Modification of membrane properties and fatty acids biosynthesis-related genes in Escherichia coli and Staphylococcus aureus: Implications for the antibacterial mechanism of naringenin

In this work, modifications of cell membrane fluidity, fatty acid composition and fatty acid biosynthesis-associated genes of Escherichia coli ATCC 25922 (E. coli) and Staphylococcus aureus ATCC 6538 (S. aureus), during growth in the presence of naringenin (NAR), one of the natural antibacterial components in citrus plants, was investigated. Compared to E. coli, the growth of S. aureus was significantly inhibited by NAR in low concentrations. Combination of gas chromatography–mass spectrometry with fluorescence polarization analysis revealed that E. coli and S. aureus cells increased membrane fluidity by altering the composition of membrane fatty acids after exposure to NAR. For example, E. coli cells produced more unsaturated fatty acids (from 18.5% to 43.3%) at the expense of both cyclopropane and saturated fatty acids after growth in the concentrations of NAR from 0 to 2.20 mM. For S. aureus grown with NAR at 0 to 1.47 mM, the relative proportions of anteiso-branched chain fatty acids increased from 37.2% to 54.4%, whereas iso-branched and straight chain fatty acids decreased from 30.0% and 33.1% to 21.6% and 23.7%, respectively. Real time q-PCR analysis showed that NAR at higher concentrations induced a significant down-regulation of fatty acid biosynthesis-associated genes in the bacteria, with the exception of an increased expression of fabA gene. The minimum inhibitory concentration (MIC) of NAR against these two bacteria was determined, and both of bacteria underwent morphological changes after exposure to 1.0 and 2.0 MIC.     

Acholeplasma laidlawii membranes: an electron spin resonance study of the influence on molecular order of fatty acid composition and cholesterol.

The effects on membrane structure of including various fatty acids and cholesterol in the growth medium of Acholeplasma laidlawii were investigated by the use of spin-labeled fatty acids. Although the order-mobility parameters varied significantly at some temperatures with the nature of the fatty acid incorporated, the value measured at the growth temperature was only slightly affected by changes in the fatty acid composition of the membranes. The data confirm previous assertions that despite a high level of incorporation of fatty acids of various chain lengths or degree of unsaturation, A. laidlawii regulates its overall membrane fluidity within close limits at the growth temperature. Incorporation of cholesterol increased the degree of order at all temperatures. The coexistence of two lipid phases, one protein-dependent, could be observed in membranes. The order-mobility parameter of spin probes proved less satisfactory for the observation of a gel to liquid crystal transition of the membrane lipid than the partition parameter of a fatty acid spin probe. Order parameters measured by fatty acid spin probes were somewhat higher than those measured by the analogous ²H nmr probes.     

Food quality of mixed bacteria–algae diets for Daphnia magna

Bacteria can comprise a large fraction of seston in aquatic ecosystems and can therefore significantly contribute to diets of filter-feeding zooplankton. To assess the effect of three heterotrophic bacteria (Flavobacterium sp., Pseudomonas sp. and Escherichia coli) on survival, growth and egg production of juvenile Daphnia magna during six-day growth experiments, five ratios of bacteria–Scenedesmus obliquus mixtures were fed. Potential growth-limiting effects mediated by essential biochemicals were assessed upon supplementation of pure bacterial diets with a sterol (cholesterol) or a polyunsaturated fatty acid (EPA). Pure bacterial diets always had detrimental effects on Daphnia. However, cholesterol supplementation of Flavobacterium sp. enhanced growth rates of Daphnia. Diets containing Pseudomonas impaired Daphnia growth even at low dietary proportions (20%), indicating their toxicity. In contrast, Daphnia grew at relative high dietary proportions of Flavobacterium sp. and E. coli (80–50%). In fact, diets containing small proportions of these heterotrophic bacteria (Flavobacterium ≤50%, E. coli 20%) even significantly increased Daphnia growth rates compared to pure algal diets, indicating a nutritional upgrading by these bacteria. Our results suggest that the relative contribution of bacteria and phytoplankton to total dietary carbon as well as their phylogenetic composition strongly influence Daphnia fitness and potentially other filter-feeding zooplankton under field conditions.     

Ethanol-induced changes in lipid composition of Escherichia coli: Inhibition of saturated fatty acid synthesis in vitro

Growth of Escherichia coli in the presence of ethanol results in the synthesis of lipids containing elevated proportions of unsaturated fatty acids. Previous in vivo experiments indicated that the ethanol-induced changes in fatty acid composition result from a preferential inhibition of saturated fatty acid synthesis. In this study, the inhibition of saturated fatty acid synthesis by ethanol was confirmed in vitro. This inhibition was not membrane mediated and resulted from a direct action of ethanol on the soluble enzymes of fatty acid synthesis. The addition of ethanol resulted in a decrease in chain length of both saturated and unsaturated acyl products in vitro. Experiments with enzymes prepared from several fatty acid synthesis mutants of E. coli indicate that β-hydroxydecanoyl-acyl carrier protein dehydrase is not the site of the ethanol inhibition of saturated fatty acid synthesis. The two condensing enzymes are the probable sites for inhibition by ethanol.     

The interaction of cholesterol with bilayers of phosphatidylethanolamine

The interaction of cholesterol with the glycerol backbone segments of phospholipids was studied in bilayers of phosphatidylethanolamine containing equimolar amounts of cholesterol. Glycerol selectively deuterated at various positions was supplied to the growth medium of Escherichia coli strain 131 GP which is defective in endogeneous glycerol synthesis. The procedure enables the stereospecific labeling of the three glycerol backbone segments of the membrane phospholipids. Phosphatidylethanolamine with wild-type fatty acid composition was purified from E. coli cells and deuterium magnetic resonance spectra were obtained either from dispersions of pure phosphatidylethanolamine or from equimolar mixtures of phosphatidylethanolamine with cholesterol. For comparative purposes 1,2-di[9,10-²H₂]elaidoyl-sn-glycero-3-phosphoethanolamine and [3-α-²H]cholesterol were synthesized in order to monitor the behavior of the fatty acyl chains and of the cholesterol molecule itself. For all deuterated segments the deuterium quadrupole splittings as well as the deuterium spin-lattice (T₁) relaxation times were measured as a function of temperature. The glycerol backbone was found to be a remarkably stable structural element of the phospholipid molecule. The quadrupole splittings of the backbone segments changed only by at most 2 kHz upon incorporation of 50 mol % cholesterol. This was in contrast to the fatty acyl chains where the same amount of cholesterol increased the quadrupole splitting by more than 20 kHz. The glycerol segments exhibited the shortest T₁ relaxation times of all CH₂ segments indicating that the glycerol backbone is the slowest motional moiety of the lipid molecule. Addition of cholesterol has no effect on the backbone motion but the fast reorientation rate of the trans-double bonds in 1,2-dielaidoyl-sn-glycero-3-phosphoethanolamine is increased dramatically.     

Cholesterol removal capability of lactic acid bacteria and related cell membrane fatty acid modifications

Milk and dairy products play an important role in a healthy diet because of their high nutritional value, even if they represent a source of lipids and cholesterol. Nowadays, some commercially hypocholesterolemic products are available, which contain lactic acid bacteria (LAB). Therefore, the aims of this study were to test and compare the cholesterol removal abilities of different LAB species and to investigate the capacity of the cholesterol to change the cellular fatty acid composition of microorganisms. Fifty-eight strains of dairy LAB were studied for their ability to remove cholesterol during 24 h of growth. Two of them, L. plantarum 885 and L. acidophilus LA-5®, showed the higher reduction capability. For these strains, the cellular fatty acid composition was studied. They showed a different behaviour, which appeared related to the needs of the cells to maintain the characteristics of membrane fluidity, but was dependent upon their original fatty acid composition. Further studies are required to better characterise the LAB strains to be used to develop fermented dairy products with reduced cholesterol content or be able to induce hypocholesterolemic effects. It will also be interesting to investigate the possible modifications of the cell membrane caused by cholesterol and its possible involvement in cell metabolism.     

Incorporation of Substrate Cell Lipid A Components into the Lipopolysaccharide of Intraperiplasmically Grown Bdellovibrio bacteriovorus

The composition of Bdellovibrio bacteriovorus lipopolysaccharide (LPS) was determined for cells grown axenically and intraperiplasmically on Escherichia coli or Pseudomonas putida. The LPS of axenically grown bdellovibrios contained glucose and fucosamine as the only detectable neutral sugar and amino sugar, and nonadecenoic acid (19:1) as the predominant fatty acid. Additional fatty acids, heptose, ketodeoxyoctoic acid, and phosphate were also detected. LPS from bdellovibrios grown intraperiplasmically contained components characteristic of both axenically grown bdellovibrios and the substrate cells. Substrate cell-derived LPS fatty acids made up the majority of the bdellovibrio LPS fatty acids and were present in about the same proportions as in the substrate cell LPS. Glucosamine derived from E. coli LPS amounted to about one-third of the hexosamine residues in intraperiplasmically grown bdellovibrio LPS. However, galactose, characteristic of the E. coli outer core and O antigen, was not detected in the bdellovibrio LPS, suggesting that only lipid A components of the substrate cell were incorporated. Substrate cell-derived and bdellovibrio-synthesized LPS materials were conserved in the B. bacteriovorus outer membrane for at least two cycles of intraperiplasmic growth. When bdellovibrios were grown on two different substrate cells successively, lipid A components were taken up from the second while the components incorporated from the lipid A of the first were conserved in the bdellovibrio LPS. The data show that substrate cell lipid A components were incorporated into B. bacteriovorus lipid A during intraperiplasmic growth with little or no change, and that these components, fatty acids and hexosamines, comprised a substantial portion of bdellovibrio lipid A.     

Correlations between fatty acid distribution in phospholipids and the temperature dependence of membrane physical state

Cytoplasmic membranes of an unsaturated fatty acid auxotroph of Escherichia coli have been studied using spin labeled hydrocarbon probes. These studies reveal that the membrane lipids undergo changes of state at critical temperatures which reflect the physical properties of the fatty acid supplement supplied to the cells during growth. The critical temperatures observed in spin labeled membranes correlate with characteristic temperatures in membrane functions. Lipid analysis reveals that fatty acid composition and distribution in membrane phospholipids are primary determinants of the temperatures at which changes of state are observed in membrane lipids. Fatty acid composition and distribution can also produce unique interactions between certain spin label probes and their lipid environment.     

Changes in fatty acid profiles of thermo-intolerant and thermo-tolerant marine diatoms during temperature stress

Fatty acid composition and degree of fatty acid saturation during temperature stress in thermo-intolerant (Phaeodactylum tricornutum) and thermo-tolerant (Chaetoceros muelleri) marine diatoms were investigated. A greater number of fatty acids were observed in C. muelleri than in P. tricornutum regardless of treatment. The major fatty acids detected were 14:0, 16:0, 16:1, 16:2, 16:3, 18:0, 18:1(n-9)c, 18:2(n-6) and 20:5(n-3) with additional fatty acids 18:1(n-9)t and 20:4(n-6) detected in C. muelleri. Short duration (2 h) temperature increase above optimal growth temperature had a greater effect on fatty acid composition in C. muelleri than in P. tricornutum and the degree of fatty acid saturation was affected more by temperature in C. muelleri than in P. tricornutum during both short and long duration (24 h) treatments. Total protein assay results suggest that P. tricornutum, but not C. muelleri, was undergoing stress under our growing conditions although lipids in both diatoms were affected by increased temperature. Immunodetection of proteins with anti-rubisco indicates that the rubisco large subunit was undergoing greater turnover in C. muelleri than in P. tricornutum. However, the integrity of rubisco as a suitable indicator of lipid status needs further study. This work supports the hypothesis that a particular temperature, and not treatment duration, has the greater effect on changes in fatty acid composition. Furthermore, changes in fatty acid composition and degree of fatty acid saturation occurred more quickly in the diatoms in response to increased temperature than previously observed in nutrient starvation studies. Since diatom lipids represent an important resource for growth and reproduction of marine animals, the rapid alteration of their lipid composition under temperatures normally encountered in marine environments warrants further study.     

UPLC/MS based method for quantitative determination of fatty acid composition in Gram-negative and Gram-positive bacteria

Quantitative fatty acid composition of microorganisms at various growth space points is required for understanding membrane associated processes of cells, but the majority of the relevant publications still restrict to the relative compositions. In the current study, a simple and reliable method for quantitative measurement of fatty acid content in bacterial biomass without prior derivatization using ultra performance liquid chromatography-electrospray ionization mass spectrometry was developed. The method was applied for investigating the influence of specific growth rate and pH on the fatty acid profiles of two biotechnologically important microorganisms — Gram-negative bacteria Escherichia coli and Gram-positive bacteria Lactococcus lactis grown in controlled physiological states. It was found that the membranes of slowly growing cells are more rigid and that the fatty acid fraction of the cells of L. lactis diminishes considerably with increasing growth rate.     

Changes in lipid composition and some pathological responses of round crucian carp gills by the exposure to acute levels of dodecylbenzenesulfonate

1. Round crucian carp (Carassius carassius grandoculis) were exposed to 7mg l⁻¹ dodecylbenzenesulfonate. All fish died within 5 hr.2. The greatest changes in fatty acid composition, especially polyunsaturated fatty acids of neutral lipids and complex lipids in gill, were observed.3. The total phospholipids were diminished, but the total cholesterol and the cholesterylester were not diminished.4. The epithelial walls of the secondary lamellae were broken and fused.     

Comparative studies on changes in lipid composition and some histological responses of gills from carp by exposure to acute level of surfactants

1. Carp (Cyprinus carpio) were exposed to four surfactants at the concentration of 0.02 mM (dodecylbenzenesulfonate (DBS); 7mg l⁻¹, sodium laurylsulfonate (LNa); 5.7mg l⁻¹, sodium stearate (SNa); 6.1 mg l⁻¹, nonylphenyl ethoxylate (EN); 13.2 mg l⁻¹).2. The changes in lipid composition of the gill were compared. No change in fatty acid composition of neutral lipids and complex lipids occurred after exposure to SNa, whereas changes did occur after DBS, EN and LNa.3. In the single case of exposure to DBS, the content of phosphatidylcholine was significantly increased as compared to control gills.4. The ratio cholesterol/total phospholipid and the ratio unsaturated fatty acid/saturated fatty acid were not changed after exposure to all surfactants tested.5. The order of gill damages was DBS > LNa > SNa > EN.     

Correlations between the thermal stability of chloroplast (thylakoid) membranes and the composition and fluidity of their polar lipids upon acclimation of the higher plant,Nerium oleander,to growth temperature

The thermal stability of excitation transfer from pigment proteins to the Photosystem II reaction center of Nerium oleander adjusts by 10 Celsius degrees when cloned plants grown at 20°C/15°C, day/night growth temperatures are shifted to 45°C/32°C growth temperature or vice versa. Concomitant with this adjustment is a decrease in the fluidity of thylakoid membrane polar lipids as determined by spin labeling. The results are consistent with the hypothesis that there is a limiting maximum fluidity compatible with maintenance of native membrane structure and function. This limiting fluidity was about the same as for a number of other species which exhibit a range of thermal stabilities. Inversely correlated shifts in lipid fluidity and thermal stability occurred during the time course of acclimation of N. oleander to new growth temperatures. Thus, the temperature at which the limiting fluidity was reached changed during acclimation while the limiting fluidity remained constant. Although the relative proportion of the major classes of membrane polar lipids remained constant during adjustments in fluidity, large changes occured in the abundance of specific fatty acids. These changes were different for the phospho- and galacto-lipids suggesting that the fatty acid composition of these two lipid classes is regulated by different mechanisms. Comparisons between membrane lipid fluidity and fatty acid composition indicate that fluidity is not a simple linear function of fatty acid composition.     

The Response of Campylobacter jejuni to Low Temperature Differs from That of Escherichia coli

Human infection with Campylobacter jejuni is often associated with the consumption of foods that have been exposed to both chilling and high temperatures. Despite the public health importance of this pathogen, little is known about the effects of cold exposure on its ability to survive a subsequent heat challenge. This work examined the effect of rapid exposure to chilling, as would occur in poultry processing, on the heat resistance at 56°C of two C. jejuni strains, 11168 and 2097e48, and of Escherichia coli K-12. Unlike E. coli K-12, whose cold-exposed cells showed increased sensitivity to 56°C, such exposure had only a marginal effect on subsequent heat resistance in C. jejuni. This may be explained by the finding that during rapid chilling, unlike E. coli cells, C. jejuni cells are unable to alter their fatty acid composition and do not adapt to cold exposure. However, their unaltered fatty acid composition is more suited to survival when cells are exposed to high temperatures. This hypothesis is supported by the fact that in C. jejuni, the ratio of unsaturated to saturated fatty acids was not significantly different after cold exposure, but it was in E. coli. The low-temperature response of C. jejuni is very different from that of other food-borne pathogens, and this may contribute to its tolerance to further heat stresses.Campylobacter jejuni is the leading cause of bacterial diarrheal disease worldwide and is the most common antecedent to the peripheral neuropathies Guillain-Barré syndrome and Miller Fisher syndrome (32, 35). In England and Wales, there were ca. 47,000 Campylobacter cases reported in 2006, a probable 10-fold underestimate of the true incidence, as the World Health Organization calculates that ca. 1% of the population of Europe will be infected each year.Most Campylobacter infections are food borne, although contaminated water and environmental exposure have also been implicated (5). Currently, identified high-risk factors include the consumption of chicken, especially when undercooked (15), barbecued meat (2), and raw or improperly pasteurized milk (10, 22). In all of these cases, the infecting Campylobacter population would have been exposed to both low and high temperatures before being consumed.Despite their importance as human pathogens, little is known about how campylobacters cope with hostile conditions in the transmission chain from animals to humans. Campylobacter presents an interesting conundrum. It is generally considered to be fragile compared to other food-borne pathogens (7) but is recognized as the leading cause of food-borne disease, and vehicles are frequently foods that have received a degree of heat exposure.Although Campylobacter has been shown to be able to mount an acid tolerance response (28), it lacks a σ³⁸ (RpoS) homologue (23) and cold shock proteins (17), which is thought to limit its ability to respond to hostile conditions common in the food chain. Despite lacking many of the classical bacterial stress responses, Campylobacter can survive for extended periods at low temperatures on key foods such as raw chicken (12). It is important to determine how the responses of Campylobacter to cold could affect its survival in other parts of the food chain. Does preexposure to cold make it more heat sensitive, as is the case for Escherichia coli and Salmonella spp. (20), or are its responses different, as work on one strain (27) suggests?The ability to deal with cold is inherently important to a wide range of mesophilic bacteria (29), and it is likely that some cold exposure tolerance mechanisms are highly conserved. One of the most important of these is a change in fatty acid composition, particularly in the outer membrane (36). Maintaining membrane homoviscosity is critical for continuing growth, and cellular membrane fatty acid composition is altered as the growth temperature changes (30). For example, during exposure to reduced temperatures, Salmonella spp. increase levels of unsaturated fatty acids (UFA), whereas Listeria spp. raise those of branched fatty acids in cellular membranes (33). Both responses increase membrane fluidity. Increased levels of cyclic fatty acids have been shown to have a role in raising the heat resistance of Pediococcus spp. (3) and to increase the stability of the membrane structure (9). Ulmer et al. (37) showed that as the growth temperature for Lactobacillus plantarum decreased, the proportion of UFA in cell membranes rose, leading to increased membrane fluidity and allowing the membrane to be maintained in a liquid gel state. A temperature-dependent shift in fatty acid production has been seen in E. coli. When the growth temperature is reduced, the enzyme activity of FabF increases, leading to an increase in the conversion of 16:1 fatty acids to 18:1 fatty acids (8, 11). Desaturation enhances the effects of the branched fatty acids produced during acclimation of bacterial cells to low temperatures (34).Although changes in fatty acid synthesis have been seen in Campylobacter coli upon a temperature downshift from 42 to 20°C (19), there is currently very little information on the effects of exposure to refrigeration conditions on the whole-cell fatty acid composition of C. jejuni. A study by Hazeleger et al. (17) compared changes in fatty acid composition in coccoid cells held at 4 or 12°C for 14 days with those in control spiral cells grown at 37°C for 24 h. They found that few significant changes in composition occurred at the lower temperatures, whereas at 25°C there were more marked changes in many fatty acids.The work reported here examines the effects of exposure to 6°C for 24 h on whole-cell fatty acid composition and heat resistance of C. jejuni to determine whether Campylobacter has a similar response to chill to the prototypic response of E. coli.     

Changes of lipid content and fatty acid composition of Schizochytrium limacinum in response to different temperatures and salinities

The growth, lipid content and fatty acid composition of Schizochytrium limacinum OUC88 at different temperatures (16, 23, 30 and 37 °C) and salinities (0, 0.9, 1.8, 2.7 and 3.6%, w/v) were analyzed. The strain grew better and lipid contents were higher at 16–30 °C and salinity at 0.9–3.6% (w/v). The adaptive responses of this microbe to temperature and salinity were mainly to regulate the degree of fatty acid unstauration to maintain the normal membrane lipid physical state. However, at 37 °C and 0 salinity, the growth of the strain was inhibited obviously and the lipid content reduced significantly and, some important changes occurred in fatty acid composition, especially the odd-numbered fatty acids 15:0 and 17:0 which amounts increased greatly. In addition, the ratio of DHA to DPA changed at different temperatures and salinities.     

A system for manipulating the membrane Fatty Acid composition of soybean cell cultures by adding tween-Fatty Acid esters to their growth medium : basic parameters and effects on cell growth

The development of a system for modifying the membrane fatty acid composition of cultured soybean cells (Glycine max [L.] Merr.) is described. Tween-fatty acid esters carrying specific fatty acids were synthesized and added to the medium of suspension cultures. Cells transferred large quantities of exogenous fatty acids from Tweens to all acylated membrane lipids; up to 50% of membrane fatty acids were exogenously derived. C15 to C20 saturated fatty acids and C16, C18, and C20 unsaturated fatty acids with either cis or trans double bonds were incorporated into lipids. Cells elongated saturated fatty acids of C16 or less, and unsaturated fatty acids with cis double bonds were further desaturated. No other types of modifications were observed. Growth ceased in cells treated with excessive concentrations of Tween-fatty acid esters, but frequently not for several days. Cessation of cell growth was correlated with changes in membrane fatty acid composition resulting from incorporation of large amounts of exogenous fatty acids into membrane lipids, although cells tolerated large variations in fatty acid composition. Maximum tolerable Tween concentrations varied widely according to the fatty acid supplied. Potential uses of this system and implications of the observed modifications on the pathway of incorporation are discussed.     

Changes in Membrane Fatty Acid Composition of Pediococcus sp. Strain NRRL B-2354 in Response to Growth Conditions and Its Effect on Thermal Resistance

Membrane fatty acid composition and thermal resistance (D value) of Pediococcus sp. were determined for mid-exponential-phase (ME) and stationary-phase (ST) cells grown in tryptic soy broth (TSB) and tryptone-glucose-yeast extract (TGY) at 28 and 37°C. As the cells entered the stationary phase of growth, the unsaturated fatty acid, C_18:1n11c, produced during the exponential phase of growth was converted to its cyclic form, C_19:0Δ9c. This shift in membrane fatty acid composition was accompanied by an increase in the D values of this bacterium. Data from this study suggest that the membrane fatty acid composition of Pediococcus sp. is dependent on the growth conditions and that membrane fatty acid composition plays a critical role in thermal resistance. Thermal inactivation curves of Pediococcus sp. cells grown in TGY at 28°C indicated the presence of a cell population that is heterogeneous in thermal resistance. The growth of this bacterium in TGY at 37°C and in TSB at 28 and 37°C resulted in cell populations that were uniform in thermal resistance with a lag time for thermal inactivation. Thermal inactivation curves of ME and ST cultures were similar. The data presented here suggest that the cell population’s uniformity of thermal inactivation is independent of the growth phase of the culture.     

The effect of hibernation on lipids of the liver mitochondrial fraction in the Yakut ground squirrel <Emphasis Type="Italic">Spermophilus undulatus</Emphasis>

We demonstrated that the level of phospholipids in the liver mitochondrial fraction is increased by 60% during the winter hibernation season in the Yakut ground squirrel S. undulatus; the phospholipid composition in sleeping animals is characterized by an increase in phosphatidylethanolamine compared with summer animals. A sharp increase in the level of cholesterol, as well as fatty acid, monoglycerides, and diglycerides was found in the mitochondrial fraction of hibernating ground squirrels in relation to summer ground squirrels. Functional changes during hibernation concern the level of phosphatidylserine (the growth in sleeping animals compared with active animals). Seasonal modification of the lipid composition of the liver mitochondria (particularly, an increase in the level of cholesterol) can play a role in the resistance of mitochondria to the seasonal increase in the level of fatty acids in the liver. Lipids of the liver mitochondrial fraction are involved in the ground squirrel adaptation to the hibernation season.     

DNA Microarray Analyses of the Long-Term Adaptive Response of Escherichia coli to Acetate and Propionate

In its natural environment, Escherichia coli is exposed to short-chain fatty acids, such as acetic acid or propionic acid, which can be utilized as carbon sources but which inhibit growth at higher concentrations. DNA microarray experiments revealed expression changes during exponential growth on complex medium due to the presence of sodium acetate or sodium propionate at a neutral external pH. The adaptive responses to acetate and propionate were similar and involved genes in three categories. First, the RNA levels for chemotaxis and flagellum genes increased. Accordingly, the expression of chromosomal fliC′-′lacZ and flhDC′-′lacZ fusions and swimming motility increased after adaptation to acetate or propionate. Second, the expression of many genes that are involved in the uptake and utilization of carbon sources decreased, indicating some kind of catabolite repression by acetate and propionate. Third, the expression of some genes of the general stress response increased, but the increases were more pronounced after short-term exposure for this response than for the adaptive response. Adaptation to propionate but not to acetate involved increased expression of threonine and isoleucine biosynthetic genes. The gene expression changes after adaptation to acetate or propionate were not caused solely by uncoupling or osmotic effects but represented specific characteristics of the long-term response of E. coli to either compound.