pH 7 (D60 = 0.76 min), indicating an interaction between the medium and pH. The L. monocytogenes survivor data were modeled to extract information on the frequency distribution of heat resistance within heat-challenged populations, and the frequency distribution characteristics of mean, mode, and variance were compared among treatment conditions. Significant differences in the frequency distribution data were compared with the D60-values. These data indicated that the presence and level of cross-protection is highly dependent on the physiological state of the cells and nutrient availability at the time of heat challenge. Such conditions should be considered to ensure that stressed pathogens in foods are destroyed or inactivated.

Ribosome inactivation for preservation: concepts and reservations

The role of the bacterial ribosome in the cellular response to environmental stress has been widely considered over last decade. Certain ribosome-associated proteins have been shown to induce conformational changes that lead to the formation of inactive forms of ribosomes that are presumed to be more stable during stationary phase. This was found to aid the survival of bacteria in this phase. Such proteins include ribosome modulation factor (RMF), YfiA and YhbH. Examining the influence of RMF on the survival of E. coli under heat, acid and osmotic stress showed that it was important for bacterial viability under these environmental pressures. However, the mechanism by which this protein exerts its effect has not been fully elucidated. The present work reviews the involvement of ribosomes in determining cell behaviour during stress. It focuses on the action of the ribosome-associated proteins and their role in inactivating ribosomes for preserving their integrity and aiding cell survival under stress.

Selection and identification of a Listeria monocytogenes target strain for pulsed electric field process optimization

Nine Listeria monocytogenes strains were treated individually with a continuous pulsed electric field (PEF) apparatus, and their sensitivities to the treatment were compared at 25 kV/cm. When cell suspensions of these strains in 0.1% NaCl (pH 7.0) were treated at 23 degrees C for 144 micro s, inactivation ranged from 0.7 to 3.7 log(10) CFU/ml. Inactivation by 72- micro s PEF treatments at 37 degrees C ranged from 0.3 to 2.5 log(10) CFU/ml. L. monocytogenes OSY-8578 was substantially more resistant than other strains when cells were PEF treated in 0.1% NaCl, whereas Scott A was one of the most sensitive strains. The superiority of OSY-8578's resistance to that of Scott A was confirmed in 50% diluted acid whey (pH 4.2). Changes in sensitivity to PEF during phases of growth were minimal in OSY-8578 and substantial in Scott A. Use of L. monocytogenes OSY-8578, therefore, is recommended in studies to optimize PEF processes that target L. monocytogenes. The nine L. monocytogenes strains were genotyped with pulsed-field gel electrophoresis (PFGE) and arbitrarily primed PCR (AP-PCR) techniques. These strains were better differentiated with PFGE than with AP-PCR. The target strain (OSY-8578) was characterized by both molecular typing techniques, but resistance to PEF, in general, was not associated with a particular genotype group.

Effect of dissolved carbon dioxide on thermal inactivation of microorganisms in milk

Postpasteurization addition of CO2 inhibits growth of certain microorganisms in dairy products, but few workers have investigated the effect of CO2 on the thermal inactivation of microorganisms during pasteurization. Concentrations of CO2 ranging from 44 to 58 mM added to raw whole milk significantly (P < 0.05) reduced the number of surviving standard plate count (SPC) organisms in milk heated over the range of 67 to 93 degrees C. A decrease in thermal survival rates (D-values) for Pseudomonas fluorescens R1-232 and Bacillus cereus ATCC 14579 spores in milk was positively correlated with CO2 concentrations (1 to 36 mM). D(50 degrees C)-values for P. fluorescens significantly decreased (P < 0.05) in a linear fashion from 14.4 to 7.2 min. D(89 degrees C)-values for B. cereus spores were significantly (P < 0.05) decreased from 5.56 min in control milk to 5.29 min in milk containing 33 mM CO2. The Weibull function was used as a model to describe the thermal inactivation of P. fluorescens, B. cereus spores, and SPC organisms in raw milk. Nonlinear parameters for the Weibull function were estimated, and survival data fitted to this model had higher R2 values than when fitted to the linear model, further providing support that the thermal inactivation of bacteria does not always follow first-order reaction rate kinetics. These results suggest that CO2 could be used as a processing aid to enhance microbial inactivation during pasteurization.

Enhanced levels of cold shock proteins in Listeria monocytogenes LO28 upon exposure to low temperature and high hydrostatic pressure

Listeria monocytogenes is a psychrotrophic food-borne pathogen that is problematic for the food industry because of its ubiquitous distribution in nature and its ability to grow at low temperatures and in the presence of high salt concentrations. Here we demonstrate that the process of adaptation to low temperature after cold shock includes elevated levels of cold shock proteins (CSPs) and that the levels of CSPs are also elevated after treatment with high hydrostatic pressure (HHP). Two-dimensional gel electrophoresis combined with Western blotting performed with anti-CspB of Bacillus subtilis was used to identify four 7-kDa proteins, designated Csp1, Csp2, Csp3, and Csp4. In addition, Southern blotting revealed four chromosomal DNA fragments that reacted with a csp probe, which also indicated that a CSP family is present in L. monocytogenes LO28. After a cold shock in which the temperature was decreased from 37 degrees C to 10 degrees C the levels of Csp1 and Csp3 increased 10- and 3.5-fold, respectively, but the levels of Csp2 and Csp4 were not elevated. Pressurization of L. monocytogenes LO28 cells resulted in 3.5- and 2-fold increases in the levels of Csp1 and Csp2, respectively. Strikingly, the level of survival after pressurization of cold-shocked cells was 100-fold higher than that of cells growing exponentially at 37 degrees C. These findings imply that cold-shocked cells are protected from HHP treatment, which may affect the efficiency of combined preservation techniques.

Cold shock induction of thermal sensitivity in Listeria monocytogenes

Cold shock at 0 to 15 degrees C for 1 to 3 h increased the thermal sensitivity of Listeria monocytogenes. In a model broth system, thermal death time at 60 degrees C was reduced by up to 45% after L. monocytogenes Scott A was cold shocked for 3 h. The duration of the cold shock affected thermal tolerance more than did the magnitude of the temperature downshift. The Z values were 8.8 degrees C for controls and 7.7 degrees C for cold-shocked cells. The D values of cold-shocked cells did not return to control levels after incubation for 3 h at 28 degrees C followed by heating at 60 degrees C. Nine L. monocytogenes strains that were cold shocked for 3 h exhibited D(60) values that were reduced by 13 to 37%. The D-value reduction was greatest in cold-shocked stationary-phase cells compared to cells from cultures in either the lag or exponential phases of growth. In addition, cold-shocked cells were more likely to be inactivated by a given heat treatment than nonshocked cells, which were more likely to experience sublethal injury. The D values of chloramphenicol-treated control cells and chloramphenicol-treated cold-shocked cells were no different from those of untreated cold-shocked cells, suggesting that cold shock suppresses synthesis of proteins responsible for heat protection. In related experiments, the D values of L. monocytogenes Scott A were decreased 25% on frankfurter skins and 15% in ultra-high temperature milk if the inoculated products were first cold shocked. Induction of increased thermal sensitivity in L. monocytogenes by thermal flux shows potential to become a practical and efficacious preventative control method.