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.     

单核细胞增生李斯特菌抗酸应激机制

单核细胞增生李斯特菌(Listeria monocytogenes,简称单增李斯特菌)是重要的人畜共患食源性病原,在青贮饲料、发酵食品、宿主胃内以及巨噬细胞吞噬体内都会遭遇酸应激。该菌有多种抗酸应激系统,如F0F1-ATPase、谷氨酸脱羧酶(Glutamate decarboxylase system,GAD)、精氨酸脱亚胺酶(Arginine deiminase,ADI)、鲱氨酸脱亚胺酶(Agmatine deiminase,Ag DI)系统等。在环境pH(pHex)4.5条件下可维持其细胞内pH(pHi)稳态,在pHex 3.5时仍能存活;用温和酸应激(pHex 4.5)预处理单增李斯特菌,可以通过酸耐受反应(Acid tolerance response)提高其在致死性酸性环境中的存活率,这一过程受σB正调控,即σB激活可以保护单增李斯特菌应对多种环境应激。因此,σB可以作为新型抗菌药物的靶标。更为重要的是,弱酸性发酵食品要严格控制李斯特菌的污染,以降低消费者的感染风险。     

The effect of acid shock on the heat resistance of Listeria monocytogenes

The effect of acid shock on the heat resistance of Listeria monocytogenes was investigated. After growth for 24 h at 30°C in tryptic soy broth containing 0.6% yeast extract, cell culture suspensions of L. monocytogenes were acidified with HCl or acetic acid over various time periods before being heated in whole milk to a temperature of 58°C. When cells were acid-shocked immediately with HCl for 1, 2 or 4 h, those acid-shocked for 1 h demonstrated the largest increase in thermotolerance as compared to control cells, when heated at 58°C in whole milk. In fact, cells acid-shocked for longer than 1 h with HCl demonstrated in some instances a decreased recovery as compared to control cells. Other types of acid-shock treatments included lowering the pH gradually either over a 4 h or a 24 h period. However, regardless of the type of acid-shock treatment, cells acid-shocked with HCl (but not acetic acid) prior to heating had significantly greater heat resistance as compared to control (non-acid-shocked) cells. It appears that acidification with HCl prior to final heating can enhance the heat resistance of L. monocytogenes.     

Survival of Listeria monocytogenes in sea water and effect of exposure on thermal resistance

Survival, recoverability and sublethal injury of two strains of Listeria monocytogenes , Scott A and an environmental strain KM, on exposure to sea water at 12·8 or 20·8 °C was determined using in situ diffusion chambers. Plate counts were used to assess recoverability and injury while 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) reduction was used to determine respiratory activity. T₉₀ values (times for 10-fold decreases in numbers of recoverable cells) on non-selective medium (trypticase soya agar with 0·6% yeast extract) at 12·8 and 20·8 °C were 61·7 and 69·2 h for L. monocytogenes Scott A, and 103·0 and 67·0 h for L. monocytogenes KM, respectively. On selective medium (Oxford agar), T₉₀ values at 12·8 and 20·8 °C were 60·6 and 56·9 h for L. monocytogenes Scott A, and 83·0 and 65·9 h for L. monocytogenes KM, respectively. With Scott A, the percentage of sublethally injured cells at 12·8 and 20·8 °C was 1·7 and 17·7%, respectively, while for KM the values were 19·0 and 1·6%, respectively. The fraction of cells reducing CTC but which were not recoverable on plating progressively increased on exposure to sea water. Listeria monocytogenes KM challenged at 58 °C showed an apparent increase in heat resistance after exposure to sea water at 20·8 °C for 7 d ( D ₅₈= 2·64 min) compared with before exposure ( D ₅₈= 1·24). This increase in thermal resistance was not apparent at temperatures greater than 63 °C, and analysis of the best-fit regression lines fitted to the thermal data obtained from the two cell populations indicated that their thermal resistance was not significantly different ( P > 0·05) over the temperature range tested (58–62 °C).     

Cold Shock Induction of Thermal Sensitivity in Listeria monocytogenes

Cold shock at 0 to 15°C for 1 to 3 h increased the thermal sensitivity of Listeria monocytogenes. In a model broth system, thermal death time at 60°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°C for controls and 7.7°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°C followed by heating at 60°C. Nine L. monocytogenes strains that were cold shocked for 3 h exhibited D₆₀ 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.     

Cold Shock and Its Effect on Ribosomes and Thermal Tolerance in Listeria monocytogenes

Differential scanning calorimetry (DSC) and fatty acid analysis were used to determine how cold shocking reduces the thermal stability of Listeria monocytogenes. Additionally, antibiotics that can elicit production of cold or heat shock proteins were used to determine the effect of translation blockage on ribosome thermal stability. Fatty acid profiles showed no significant variations as a result of cold shock, indicating that changes in membrane fatty acids were not responsible for the cold shock-induced reduction in thermal tolerance. Following a 3-h cold shock from 37 to 0°C, the maximum denaturation temperature of the 50S ribosomal subunit and 70S ribosomal particle peak was reduced from 73.4 ± 0.1°C (mean ± standard deviation) to 72.1 ± 0.5°C (P ≤ 0.05), indicating that cold shock induced instability in the associated ribosome structure. The maximum denaturation temperature of the 30S ribosomal subunit peak did not show a significant shift in temperature (from 67.5 ± 0.4°C to 66.8 ± 0.5°C) as a result of cold shock, suggesting that either 50S subunit or 70S particle sensitivity was responsible for the intact ribosome fragility. Antibiotics that elicited changes in maximum denaturation temperature in ribosomal components also elicited reductions in thermotolerance. Together, these data suggest that ribosomal changes resulting from cold shock may be responsible for the decrease in D value observed when L. monocytogenes is cold shocked.     

Identification of Listeria monocytogenes genes involved in salt and alkaline-pH tolerance

The capacity of Listeria monocytogenes to tolerate salt and alkaline stresses is of particular importance, as this pathogen is often exposed to such environments during food processing and food preservation. We screened a library of Tn917-lacZ insertional mutants in order to identify genes involved in salt and/or alkaline tolerance. We isolated six mutants sensitive to salt stress and 12 mutants sensitive to salt and alkaline stresses. The position of the insertion of the transposon was located in 15 of these mutants. In six mutants the transposon was inserted in intergenic regions, and in nine mutants it was inserted in genes. Most of the genes have unknown functions, but sequence comparisons indicated that they encode putative transporters.     

The effect of osmotic shock and subsequent adaptation on the thermotolerance and cell morphology of Listeria monocytogenes

The relationship between the time of exposure to different levels of NaCl and the corresponding changes in thermotolerance and cell morphology of Listeria monocytogenes was investigated. The kinetics of the increase in thermotolerance, after an osmotic upshift, showed a very rapid initial response (<2 min) followed by a more gradual increase whereby cells, after 4 h exposure at 30°C, became nearly as heat resistant as those grown for 48 h under the same conditions. Cells grown in media with 0.09 mol l⁻¹ NaCl subjected to a short osmotic up-shock in media containing 0.5, 1.0 or 1.5 mol l⁻¹ NaCl showed a 1.3, 2.5 and 8-fold increase in thermotolerance, respectively. Osmotic adaptation, signified by growth at the higher NaCl concentration, however, resulted in a 2- to 3-fold additional increase in thermotolerance. An osmotic down-shock caused a very rapid loss of thermotolerance (<5 min). Osmotic shock and adaptation experiments were also performed in minced beef where similar changes in thermotolerance were observed. Cell morphology was markedly affected by the osmolarity of the growth medium. Cells grown in media containing 1.5 mol l⁻¹ NaCl became up to 50 times longer than cells grown in media with 0.09 mol l⁻¹ NaCl, but no direct link to thermotolerance could be made.     

Effect of food processing-related stresses on acid tolerance of Listeria monocytogenes

Stationary-phase cells of Listeria monocytogenes grown in glucose-free or glucose-containing media were exposed for 90 min to various stresses, including acid stress (pH 4.0 to 7.0), osmotic stress (10.5 to 20.5% NaCl), and various temperatures (-5 to 50 degrees C), and were further exposed to pH 3.5. Exposure to a mildly acidic (pH 5.0 to 6.0) environment provided protection of the pathogen against acid upon subsequent exposure. This adaptive response, however, was found to be strongly dependent on other environmental conditions during the shock, such as temperature or the simultaneous presence of a second stress factor (NaCl). Growth of L. monocytogenes in the presence of glucose resulted in enhanced survival of the pathogen at pH 3.5. Sublethal stresses other than acidic stresses, i.e., osmotic, heat, and low-temperature stresses, did not affect the acid resistance of L. monocytogenes (P > 0.5). More-severe levels of these stresses, however, resulted in sensitization of the pathogen to acid.     

Characterization of glycine betaine porter I from Listeria monocytogenes and its roles in salt and chill tolerance

Listeria monocytogenes is a pathogenic bacterium that can grow at low temperatures and elevated osmolarity. The organism survives these stresses by the intracellular accumulation of osmolytes: low-molecular-weight organic compounds which exert a counterbalancing force. The primary osmolyte in L. monocytogenes is glycine betaine, which is accumulated from the environment via two transport systems: glycine betaine porter I, an Na(+)-glycine betaine symporter; and glycine betaine porter II, an ATP-dependent transporter. The biochemical characteristics of glycine betaine porter I were investigated in a mutant strain (LTG59) lacking the ATP-dependent transporter. At 4% NaCl, glycine betaine uptake in LTG59 was about fivefold lower than in strain DP-L1044, which has both transporters, indicating that the ATP-dependent transporter is the primary means by which glycine betaine enters the cell. In the absence of osmotic stress, cold-activated uptake by both transporters was most rapid between 7 and 12 degrees C, but a larger fraction of the total uptake was via the ATP-dependent transporter than was observed under salt-stressed conditions. Twelve glycine betaine analogs were tested for their ability to inhibit glycine betaine uptake and growth of stressed cultures. Carnitine, dimethylglycine, and gamma-butyrobetaine appear to inhibit the ATP-dependent transporter, while trigonelline and triethylglycine primarily inhibit glycine betaine porter I. Triethylglycine was also able to retard the growth of osmotically stressed L. monocytogenes grown in the presence of glycine betaine.     

The effect of culture growth phase on induction of the heat shock response in Yersinia enterocolitica and Listeria monocytogenes

The effect of culture growth phase on induction of the heat shock response in Yersinia enterocolitica and Listeria monocytogenes, was examined. Exponential or stationary preconditioned cultures were heat shocked and survivor numbers estimated using selective and overlay/resuscitation recovery techniques. The results indicate that prior heat shock induced increased heat resistance in both micro-organisms to higher heat treatments. Heat-shocked cells of each micro-organism were able to survive much longer than non-heat-shocked cells when heated at 55 degrees C. The size of the change in heat resistance between heat-shocked and non-heat-shocked cells was greatest for exponential cultures (X:X). Results indicate that the overall relative thermal resistance of each pathogen was dependent on cell growth phase. Stationary cultures (S:S) were significantly (P < 0.01) more thermotolerant than exponential cultures (X:X) under identical processing conditions. Under most conditions, the use of an overlay/resuscitation recovery medium resulted in higher D-values (P < 0.05) compared with a selective recovery medium.     

The potentiating action of Listeria monocytogenes in relation to endotoxic shock

The infection of mice with Listeria cells 15 hours prior to the inoculation of endotoxin sharply increased its lethal action. Blood taken from the mice at the peak of the development of shock was toxic for adrenalectomized recipients. This potentiating effect was blocked by hydrocortisone. Carrageenan prolonged the survival time of such animals, but did not change the course of endotoxin shock, as well as toxicosis, caused by listerial aqueous extract in adrenalectomized mice. This potentiating phenomenon plays supposedly a certain role in the genesis of toxicosis in mixed infections (listeriosis in combination with other infections).     

Comparative proteomic analysis of Listeria monocytogenes tolerance to bile stress

The ability of the food-borne pathogen Listeria monocytogenes to tolerate bile is critical to its successful infection and colonization in the human gastrointestinal tract. Using comparative proteomics, a total of 48 proteins were identified in this study in the presence of moderate (0.3 %) or high (3 %) level of bile salts in the wild-type strain EGD. Identified proteins fell into 14 functional categories covering most of the biochemical functions of bacterial cells, indicating that there were complex physiological mechanisms involved in L. monocytogenes tolerance of bile stress. Among them, 16, 14, and 18 proteins were expressed differently in the isogenic deletion mutants of L. monocytogenes EGDΔsigB, EGDΔprfA, and EGDΔprfAΔsigB, respectively, compared with their parent strain EGD at corresponding concentrations of bile salts. All proteins identified in EGDΔsigB and EGDΔprfAΔsigB were all down-expressed in the presence of bile salts, whereas several proteins were up-expressed in EGDΔprfA, in particular at the high level of bile (3 %), indicating that SigB plays an essential positive role in L. monocytogenes tolerance of bile stress and that the negative effect of PrfA may facilitate its survival in bile in the gastrointestinal tract before its successful colonization and invasion.     

Prevention of allograft tolerance by bacterial infection with Listeria monocytogenes

Exposure to certain viruses and parasites has been shown to prevent the induction of transplantation tolerance in mice via the generation of cross-reactive memory T cell responses or the induction of bystander activation. Bacterial infections are common in the perioperative period of solid organ allograft recipients in the clinic, and correlations between bacterial infections and acute allograft rejection have been reported. However, whether bacterial infections at the time of transplantation have any effect on the generation of transplantation tolerance remains to be established. We used the Gram-positive intracellular bacterium Listeria monocytogenes (LM) as a model pathogen because its effects on immune responses are well described. Perioperative LM infection prevented cardiac and skin allograft acceptance induced by anti-CD154 and donor-specific transfusion in mice. LM-mediated rejection was not due to the generation of cross-reactive T cells and was largely independent of signaling via MyD88, an adaptor for most TLRs, IL-1, and IL-18. Instead, transplant rejection following LM infection was dependent on the expression of the phagosome-lysing pore former listeriolysin O and on type I IFN receptor signaling. Our results indicate that bacterial exposure at the time of transplantation can antagonize tolerogenic regimens by enhancing alloantigen-specific immune responses independently of the generation of cross-reactive memory T cells.     

单增李斯特菌生物膜及其形成机制的研究进展

单增李斯特菌(Lm)是重要的人兽共患食源性病原菌。Lm生物膜与其致病性和耐药性密切相关。影响Lm生物膜形成的关键因子有鞭毛糖蛋白、胞外基质和群体感应系统等。鞭毛糖蛋白能促进菌体聚集,从而直接影响生物膜的形成。胞外DNA参与Lm粘附和生物膜早期的形成,并与胞外多糖和胞外结合蛋白一起构成生物膜胞外基质。Lm的Agr群体感应系统正调控生物膜形成,是一种集合毒力因子、耐药因子和生物膜的整体水平调控网络体系。     

Effect of prior heat shock on heat resistance of Listeria monocytogenes in meat

The effect of prior heat shock on the thermal resistance of Listeria monocytogenes in meat was investigated. A sausage mix inoculated with approximately 10(7) L. monocytogenes per g was initially subjected to a heat shock temperature of 48 degrees C before being heated at a final test temperature of 62 or 64 degrees C. Although cells heat shocked at 48 degrees C for 30 or 60 min did not show a significant increase in thermotolerance as compared with control cells (non-heat shocked), bacteria heat shocked for 120 min did, showing an average 2.4-fold increase in the D64 degrees C value. Heat-shocked cells shifted to 4 degrees C appeared to maintain their thermotolerance for at least 24 h after heat shock.     

Growth and thermal inactivation of Listeria monocytogenes in cabbage and cabbage juice

Studies were done to determine the interacting effects of pH, NaCl, temperature, and time on growth, survival, and death of two strains of Listeria monocytogenes. Viable population of the organism steadily declined in heat-sterilized cabbage stored at 5 degrees C for 42 days. In contrast, the organism grew on raw cabbage during the first 25 days of a 64-day storage period at 5 degrees C. Growth was observed in heat-sterilized unclarified cabbage juice containing less than or equal to 5% NaCl and tryptic phosphate broth containing less than or equal to 10% NaCl. Rates of thermal inactivation increased as pH of clarified cabbage juice heating medium was decreased from 5.6 to 4.0. At 58 degrees C (pH 5.6), 4 X 10(6) cells/mL were reduced to undetectable levels within 10 min. Thermal inactivation rates in clarified cabbage juice (pH 5.6) were not significantly influenced by the presence of up to 2% NaCl; however, heat-stressed cells had increased sensitivity to NaCl in tryptic soy agar recovery medium. Cold enrichment of heat-stressed cells at 5 degrees C for 21 days enhanced resuscitation. Results indicate that L. monocytogenes can proliferate on refrigerated (5 degrees C) raw cabbage which, in turn, may represent a hazard to health of the consumer. Heat pasteurization treatments normally given to cabbage juice or sauerkraut would be expected to kill any L. monocytogenes cells which may be present.