Glycine betaine and proline are the principal compatible solutes ofStaphylococcus aureus

The foodborne pathogenStaphylococcus aureus is distinguished by its ability to grow within environments of extremely high osmolarity (e.g., foods with low water activity values). In the present study, we examined the accumulation of intracellular organic solutes withinS. aureus strain ATCC 12600 when cells were grown in a complex medium containing high concentrations of NaCl. Consistent with previous reports [Measures JC (1975) Nature 257:398–400; Koujima I, et al. (1978) Appl Environ Microbiol 35:467–470; and Anderson CB, Witter LD (1982) Appl Environ Microbiol 43:1501–1503], intracellular proline was found to accumulate to high concentrations. However, NMR spectroscopy of cell extracts revealed glycine betaine to be the predominant intracellular organic solute accumulated within cells grown at high osmolarity. In additional experiments, we examined the growth rate ofS. aureus in a defined medium of high osmolarity and found it to be stimulated significantly by the presence of either exogenous proline or glycine betaine. Highest growth rates were obtained when the defined medium was supplemented with glycine betaine.     

Effect of age on the membrane lipid composition of Streptococcus sanguis

The cell membrane of Streptococcus sanguis contains three classes of lipid: neutral lipid, glycolipid and phospholipid. A striking difference in membrane lipid composition between cells in the exponential and in the stationary phases of growth was observed. During the exponential phase, approx. 37–45%, 14–19% and 37–45% of the lipids synthesized were found to be neutral lipid, glycolipid and phospholipid, respectively. The amount of lipid synthesized reached a maximum at the early stationary phase. The amount of phospholipid drastically declined thereafter and that of neutral lipid slightly declined. In contrast, the amount of glycolipid markedly increased and exceeded the amount of phospholipid. The phospholipid present during the exponential phase was found to be mainly phosphatidylglycerol (82–88%) and a small amount of cardiolipin (12–18%). At the stationary phase, the amount of phosphatidylglycerol greatly decreased and reached approx. 16% of that in the early stationary phase, while cardiolipin steadily increased and became the major phospholipid in the late stationary phase. The glycolipid was found to be composed of mainly mono- and diglucosyldiglycerides. At the end of the experiment (after 8 h incubation), the distribution of lipids was found to be: neutral lipid, 46%; glycolipid (monoglucosyldiglyceride, 28%; diglucosyldiglyceride, 13%) 41%; and phospholipid (phosphatidylglycerol, 3%, cardiolipin, 8%) 13%.     

Cyclic beta-1,6-1,3 glucans ofBradyrhizobium: Functional analogs of the cyclic beta-1,2-glucans ofRhizobium?

We have previously shown that species ofBradyrhizobium synthesize a novel class of cyclic beta glucans which contains both beta-1,6 and beta-1,3 glycosidic linkages [Miller KJ, Gore RS, Johnson R, Benesi AJ, Reinhold VN (1990) J Bacteriol 172:136–142]. In the present study we show that these cell-associated glucans are localized within the periplasmic compartment and that the biosynthesis of these glucans is osmotically regulated.     

The effects of temperature on the fatty acid and phospholipid composition of four obligately psychrophilicVibrio Spp.

The free fatty acid and phospholipid composition of 4 psychrophilic marineVibrio spp. have been determined in chemostat culture with glucose as the limiting substrate over a temperature range 0–20°C. All the isolates show maximum glucose and lactose uptake at 0°C and this correlates with maximum cell yield. None of the isolates contain fatty acids with a chain length exceeding 17 carbon atoms.Vibrio AF-1 andVibrio AM-1 respond to decreased growth temperatures by synthesizing increased proportions of unsaturated fatty acids (C15:1, C16:1 and C17:1) whereas inVibrio BM-2 the fatty acids undergo chain length shortening. The fourth isolate (Vibrio BM-4) contains high levels (60%) of hexadecenoic acid at all growth temperatures and the fatty acid composition changes little with decreasing temperature. The principal phospholipid components of the four psychrophilic vibrios were phosphatidylserine, phosphatidylglycerol, phosphatidylethanolamine and diphosphatidylglycerol. Lyso-phosphatidylethanolamine and 2 unknown phospholipids were additionally found inVibrio AF-1. The most profound effect of temperature on the phospholipid composition of these organisms was the marked increase in the total quantities synthesized at 0°C. At 15°C phosphatidylglycerol accumulated in the isolates as diphosphatidylglycerol levels decreased. Additionally inVibrio BM-2 andVibro BM-4 phosphatidylserine accumulates as phosphatidylethanolamine biosynthesis was similarly impaired. The observed changes in fatty acid and phospholipid composition in these organisms at 0°C may explain how solute transport is maintained at low temperature.Abbreviations PS Phosphatidylserine - PE phosphatidylethanolamine - PG phosphatidylglycerol - DPG diphosphatidylglycerol - lyso PE lysophosphatidylethanolamine     

Fatty acid and phospholipid composition of five psychrotrophic Pseudomonas spp. grown at different temperatures

The free fatty acid and phospholipid composition of 5 psychrotrophic marine Pseudomonas spp. have been determined in chemostat culture with glucose as the limiting substrate over the range 0–20°C. The predominant fatty acid present in all the isolates was hexadecenoic acid (C16:1) together with lesser quantities of octadecenoic acid (C 18:1) whilst none contained acids with chain lengths exceeding 18 carbon atoms. Decreasing the growth temperature from 20°C to 0°C resulted in little significant change in fatty acid composition. The principal phospholipid components of the five psychrotrophic pseudomonads have been identified as phosphatidylserine, phosphatidylglycerol, phosphatidylethanolamine and diphosphatidylglycerol. Decreasing the growth temperature did not elicit significant changes either in the total quantities of phospholipid synthesized or in the concentration of individual phospholipid components in any of the isolates. All the psychrotrophs showed maximum glucose uptake between 15°C and 20°C and the rate decreased rapidly as the temperature was decreased towards 0°C.Abbreviations PS Phosphatidylserine - PE phosphatidylethanolamine - PG phosphatidylglycerol - DPG diphosphatidylglycerol     

Modification of cell membrane lipids inMicrococcus lysodeikticus induced by pantoyl lactone

Summary Growth ofMicroccoccus lysodeikticus in the presence of pantoyl lactone brings about both qualitative and quantitative changes in cell membrane lipids. Significant amounts of the two major phospholipids (phosphatidylglycerol and diphosphatidylglycerol) are converted to lyso forms; the largest conversion occurs in the phosphatidylglycerol. In addition, amounts of several phospholipid fatty acids are changed. Physical alteration of the cell membrane can be demonstrated using differential scanning calorimetry. Although growth and transport are significantly inhibited when pantoyl lactone is present, cells possessing altered cell membrane phospholipids and phospholipid fatty acids, brought about by growth in the presence of pantoyl lactone, transportd-alanine,l-glutamic andl-aspartic acid normally when washed free of the pantoyl lactone.     

Proton conductance through phospholipid bilayers: Water wires or weak acids?

The proton/hydroxide (H⁺/OH^–) permeability of phospholipid bilayer membranes at neutral pH is at least five orders of magnitude higher than the alkali or halide ion permeability, but the mechanism(s) of H⁺/OH^– transport are unknown. This review describes the characteristics of H⁺/OH^– permeability and conductance through several types of planar phospholipid bilayer membranes. At pH7, the H⁺/OH^– conductances (G _H/OH) range from 2–6 nS cm^–2, corresponding to net H⁺/OH^– permeabilities of (0.4–1.7)×10^–5 cm sec^–1. Inhibitors ofG _H/OH include serum albumin, phloretin, glycerol, and low pH. Enhancers ofG _H/OH include chlorodecane, fatty acids, gramicidin, and voltages >80 mV. Water permeability andG _H/OH are not correlated. The characteristics ofG _H/OH in fatty acid (weak acid) containing membranes are qualitatively similar to the controls in at least eight different respects. The characteristics ofG _H/OH in gramicidin (water wire) containing membranes are qualitatively different from the controls in at least four different respects. Thus, the simplest explanation for the data is thatG _H/OH in unmodified bilayers is due primarily to weakly acidic contaminants which act as proton carriers at physiological pH. However, at low pH or in the presence of inhibitors, a residualG _H/OH remains which may be due to water wires, hydrated defects, or other mechanisms.     

Chemical and cytological changes during the autolysis of yeasts

Summary Cell suspensions ofSacharomyces cerevisiae, Kloeckera apiculata andCandida stellata were autolyzed in phosphate buffer, pH 4.5, for up to 10 days. Cell dry weights decreased by 25–35% after 10 days. Based on initial cell dry weight, the soluble autolysate consisted of: carbohydrate (principally polysaccharide) 3–7%; organic acids 3–6%; protein 12–13%; free amino acids 8–12%; nucleic acid products 3–5%; and lipids 1–12%. The main organic acids in autolysates were propionic, succinic and acetic and the main amino acids were phenylalanine, glutamic acid, leucine, alanine and arginine. Approximately 85–90% of cellular RNA and 25–40% of cellular DNA were degraded during autolysis. Both neutral lipid and phospholipid components were degraded, with neutral lipids but not phospholipids being found in autolysates. Scanning and transmission electron micrographs showed retention of cell wall structure and shape during autolysis, but there was extensive intracellular disorganization withinS. cerevisiae andC. stellata. There were differences in the autolytic behavior ofK. apiculata compared withS. cerevisiae andC. stellata.This paper is dedicated to Professor Herman Jan Phaff in honor of his 50 years of active research which still continues.     

Size-frequency estimates of secondary production byMysis relicta in Lakes Michigan and Huron

Data from five Great Lakes studies ofMysis relicta populations were reanalyzed to calculate secondary production estimates using the size-frequency method. Production estimates (P) ranged from 0.25 to 3.2 g dry weight m^–2 yr^–1. Average annual biomass {xxB} and mean annual density (xxD) were 0.11–1.11 g dry weight/ m² and 25–434 animals/ m², respectively. P:{xxB} ratios varied only between 2.2 and 3.3. Maximum and minimum biomass values within a study varied by a factor of 519 for one study but by less than 17 for the others. Highest estimates of P, {xxB} and {xxD} were calculated for collections from a 50-m station in Lake Michigan despite the larger populations suspected to be present at greater depths sampled in the other studies. These conservative estimates provide a basis for scaling trophic interactions involvingM. relicta and emphasize findings by previous workers that night-time sampling with vertical net hauls is the best available technique for quantitative studies ofM. relicta populations in the Great Lakes.     

Phase behavior of hydrated lipid bilayer composed of binary mixture of phospholipids with different head groups

Phase behavior of hydrated lipid bilayer was investigated for the mixtures of two phospholipid species chosen from phosphatidic acid (PA), phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylglycerol (PG) with the same acyl chains. The pseudo-binary phase diagrams constructed by a differential scanning calorimetry (DSC) were analyzed based on a thermodynamic model applying the Bragg–Williams approximation for non-ideality of mixing. The interchange energy parameters, ρ₀, derived from this approach were positive for all mixture systems in both gel and liquid–crystalline phase bilayers, and increased in the order PG/PE<PC/PA<PC/PE<PG/PA with a few exception. This suggests that the energetical disadvantage for the mixed-pair formation relative to the like-pair formation in the hydrated bilayer increases in this order. In addition, the ρ₀ values increased with the increase in the acyl chain length of the phospholipids. These experimental results were discussed in terms of an intermolecular interaction of the phospholipid species in hydrated bilayer.     

Lipids and fatty acids of two pelagic cottoid fishes (Comephorus spp) endemic to Lake Baikal

Matured females of two Lake Baikal endemic fish species, Comephorus baicalensis and Comephorus dybowski, have been investigated for lipid of the whole body and specific tissues (liver, muscles, ovaries), phospholipid classes and fatty acids of neutral and polar lipids. Total lipid in the body (38.9% fresh weight), liver (23.5%) and muscles (14.5%) of C. baicalensis were greater than those of C. dybowski (4.7, 8.7 and 2.6%, respectively); only their ovaries were similar (5.3 and 5.6% lipid, respectively). In both species, phosphatidylcholine and phosphatidylethanolamine were the major phospholipids, ranging from 60.7 to 75.1% of total phospholipid and 14.5–25.7%, respectively. In most cases, monounsaturated fatty acids (MUFA) were the major fatty acid group in C. baicalensis, whereas polyunsaturated fatty acids (PUFA) were the major group in C. dybowski. The MUFA 18:1(n-9) prevailed over other fatty acids in C. baicalensis and varied from 19% in polar lipids of muscles to 56.1% in neutral lipids of muscles. In polar lipid of C. dybowski, the PUFA 22:6(n-3) prevailed over other fatty acids in muscles and ovaries, while 16:0 dominated polar liver lipids and neutral lipids of all tissues. Other major fatty acids included 16:1(n-7), 18:1(n-7), and 20:5(n-3). Values of the (n-3)/(n-6) fatty acid ratio for neutral lipids of C. baicalensis (0.5–0.9) are well below the range of values characteristic either for marine or freshwater fish, while these values for polar lipids (1.6–1.8) are in the range typical of freshwater fish. Neutral lipid fatty acid ratios in C. dybowski (2.5–3.1) allow it to be assigned to freshwater fish, but polar lipids (2.8–3.7) leave it intermediary between freshwater and marine fish.     

New species ofIsospora (Apicomplexa: Eimeriidae) from passeriform birds of Hawaii

Three new species ofIsospora (Apicomplexa: Eimeriidae) are described from the faeces of passeriform birds of Hawaii.Isospora lyonensis n. sp. is described from the faeces ofLonchura punctulata (Passeriformes: Estrildidae) and has spherical-subspherical oocysts, 24×23 (21–27×21–26.5), with a thick, bi-layered wall c.1.5 thick. A micropyle and oocyst residuum are absent, but three or more polar granules are present. Oocysts ofI. mejiro n. sp. were found in the faeces ofZosterops japonicus (Passeriformes: Zosteropidae) and are spherical to subspherical, 28.5×27 (25–32×25–29.5), with a thick, bi-layered wall c.1.5 thick. A micropyle and oocyst residuum are absent, but a large polar granule is present.I. manoaensis n. sp. was also found inZ. japonicus and has spherical-subspherical oocysts, 28×26.5 (25.5–30.5×22.5–29), and a thick, bi-layered wall c.1.5 thick. A micropyle and oocyst residuum are absent, but a polar granule consisting of numerous splinter-like fragments is present. In addition to the above new species, oocysts ofI. brayi Levine, Van Riper & Van Riper, 1980 were also found inZ. japonicus.     

Kinetics of spore coat protein synthesis byClostridium perfringens type A

Spore coat proteins of several strains ofClostridium perfringens were analyzed by immunoblotting with antisera against two major spore coat proteins, one [34-kilodalton (kDa)] from an enterotoxin-positive and one (19-kDa) from an enterotoxin-negative (ent^–) strain of this organism. The results indicated that spore coat proteins from many strains ofC. perfringens were immunologically related regardless of their ability to produce enterotoxin, but were not immunologically related to enterotoxin. The kinetics of synthesis and deposition of two major spore coat proteins differed depending upon the strain. Coat protein synthesis was sporulationspecific, since coat protein was not detected in vegetative cell extracts. There was no similarity between the amino acid composition of either coat protein and enterotoxin. These results suggest that, contrary to previous reports (W.R. Frieben and C.L. Duncan, Eur J Biochem 39:393–410, 1973), enterotoxin synthesis is not closely related to spore coat protein synthesis in this organism.     

Electrophysiological characterization of the multipolar thermoreceptors in the "fire-beetle" Merimna atrata and comparison with the infrared sensilla of Melanophila acuminata (both Coleoptera,Buprestidae)

A thermosensitive multipolar neuron innervates each of the four abdominal receptors of the Australian buprestid beetle Merimna atrata. The neuron is spontaneously active within a broad range of body temperatures (tested between 10°C and 40°C). We heated the receptors with a red diode laser (=0.66 µm) at intensities ranging from 5.3 mW cm^–2 up to 1.3 W cm^–2. In general, warming caused an increase of receptor activity. Peak discharge frequencies were reached 100–300 ms after onset of irradiation. After peak frequencies were reached, distinct adaptation took place within seconds. A linear increase in irradiation intensity caused an exponential increase in peak frequencies. Lowest threshold was found to be at 40 mW cm^–2 where latencies were 47 ms. At the highest intensity tested (1.3 W cm^–2), peak frequencies increased up to about 300 Hz and latencies decreased to 24 ms. Considering the pyrophilous behaviour of Merimna and the morphological data from previous studies, our results support the hypothesis that the abdominal receptors are infrared receptors. We also recorded the responses of the photomechanic infrared sensilla of Melanophila acuminata under the same experimental conditions. These results show that the photomechanic sensillum of Melanophila has a higher sensitivity, and that the latencies are considerably shorter.     

Microalgae for use in tropical aquaculture II: Effect of salinity on growth,gross chemical composition and fatty acid composition of three species of marine microalgae

The influence of salinity on the growth, gross chemical composition and fatty acid composition of three species of marine microalgae,Isochrysis sp.,Nannochloropsis oculata andNitzschia (frustulum), was investigated. There was no significant change in growth rate ofIsochrysis sp. andN. (frustulum) over the experimental range of salinity (10–35 ppt), whileN. oculata had a significantly slower growth rate only at 35 ppt. The ash content of all three species increased with increasing salinity. Two species,Isochrysis sp. andN. oculata, showed significant linear increases in total lipid content with increasing salinity over the range 10 to 35 ppt.N. (frustulum) showed significant linear decrease in total lipids, with the highest percentage at low salinity within the range 10–15 ppt. Variation in salinity had only a slight effect on the total protein, the soluble carbohydrate and chlorophylla content of all species. All species responded to change in salinity by modifying their cellular fatty acid compositions. Significant positive correlations were observed between increase in salinity and increase in the percentage ofcis-9-hexadecenoic acid [16:1 (n-7)] over the entire experimental range inN. (frustulum) and between 25–35 ppt inN. oculata. There were curved relationships between salinity and percentage of hexadecanoic acid [16:0] inN. oculata andN. (frustulum), with maxima within the range 25–30 ppt for both species. A curved relationship was found between salinity and percentage of eicosapentaenoic acid [20–5(n-3)], forN. (frustulum), with lowest percentages of the fatty acid within the range 25–30 ppt. There was no consistent pattern in the percentages of other major fatty acids as functions of salinity. The Northern Territory isolateN. (frustulum) was unusual in having a substantial increase in total fatty acids with decreasing salinity (85 mg g^–1 dry wt at 10 ppt compared with 33 mg g^–1 at 35 ppt). The optimum salinities for the production of maximum amount of lipids and the essential fatty acids 20:5(n-3) and/or 22:6(n-3) were as follows:25 ppt forIsochrysis sp. [22:6(n-3)]; 20–30 ppt forN. oculata [20:5(n-3)]; 10–15 ppt forN. (frustulum) [20:5(n-3) and 22:6(n-3)].Author for correspondence     

Heterologous gene expression in <Emphasis Type="Italic">Thermus thermophilus</Emphasis>: β-galactosidase,dibenzothiophene monooxygenase,PNB carboxy esterase, 2-aminobiphenyl-2,3-diol dioxygenase,and chloramphenicol acetyl transferase

Enzymes from thermophiles are preferred for industrial applications because they generally show improved tolerance to temperature, pressure, solvents, and pH as compared with enzymes from mesophiles. However, nearly all thermostable enzymes used in industrial applications or available commercially are produced as recombinant enzymes in mesophiles, typically Escherichia coli. The development of high-temperature bioprocesses, particularly those involving cofactor-requiring enzymes and/or multi-step enzymatic pathways, requires a thermophilic host. The extreme thermophile most amenable to genetic manipulation is Thermus thermophilus, but the study of expression of heterologous genes in T. thermophilus is in its infancy. While several heterologous genes have previously been expressed in T. thermophilus (Fridjonsson et al. in J Bacteriol 184:3385–3391, 2002, Koyama et al. in Appl Environ Microbiol 56:2251–225, 1990, Lasa et al. in J Bacteriol 174:6424–6431, 1992, Mathew et al. in Appl Environ Microbiol 58:421–425, 1992, Takagi et al. in J Ind Microbiol Biotechnol 23:214–217, 1999, Tamakoshi et al. in Extremophiles 5:17–22 2001), the data reported here include the first examples of the functional expression of a gene from an archaeal hyperthermophile (bglA from Pyrococcus woesei), a cofactor-requiring enzyme (dszC from Rhodococcus erythropolis IGTS8), and a two-component enzyme (carBa and carBb from Sphingomonas sp. GTIN11). A thermostable derivative of pnbA from Bacillus subtilis was also expressed, further expanding the list of genes from heterologous hosts that have been expressed in T. thermophilus.     

Sarcophagid larval proteins: Partial sequence homologies among three cuticle proteins and related structures of drosophilids

Summary Three structural proteins from the larval cuticle ofSarcophaga bullata have been sequenced at the amino terminus for 30–40 residues. We observed a high degree of homology with related proteins ofDrosophila melanogaster, based on the previous findings of M. Snyder, J. Hirsh, and N. Davidson [(1981) Cell 25:165–177].S. bullata protein SC1 had 65% homology withDrosophila isolate CP1, and SC6 showed 49% homology with CPX and 54% with CP2a. The three sarcophagid polypeptides also resembled each other with respect to mapped products of tryptic cleavage. The sites of posttranslational arylation required for puparium formation, namely histidyl and lysyl residues, were asymmetrically distributed in the sarcophagid samples. In SC1 the bulk of the loci of putative crosslinks lay beyond the 43-residue fragment. In SC6 half the histidines fell within the first 25% of the primary chain.     

Slow low-threshold potassium current inHelix pomatia neurons

A low-threshold outward current was studied in the neurons ofHelix pomatia at –70 to –30 mV using a two-electrode voltage clamp technique. In addition to the well-known A current (I _A), a slower outward current calledI _As (slow) was revealed. Activation and inactivation times ofI _As at –40 mV ranged from 90 to 120 msec and from 3 to 5 sec, respectively. The current recovered within 2 to 5 sec after inactivation at –120 mV. Analysis of changes in the reversal potential ofI _As caused by an increase in external potassium concentration suggests a potassium origin forI _As. The curves ofI _As stationary activation and inactivation fit the Boltzmann equation. Deriving from an activation curve, the activation potential for a half-maximum current,, is –40 mV, and the slope factor,k, is –9.8 mV, while those values for the inactivation curve are –84 mV (a half-maximum inactivation) and 7.5 mV.I _As is blocked by 4-aminopyridine (1–30 µM), tetraethylammonium (1 mM), and Ba²⁺ (1 mM), but is resistant to Cs⁺ (1 mM). PeakI _As is not affected either by substitution of external Ca²⁺ for Mg²⁺ or by application of Cd²⁺ (0.5–1.0 mM). The results suggest that activation ofI _As does not require Ca²⁺ entry into the cell.Neirofiziologiya/Neurophysiology, Vol. 25, No. 6, pp. 427–432, November–December, 1993.     

Two strains ofVibrio cholerae non-O1 possessing somatic (O) antigen factors in common withV. cholerae serogroup O139 synonym “Bengal”

Two strains (O22 reference strain, 169–68, and strain 490–93 isolated from a patient with diarrhea in Thailand) ofVibrio cholerae non-O1 possessing somatic (O) antigen factors in common withV. cholerae O139 synonym Bengal are described. The O antigens of these two strains were closely related to that ofV. cholerae O139 in an a,b-a,c type of relationship, but were not completely identical with serogroup O139. Therefore, both these strains are not classified into the O139 serogroup ofV. cholerae, because they have their own major antigens. As the strain 490–93 could not be placed into any of the 154 established O serogroups ofV. cholerae, this strain was assigned to a new serogroup, O155. For practical use, the diagnostic antiserum prepared against the O139 reference strain (MO45, ATCC 51394) ofV. cholerae must be absorbed with reference strains 169–68 and 490–93 representing serogroups O22 and O155 ofV. cholerae to remove cross-reacting agglutinins of the O22 and O155 strains, respectively.     

Phospholipids of marine proteobacteria of the genusPseudoalteromonas

The study of the phospholipid composition of 14 type strains of marine proteobacteria of the genusPseudoalteromonas showed that phospholipids are the main polar lipid constituents of membranes in these proteobacteria. The phospholipid patterns of the strains studied were found to be similar and involved five phospholipids typical of gram-negative bacteria, namely, phosphatidylethanolamine, phosphatidylglycerol, bisphosphatidic acid, lysophosphatidylethanolamine, and phosphatidic acid. The major phospholipids were phosphatidylethanolamine and phosphatidylglycerol, which add up to 89–97% of the total phospholipids; bisphosphatidic acid was dominant among minor phospholipids. The prevalence of phosphatidylethanolamine (62–77% of the total phospholipids) and the absence of diphosphatidylglycerol are the characteristic features of most bacteria of this genus. As inEscherichia coli, the phospholipid composition of the marine proteobacteria depended on the presence of magnesium in the medium.