Lag Phase Of Cells Research Articles

Allelopathic compounds of a red tide dinoflagellate have species-specific and context-dependent impacts on phytoplankton

The use of chemical compounds to suppress the growth of competitors is a competitive strategy known as allelopathy that can be readily observed with many phytoplankton species in lab- oratory studies. However, it is unclear how these allelopathic interactions are altered when the com- plexity of the system is increased to more closely mimic natural conditions. In the present study, we conducted laboratory experiments to decipher how the identity, abundance, and growth stage of competitors affect the outcome of allelopathic interactions with the red tide dinoflagellate Karenia brevis. Multiple chemical compounds produced by K. brevis were found to inhibit the growth of 4 phytoplankton competitors, although these competitors were susceptible to different combinations of compounds. We found that physiological state and cell concentration of competitors were important determinants of allelopathy, with early-stage (lag phase) cells more vulnerable to allelopathic effects than later growth stages for the diatom Skeletonema grethae. Despite being allelopathic to multiple competitors in the laboratory, in a microcosm experiment using plankton field assemblages, extracel- lular extracts of 2 strains of K. brevis had no effects on some taxa although they stimulated the growth of some diatoms. This suggests that in a species-rich ecological community under oligotrophic condi- tions, the relative importance of K. brevis allelopathy may not be as high as most laboratory studies predict.

Influence of temperature and sakacin A concentration on survival ofListeria innocua cultures

The antimicrobial activity of sakacin A, a bacteriocin produced byLactobacillus sakei, was investigated at 30, 10 and 4°C againstListeria innocua in stationary and lag phases of growth. When sakacin was added toListeria innocua cells in stationary phase, two death kinetics were observed. Populations ofListeria innocua were reduced up to threelog cycles when sakacin A was increased from 0 (control) to 1600 AU ml−1. When sakacin A was added toListeria innocua cells in lag phase, lag time, inhibition time and rate were proportionally extended, and maximum population decreased when employing higher bacteriocin levels. Sakacin A was found to influenceListeria innocua growth, a microorganism able to grow at 4 °C. At refrigerated temperatures, the addition of sakacin A was found to inhibitListeria innocua cell growth. Sakacin A may be considered a promising molecule to be used as antimicrobial agent to preserve the shelf life of refrigerated foods.

Monitoring growth phase-related changes in phosphatidylcholine-specific phospholipase C production, adhesion properties and physiology of Bacillus cereus vegetative cells

The physiological status and metabolic heterogeneity of Bacillus cereus cells within a culture during an 8-h batch fermentation process was measured using flow cytometry (FCM). Concurrently, production of the toxin, PC-PLC, and the extent of cell adhesion of live and dead cells were monitored using novel fluorescent assays. Flow cytometry analysis detected growth phase-related changes in the physiological profiles of cells over the course of the fermentation, with variation in the percentage of cells displaying membrane damage and intracellular esterase and redox activities. As the exponential phase proceeded, populations became more uniform in terms of protein content as measured using FCM in tandem with a cell tracking dye, with the majority of cells becoming membrane intact, esterase positive and redox active. PC-PLC activity appeared strongly related to cell density. Permeabilisation of cells was accompanied by a loss in adherent properties, while 25-100% of cells with intracellular esterase activity possessed adhesion properties. Cells in late exponential phase appeared to have reduced adherence properties compared to cells in early exponential or lag phase. As well as demonstrating the utility of FCM for measuring heterogeneity in terms of cell physiological status throughout the course of batch cultures, the methods utilised in this study could be used to relate processes such as toxin production or cell adhesion to cell physiological state.

Characterization of growth‐phase‐specific responses to cold in Arabidopsis thaliana suspension‐cultured cells

To understand complex responses of plant cells to low temperatures, suspension-cultured cells of Arabidopsis thaliana (line T87) were characterized during cold treatment. Freezing tolerance of cells collected at the lag or log phase was quite different: an increase in freezing tolerance during cold treatment was only detectable with cells at the lag phase. Although there were little differences in the osmolality of cells at the two growth phases, sugar content increased during cold treatment only in lag phase cells. Abscisic acid (ABA) content was greater at the lag phase than at the log phase throughout the cold treatment, and increased only in lag phase cells. Interestingly, ABA treatment resulted in an increase in freezing tolerance only in lag phase cells. Expression of cold-responsive genes such as DREB1A/CBF3, COR15a and RD29A occurred in cells at the two growth phases; however, the extent of the induction of COR15a and RD29A was somewhat greater in cells at the lag phase than at the log phase. Microarray analysis revealed that cold-regulated genes (COR genes) were categorized into three groups: up- or down-regulated consistently during cold treatment, transiently after 1 d and later after 2 d of cold treatment. Among genes that were up-regulated throughout or transiently after 1 d of cold treatment only in lag phase cells, functions in upstream of signal transduction such as kinase and transcription factor were often deduced. These results collectively suggest that these genes seem to be associated with induction of freezing tolerance by cold treatment in cells at the lag phase.

A mathematical model of Saccharomyces cerevisiae growth in response to cadmium toxicity

Microbial growth can be described using models derived by differential equations, but available mathematical models have yet to adequately describe lag phase related cell growth or cell mortality in response to chemical toxicity. Lag phase cell behavior, however, dictates the onset of exponential growth and the number of actively growing cells available to initiate exponential growth, important factors in the success of remediation efforts. In this study, a five-parameter polynomial ratio (PR) model was used to characterize the growth, from lag through stationary phase, of the yeast Saccharomyces cerevisiae in response to cadmium toxicity. The PR model used in this study has the advantages over standard mathematical models in the ability to represent the initial cell mortality observed when S. cerevisiae is exposed to increasing cadmium levels, up to 12 mg/l Cd, as well as following cell recovery and growth to stationary levels.

Quantitative response of trehalose and glycerol syntheses by Candida krusei to osmotic stress of the medium

The effect of osmotic stress on the biosyntheses of trehalose and glycerol by Candida krusei was quantitatively studied. With the increase of initial osmotic pressure (IOP) between 959 and 2108 mOsm kg −1, the cell growth and trehalose accumulation were decreased; the extracellular and intracellular glycerol syntheses were enhanced with an increase of IOP from 1320 to 1678 mOsm kg −1, and leveled off at an even higher IOP (i.e. 2108 mOsm kg −1). The cellular response to medium osmotic pressure was also dependent on the physiological state of cells (growth phase). The cells in exponential growth phase survived better than those in stationary and lag phases. For the exponential phase cells, the glycerol concentration doubled and the trehalose content increased 19% compared to the control. In contrast, for the lag phase cells, the trehalose content decreased while the glycerol synthesis was enhanced. The results are considered helpful to the further understanding of the metabolism of the osmophilic yeast under osmotic environments and the work may be useful for quantitative manipulation of the metabolites’ production on a large scale.

THE INFLUENCE OF PRE-CULTURAL CONDITIONS ON THE INDIVIDUAL CELL LAG PHASE AND GENERATION TIME OF LISTERIA MONOCYTOGENES

In this study, we assessed the impact of pre-cultural temperature and pH on the lag phase of individual Listeria monocytogenes cells at 7 degrees C. In a first step, the pre-cultural temperature effect (37, 15, 10, 7, 4 and 2 degrees C) was investigated on a subsequent growth at 7 degrees C and pH 7.4. In a second step, low pre-cultural temperatures (10, 7 and 4 degrees C) were combined with a controlled pH at 7.4 and 5.7 with a subsequent growth at 7 degrees C and different pH values (7.4, 6.0 and 5.5). Growth was monitored by OD measurements at 600 nm. For all temperature-pH combinations, the individual cell lag phase and the subsequent growth rate were determined using a three-phase linear growth model. Around 100 replications were made for each set of conditions. The results were plotted as histograms, and distributions were fitted to these data. It was observed that at low pre-cultural temperatures, a high proportion of L. monocytogenes cells was able to grow with almost no lag phase. Regarding the pH effect, the pH transition from the pre-cultural to a growth media was proportional to the mean values of the lag phases. There was no remarkable effect observed on the generation time. Regarding the statistical distributions, the exponential distribution yielded the best fit in most cases, as the highest probability was observed for very small lag phases and decreased with lag phase increase. To apply the results obtained at an industrial level, a simulation in @RISK to predict the distribution of the lag phase of L. monocytogenes at different transitions of temperature was performed. These findings have important consequences to estimate the possible risks concerned L. monocytogenes for refrigerated food products.

CumA, a gene encoding a multicopper oxidase, is involved in Mn2+ oxidation in Pseudomonas putida GB-1.

Pseudomonas putida GB-1-002 catalyzes the oxidation of Mn2+. Nucleotide sequence analysis of the transposon insertion site of a nonoxidizing mutant revealed a gene (designated cumA) encoding a protein homologous to multicopper oxidases. Addition of Cu2+ increased the Mn2+-oxidizing activity of the P. putida wild type by a factor of approximately 5. The growth rates of the wild type and the mutant were not affected by added Cu2+. A second open reading frame (designated cumB) is located downstream from cumA. Both cumA and cumB probably are part of a single operon. The translation product of cumB was homologous (level of identity, 45%) to that of orf74 of Bradyrhizobium japonicum. A mutation in orf74 resulted in an extended lag phase and lower cell densities. Similar growth-related observations were made for the cumA mutant, suggesting that the cumA mutation may have a polar effect on cumB. This was confirmed by site-specific gene replacement in cumB. The cumB mutation did not affect the Mn2+-oxidizing ability of the organism but resulted in decreased growth. In summary, our data indicate that the multicopper oxidase CumA is involved in the oxidation of Mn2+ and that CumB is required for optimal growth of P. putida GB-1-002.

Effect of Growth Stage and Processing Temperature on the Inactivation of E. coli by Pulsed Electric Fields

The effect of growth stage and processing temperature on the inactivation of Escherichia coli subjected to pulsed electric fields was studied. Simulated milk ultrafiltrate (SMUF) inoculated with E. coli was subjected to high-intensity exponentially decaying or square-wave pulses with a field strength of 36 kV/cm and pulse duration of 2 μs at selected temperatures ranging between 3 and 40°C. The rate of inactivation increased with an increase in the processing temperature. Furthermore, square-wave pulses were more lethal than exponentially decaying pulses. At 7°C after 100 μs, square-wave pulses produced a 99% decrease while exponential decaying pulses produced a 93% decrease in bacterial cell population. Cells harvested at lag, log, and stationary phases were subjected to 2 and 4 pulses with an electric field intensity of 36 kV/cm at 7°C. Logarithmic-phase cells were more sensitive than stationary- and lag-phase cells to the pulsed electric field treatment.

Autocrine Interleukin-1β Regulates Both Proliferation and Apoptosis in EL4-6.1 Thymoma Cells

We demonstrate here that EL4-6.1 cells, a mouse thymoma that expresses high levels of membrane interleukin (IL)-1 receptors, produce IL-1 beta as an autocrine regulatory factor. Endogenous IL-1 beta sustains both proliferation and apoptosis: during the exponential phase, it mainly promotes proliferation, while during the plateau phase of cell growth, it induces death by apoptosis. Additionally, we show that exogenous IL-1 beta added to EL4-6.1 cells in lag phase induces apoptosis in a portion of the cells and proliferation in the remaining cells. Therefore, IL-1 beta can exert two completely opposite effects on a single cell type, depending on the state of the target cell.

Novel nucleoside transport inhibitors of natural origin.

The salvage pathway of nucleotide biosynthesis is one of the attractive targets for cancer chemotherapy. Dipyridamole, a nucleoside transport inhibitor, may be used for blocking the salvage pathways. Studies have demonstrated that dipyridamole can reduce the rescue effect of exogenous nucleosides and potentiated the cytotoxicity of acivicin, an antimetabolite of de novo pathways of nucleotide biosynthesis, in hepatoma cells1, 2. The inhibitory effect of dipyridamole was cell growth phase -dependent. Cultured hepatoma cells in lag and log phase were highly sensitive to dipyridamole; by contrast, cells in stationary phase were insensitive3, 4. The combination of acivicn and dipyridamole administered in rats bearing s.c. transplanted hepatoma yielded a summation effect, decreasing NTP and dNTP pools5. The synergistic effect of dipyridamole and acivicin was also demonstrated in colon cancer cells6. There have been reports that dipyridamole potentiated the cytotoxicity of several antimetabolites including methotrexate, 5-fluorouracil and N-phosphonacetyl-L-aspartate (PALA)7, 8, 9. In addition to potentiating antimetabolites, dipyridamole was found to enhance the cytotoxicity of doxorubicin, etoposide, vincristine and mitoxantrone10, 11, 12. These synergistic effects of dipyridamole and drugs which are not antimetabolites were due to, at least in part, increased intracellular concentrations of anticancer drugs. Drug sensitivity of multidrug resistant cells may be modulated by dipyridamole13. The use of dipyridamole in cancer chemotherapy has drawn much attention and clinical trials have been underway14, 15. The study of dipyridamole indicates that blocking nucleoside salvage pathways by inhibition of nucleoside transport may be one of the effective ways in chemotherapy. By examination of a series of compounds of natural origin, we have found that green tea polyphenols (GP), antibiotics C3368-A (CA) and C3368-B (CB) are highly active in blocking nucleoside transport in cancer cells. Investigations have demonstrated that GP, CA or CB show synergistic effects with anticancer drugs.

Culture characteristics and orthophosphate excretion of a marine oligotrich ciliate, Strombidium sulcatum, fed heat-killed bacteria

Ciliate microzooplankton have long been hypothesized as probably being important in nutrient regeneration. However, few estimates of excretion rates exist, especially with regard to phosphorus, identified as the limiting nutrient in many marine and estuarine systems. The utility of using heat-killed prey, which avoids prey uptake of excretory products, to measure excretion in feeding ciliates was examined. The growth characteristics of Strombidium sulcatum fed heat-killed vs live bacteria were investigated and orthophosphate excretion rates were estimated. Organismal and orthophosphate concentrations were monitored over an 8 d incubation period in triplicate solutions of live bacteria, live bacteria plus ciliates, heat-killed bacteria, and heat-killed bacteria plus ciliates. Ciliate cultures fed heat-killed or live bacteria were indistinguishable in terms of the duration of different growth phases, maximum growth rates, maximum concentrations and average cell dry weights based on volume estimates. Calculated orthophosphate excretion rates of S. sulcatum, based on changes in concentrations of orthophosphate in cultures fed heat-killed bacteria, were highest for lag-phase cells, 13 x 10(-6) mug-at. ciliate-1 h-1 (24.6 mug P mg-1 dry wt h-1) and decreased to zero for late stationary-phase cells.

Detection of SOS Response Induced by .GAMMA.-Irradiation Using umu-Test, a Bacterial Short-term Test.

The genotoxic effects of γ-irradiation were examined using the umu-test, a bacterial short-term test for the detection of genotoxic agents. The principle of the umu-test is based on the induction of the SOS function against DNA damage in Salmonella typhimurium (TA1535/pSK1002 and TA4107/ pSK1002) ; the test measures colorimetrically the expression of umu gene involved directly in SOS mutagenesis. The strains, irradiated aerobically in phosphate buffer, showed a phasic response to γ-irradiation during their growth cycles. The SOS response was the highest level in the lag phase, suggesting that the irradiated lag-phase cells are very sensitive to γ-irradiation and survive via mutation.The hydroxyl radical scavengers, dimethylsulfoxide (DMSO) and ethanol, decreased the SOS response in a dose-related manner. DMSO (0.5 M) added into the aerobic cell suspension decreased the SOS response by about 50% in TA1535/pSK1002 and by about 40% in TA4107/pSK1002 ; the similar proportion was shown in the anaerobic cell suspension containing the scavenger. In addition, the SOS response induced under the aerobic condition was reduced by about 60% in the anaerobic cell suspension containing 0.5 M DMSO. The observations suggest that the SOS response is induced similarly by the direct excitation and the indirect effect by hydroxyl radicals generated during the γ-irradiation of aqueous solution.These results obtained using umu-test were consistent with the phasic response and the irradiation effect reported previously using the survival and DNA strand breaks of bacteria.

Destruction, Injury, and Repair of Listeria Species Exposed to Sanitizing Compounds

The efficacy of four commonly used dairy plant sanitizers against Listeria coupled with an examination of ability of these sanitizers to induce injury was investigated. Listeria monocytogenes F 5069, F 5027, and Listeria innocua CWD 350 were tested against different concentrations of sanitizers (for periods of 30 s and 2 min) which included two quaternary ammonium compounds (QAC), an acid anionic sanitizer, and a chlorine-containing sanitizer. According to the Sanitizer Test of the Association of Analytical Chemists, all sanitizers showed efficacy of >99.999% against all test organisms, regardless of the type of sanitizer, its concentration, or exposure time. Two enrichment broths consisting of the University of Vermont (UVM), which is currently used for selective enrichment of Listeria, and Listeria repair broth (LRB), which is a nonselective highly nutritious medium, were used to determine the ability of standard enrichment procedures to recover injured Listeria. Use of these enrichment procedures suggests that sanitizers may induce injury rather than death as indicated by recovery of the organisms in LRB versus failure of growth in UVM, or through variation in the lag phase and viable cell count between both broths. The extent of cell death, injury, and repair was found to be affected by the type and concentration of sanitizer, exposure time, bacterial strain, and the enrichment procedure. QAC were the most effective while the acid anionic sanitizer was the least effective. The lethal effect of the sanitizer was found to increase by increasing its concentration or exposure time. L. monocytogenes F 5027 was the most resistant of the strains tested to QAC and the acid anionic sanitizer, while L. innocua was more resistant to the chlorine sanitizer. In all instances LRB proved to be superior to UVM in promoting repair and growth of sanitizer-injured cells.

Carbohydrate utilization by Clostridium thermocellum: Importance of internal pH in regulating growth

Clostridium thermocellum can utilize cellulose, cellobiose, glucose, and fructose as carbon sources, although initial growth on the monosaccharides from an inoculum grown on cellobiose occurs only after a long lag period of 180–200 h. The lag phase, during which no growth occurs, cannot be due to lack of uptake and phosphorylation of glucose and fructose by cellobiose-grown cells as measured by 13C- and 31P-NMR spectroscopy. ATP levels and Δψ, the transmembrane electrical potential, are comparable in growing cells and lag phase cells. Of the parameters measured, the only significant difference between cellobiose-grown cells incubated with monosaccharides versus such cells incubated with cellobiose is that the internal pH (and the pH gradient) are lower in the nongrowing cultures. Assuming that the value of the internal pH (or ΔpH) is critical for growth, the following alternative hypotheses are proposed: either the breakdown and internalization of the disaccharide cellobiose contributes to the formation of the essential pH gradient, or the transport of glucose or fructose via proton symport dissipates the pH gradient and effectively prevents growth of these cells .

Isolation and Characterization of PEP3, a Gene Required for Vacuolar Biogenesis in Saccharomyces cerevisiae

The Saccharomyces cerevisiae PEP3 gene was cloned from a wild-type genomic library by complementation of the carboxypeptidase Y deficiency in a pep3-12 strain. Subclone complementation results localized the PEP3 gene to a 3.8-kb DNA fragment. The DNA sequence of the fragment was determined; a 2,754-bp open reading frame predicts that the PEP3 gene product is a hydrophilic, 107-kDa protein that has no significant similarity to any known protein. The PEP3 predicted protein has a zinc finger (CX2CX13CX2C) near its C terminus that has spacing and slight sequence similarity to the adenovirus E1a zinc finger. A radiolabeled PEP3 DNA probe hybridized to an RNA transcript of 3.1 kb in extracts of log-phase and diauxic lag-phase cells. Cells bearing pep3 deletion/disruption alleles were viable, had decreased levels of protease A, protease B, and carboxypeptidase Y antigens, had decreased repressible alkaline phosphatase activity, and contained very few normal vacuolelike organelles by fluorescence microscopy and electron microscopy but had an abundance of extremely small vesicles that stained with carboxyfluorescein diacetate, were severely inhibited for growth at 37 degrees C, and were incapable of sporulating (as homozygotes). Fractionation of cells expressing a bifunctional PEP3::SUC2 fusion protein indicated that the PEP3 gene product is present at low abundance in both log-phase and stationary cells and is a vacuolar peripheral membrane protein. Sequence identity established that PEP3 and VPS18 (J. S. Robinson, T. R. Graham, and S. D. Emr, Mol. Cell. Biol. 11:5813-5824, 1991) are the same gene.

Isolation and characterization of PEP3, a gene required for vacuolar biogenesis in Saccharomyces cerevisiae.

The Saccharomyces cerevisiae PEP3 gene was cloned from a wild-type genomic library by complementation of the carboxypeptidase Y deficiency in a pep3-12 strain. Subclone complementation results localized the PEP3 gene to a 3.8-kb DNA fragment. The DNA sequence of the fragment was determined; a 2,754-bp open reading frame predicts that the PEP3 gene product is a hydrophilic, 107-kDa protein that has no significant similarity to any known protein. The PEP3 predicted protein has a zinc finger (CX2CX13CX2C) near its C terminus that has spacing and slight sequence similarity to the adenovirus E1a zinc finger. A radiolabeled PEP3 DNA probe hybridized to an RNA transcript of 3.1 kb in extracts of log-phase and diauxic lag-phase cells. Cells bearing pep3 deletion/disruption alleles were viable, had decreased levels of protease A, protease B, and carboxypeptidase Y antigens, had decreased repressible alkaline phosphatase activity, and contained very few normal vacuolelike organelles by fluorescence microscopy and electron microscopy but had an abundance of extremely small vesicles that stained with carboxyfluorescein diacetate, were severely inhibited for growth at 37 degrees C, and were incapable of sporulating (as homozygotes). Fractionation of cells expressing a bifunctional PEP3::SUC2 fusion protein indicated that the PEP3 gene product is present at low abundance in both log-phase and stationary cells and is a vacuolar peripheral membrane protein. Sequence identity established that PEP3 and VPS18 (J. S. Robinson, T. R. Graham, and S. D. Emr, Mol. Cell. Biol. 11:5813-5824, 1991) are the same gene.

Enhancement of 5-fluorouracil's anticancer activity by dipyridamole

Although the interaction between FUra and DP in HCT 116 cells is fairly complex, data from other investigators indicate that in cell lines in which inhibition of TS is growth limiting at relatively low concentrations of fluoropyrimidines, DP appears to augment the cytotoxicity of FUra and FdUrd by blocking the salvage of dThd (Miller et al., 1987; Schwartz et al., 1987). The previous in vitro data regarding the ability of DP to modulate the toxicity of fluoropyrimidines was obtained in exponentially growing cells. An additional observation that warrants consideration is a report that the inhibition of nucleoside incorporation by DP changed as a function of time in culture (Zhen et al., 1986). Hepatoma 3924A cells in lag and log phase were highly sensitive to DP with IC50 values for dThd incorporation of 0.2 and 0.32 microM, respectively. In contrast, stationary phase cells were insensitive to DP (IC50 = 38.9 microM). Amphotericin B, an antifungal agent which perturbs cell membranes, restored the sensitivity to DP in stationary cells. Several investigators have presented information on the effect of DP on fluoropyrimidines in normal tissues. Lee and Park (1987) examined the effect of DP on FUra and MTX toxicity in a soft-agar cloning assay against two human cancer cell lines and on pooled normal human bone marrow (CFU-C). DP (1 microM) potentiated the action of both MTX (0.1 microM) and FUra (5 microM) on Hep-2 (epidermoid carcinoma), MCF-7 (breast carcinoma) and CFU-C in medium supplemented with either non-dialyzed or dialyzed serum. Woodcock et al. (1987) incubated gallbladder mucosa, obtained from patients undergoing elective surgery for cholelithiasis, with control medium or varying concentrations of DP for 1 hr, and then exposed the mucosal cells to 2.5 microCi [3H]-FdUrd (2.5 microM). After 1 hr, the uptake of FdUrd into the tissue was inhibited to 49% and 42% of control by 0.1 microM and 1 microM, respectively.

Effect of continuous heat stress on cell growth and protein synthesis in Aedes albopictus.

Aedes albopictus (clone C6/36) cells, which normally grow at 28 degrees C, were maintained at a supraoptimal temperature of 37 degrees C. The effect of continuous heat stress (37 degrees C) on cell growth was analyzed as were the modifications occurring with protein synthesis during short- and long-term heat stress. We observed that cells in lag or exponential growth phase, present inhibition of cell growth, and cells in the lag phase showed more sensitivity to death than cells growing exponentially. During the first hour of exposing the cells to 37 degrees C, they synthesized two heat shock proteins (hsps) of 82 kd and 70 kd, respectively, concomitant with inhibition of normally produced proteins at control temperature (28 degrees C). However, for incubations longer than 2 hr at 37 degrees C, a shift to the normal pattern of protein synthesis occurred. During these transitions, two other hsps of 76 kd and 90 kd were synthesized. Pulse chase experiments showed that the 70-kd hsp is stable at least for 18 hr, when the cells are returned to 28 degrees C. However, if cells were incubated at 37 degrees C, the 70-kd hsp is stable for at least 48 hr. The 70-kd hsp was localized in the cytoplasmic and in the nuclear compartment. Our results indicate a possible role of hsp 70-kd protein in the regulation of cell proliferation.

Hyperthermic Sensitivity and Growth Stage in Escherichia coli

Hyperthermic sensitivities of Escherichia coli B/r and Bs-1 were determined for lag-, midlog-, and stationary-phase cells at 47, 48, and 49 degrees C. In both strains midlog-phase cells were strikingly more heat sensitive (100-fold greater killing after 4 h at 48 degrees C) than stationary-phase cells, with intermediate sensitivity for lag-phase cells. In contrast to the reported difference in the radiation sensitivity between these two strains, very little difference in heat sensitivity was seen. Patterns of fatty acid composition of both strains were very similar at each phase of growth. From midlog to stationary phase, 16:1 and 18:1 unsaturated fatty acids decrease from 16 and 30% to 0.5 and 3%, respectively, while the C17 and C19 cyclopropane fatty acids increase from 7 and 3% to 22 and 25%, respectively. Concomitant with these changes in fatty acid composition, substantially higher membrane microviscosity values were recorded for stationary-phase cells. Total membrane microviscosity was positively associated with the C17 and C19 cyclopropane fatty acid composition and with cell survival following hyperthermia. In contrast to hyperthermic sensitivity, radiation survival differences between B/r and Bs-1 are little affected by growth stage. We propose that these results are consistent with a critical influence of membrane lipids on cellular hyperthermic sensitivity and further that the target sites for radiation and hyperthermia are different in these cells.