Mitochondrial presequence inserts differently into membranes containing cardiolipin and phosphatidylglycerol

The interaction of the 25-residue presequence of yeast cytochrome oxidase subunit IV with lipid bilayers composed of phosphatidylglycerol, cardiolipin, or their (1:4) mixtures with phosphatidylcholine has been studied by spin-label ESR spectroscopy. Binding of the presequence progressively broadens the gel-to-fluid phase transition of dimyristoylphosphatidylglycerol bilayers, leading to abolition of the transition at a peptide/lipid ratio of > or = 1:5 mol/mol. The mobility of phosphatidylglycerol spin-labeled at the 5-position of the sn-2 chain is decreased in both gel and fluid phases on binding the presequence, with a progressively increasing ESR spectral anisotropy in the fluid phase. The ESR spectra of phosphatidylglycerol spin-labeled at the 14-position of the sn-2 chain contain a second motionally restricted component, in addition to the fluid bilayer spectral component, that arises from direct interaction of the bound presequence with the lipid chains. The proportion of this motionally restricted component is greater for dioleoylphosphatidylglycerol bilayers (corresponding to 2-3 lipids per peptide) than for cardiolipin bilayers (1-2 lipids/peptide), and this component is present also in the mixed bilayers containing 80% phosphatidylcholine. The ESR spectra of the presequence spin-labeled with a maleimide derivative at cysteine-19 evidence high mobility in solution and a very strong reduction in mobility on binding to bilayers containing negatively charged lipids. At low peptide to lipid ratios, the ESR spectra of the spin-labeled presequence sense the phase transition of dimyristoylphosphatidylglycerol.(ABSTRACT TRUNCATED AT 250 WORDS)

Reversibility of the binding of cytochrome c to liposomes. Implications for lipid-protein interactions

Studies on the membrane binding of cytochrome c revealed liposome-associated and soluble cytochrome c not to be in rapid equilibrium. In brief, cytochrome c attached to pyrene phospholipid-labeled, fluorescent liposomes containing either 17.6 mol % cardiolipin (CL) or 30 mol % egg phosphatidylglycerol (PG) is practically not at all or very slowly, respectively, detached by a subsequently added excess (up to 20-fold) of nonlabeled liposomes containing these acidic lipids. Cytochrome c was fully dissociated from PG-containing liposomes by increasing the ionic strength by NaCl, whereas dissociation from CL-containing membranes was less complete, presumably because of the scavenging of the protein within inverted intramembrane micelles. Importantly, the apparent irreversibility of the binding of cytochrome c to liposomes is strongly dependent on the structure of the acidic phospholipid. Cytochrome c bound to lyso-PG/PC liposomes could be dissociated with an excess of nonlabeled PG-containing liposomes. Cytochrome c was also efficiently bound to membranes containing the negatively charged dicetylphosphate yet could be readily dissociated by nonlabeled PG-containing liposomes. We conclude both proper geometry of the phosphate group and the presence of two acyl chains to be required for the tight binding of cytochrome c to acidic phospholipids. These data provide evidence for the membrane association of cytochrome c by an acidic phospholipid in the extended conformation (Kinnunen, P. K. J., Köiv, A., Lehtonen, J. Y. A., Rytömaa, M., and Mustonen, P. (1994) Chem. Phys. Lipids 73, 181-207) in which one of the acyl chains of the lipid becomes accommodated within a hydrophobic cavity of the protein. Based on the crystal structure of cytochrome c we putatively assign the invariant Asn-52 (horse heart cytochrome c) as the site liganding the protonated phosphate of the lipid, whereas Lys-72 and -73 should bind the deprotonated form.

The F0 complex of the ATP synthase of Escherichia coli contains a proton pathway with large proton polarizability caused by collective proton fluctuation

The F0 complex of the Escherichia coli ATP synthase embedded into cardiolipin liposomes was studied by FT-IR spectroscopy. For comparison, respective studies were performed with dried F0 liposomes and with F0 liposomes treated with N,N'-dicyclohexyl-carbodiimide (DCCD), which binds to Asp-61 of subunit c. Furthermore, the effect of H2O-->D2O exchange on the infrared spectrum was investigated. With F0 liposomes an infrared continuum is observed beginning at about 3000 cm-1 and extending toward smaller wavenumbers. In the DCCD-treated sample, this continuum is no longer observed. It vanishes also with drying of the liposomes. After H2O-->D2O exchange, this infrared continuum begins at about 2350 cm-1 and is less intense. All of these results demonstrate that a proton pathway in native F0 is present, in which the protons are shifted in a hydrogen-bonded chain with large proton polarizability due to collective proton tunneling. With the D2O-hydrated system, deuteron polarizability due to collective deuteron motion is observed, but the polarizability due to collective deuteron motion is smaller. Such pathways are very efficient, because they conduct protons or deuterons within picoseconds. These pathways lose their polarizability if the F0 complex is blocked by DCCD or if the liposomes are dried. On the basis of our results on the proton polarizability of hydrogen bonds and hydrogen-bonded systems and on the basis of structural data from the literature, the nature of the proton pathway of the F0 complex of E. coli is discussed.

SecA of Escherichia coli traverses lipid bilayer of phospholipid vesicles

SecA protein of Escherichia coli, when added externally to the vesicles composed of phosphatidylethanolamine, dioleoylphosphatidylglycerol and cardiolipin, was found to be fragmented by trypsin encapsulated within the vesicles. In the presence of ATP or its non-hydrolyzing analogue, ATP-gamma S, the number of fragments and extent of hydrolysis occurred much less than in the absence of these compounds. When ADP was added, however, the hydrolysis products were similar to those when no nucleotide was present. Quenching of SecA fluorescence by vesicle-entrapped iodide corroborated the digestion results. These experiments demonstrated that the SecA protein traverses the lipid bilayer and its membrane topology depends on the kind of nucleotide present.

Transformation of actin-encapsulating liposomes induced by cytochalasin D

Liposomes encapsulating actin filaments were prepared by swelling at 0 degrees C lipid film consisting of a mixture of dimyristoyl phosphatidylcholine and cardiolipin (equal amounts by weight) in 100 microM rabbit skeletal muscle actin and 0.5 mM CaCl2 followed by polymerization of actin at 30 degrees C. Liposomes initially assumed either disk or dumbbell shape, but when cytochalasin D was added to the medium surrounding the liposomes, they were found to become spindle shaped. Liposomes containing bovine serum albumin that were given cytochalasin D and actin-containing liposomes that were given dimethylformamide, the solvent for cytochalasin D, did not transform. These results indicated actin-cytochalasin interaction is involved in the transformation process. Falling-ball viscometry and sedimentation analysis of actin solution indicated that cytochalasin cleaved actin filaments and caused depolymerization. The observation of polarized fluorescence of encapsulated actin labeled with acrylodan indicated that the actin filaments in the transformed liposomes aligned along the long axis of the liposomes. Because the actin filaments in the disk- or dumbbell-shaped liposomes formed bundles running along the liposome contour, the transformation was likely to be accompanied by the change in the actin filament arrangement in the liposomes, which was induced by actin-cytochalasin interaction.

Reconstitution of hepatitis B surface antigen proteins into phospholipid vesicles

Hepatitis B surface antigen (HBsAg), devoid of 75% of its total lipids has been reconstituted with several phospholipids by the detergent dialysis method, using the non-ionic detergent beta-D-octyl glucoside. Upon reconstitution with both neutral and acidic phospholipids, HBsAg particles had the same morphology and, as indicated by trypsin hydrolysis, the topology of the surface proteins was maintained. However, only negatively charged phospholipids were able to completely revert the conformational changes which had been induced by removal of the lipids. The helical content, as indicated by CD techniques, and the antigenic activity, as measured by binding to polyclonal antibodies, of HBsAg reconstituted with acidic phospholipids were practically identical to those of the native antigen. Cholesterol had no effect on the antigenic activity recovered by reconstitution with any of the phospholipids.

Comparison of electron transfer kinetics between redox proteins free in solution and electrostatically complexed to a lipid bilayer membrane

The second-order rate constants obtained in solution for the reduction of horse cytochrome c (cytc; net charge +7) by either Clostridium beijerinckii flavodoxin semiquinone (Fld; net charge -16) or reduced spinach ferredoxin (Fd; net charge -15) decrease monotonically with increasing ionic strength, as expected for reactions between oppositely charged species. Although the rate constant for the Fld reaction is almost two orders of magnitude larger at low ionic strength than that for Fd, the values extrapolated to infinite ionic strength are closely similar, indicating comparable reactivities when electrostatic effects are eliminated. Furthermore, Fld has a much larger value for the electrostatic interaction energy, and thus a larger apparent active site charge, than does Fd, accounting for the rate constant disparity at low ionic strength. Electrostatically binding cytc at low ionic strength to a negatively charged lipid bilayer vesicle (membranes containing mixtures of egg phosphatidylcholine (PC) and cardiolipin (CL)) results in a marked decrease of the observed electron transfer rate constant (k(obs)) for reduction of the cytochrome by both Fld and Fd. The magnitude of this decrease is proportional to the mole percent of CL present in the membrane (10- to 20-fold change over 5-60 mol%). With Fld, k(obs) decreases monotonically with increasing ionic strength at a fixed CL concentration. With Fd an increase in k(obs) occurs as the ionic strength is increased, which maximizes at intermediate ionic strength at a value larger than that obtained in the absence of lipid vesicles. When Fld is electrostatically bound to a positively charged vesicle composed of 40 mol% dioctadecyldimethylammonium ion (DODAC) and 60 mol% PC, again k(obs) for electron transfer to cytc is decreased over that obtained in solution, and the magnitude is diminished monotonically by increasing ionic strength. In contrast, k(obs) for electron transfer from Fd to cytc is unaffected by the presence of the positively charged membrane. The implications of these results for the role of membrane surface charge in modulating protein-protein interactions is discussed.

[Use of the competitive analysis method for studying protein-lipid interactions]

Competitive binding of fluorescent probe DSM and ribonuclease to liposomes, consisted of phosphatidylcholine and diphosphatidylglycerol has been investigated. Association constants and binding stoichiometry for probe and protein has been estimated.

Recurrence of yaws outbreak in Thailand, 1990

In November 1990, an outbreak of yaws was reported in a rural village of southern Thailand. A survey consisting of history taking, physical examination and blood tests for VDRL and TPHA was undertaken in the whole village and in the primary school of the village. A yaws case was defined as a person who had a positive VDRL confirmed by TPHA without history of other treponemal infection or post-treatment of yaws. Fifty-four cases, an attack rate of 23%, were found in the age range from 2 to 79 years old. The epidemic curve started with a few cases in 1989 and early 1990 with an increasing number of cases from June, and reached its peak in November. Half of the cases (53.7%) were children under 15 years. Male cases were higher than female (1.5:1). A survey of 105 primary school students found an attack rate of 32% (34/105) and the prevalence was higher in younger classes. Benzathine penicillin injection was given to cases for treatment. From this investigation, the Ministry of Public Health was alerted about the recurrence of yaws. Strengthening health education and surveillance activities was recommended in adjacent districts and provinces. The extent of the problem was assessed through surveys among school children and expanded to their family members if cases of yaws were found.

Crystallization of mitochondrial creatine kinase on negatively charged lipid layers

Polymorphic forms of crystals of mitochondrial creatine kinase (Mi-CK) octamers were generated by the lipid-layer technique, using cardiolipin as interphase adhesion matrix. Depending on the protein and lipid concentration, different types of monolayers and 3-D stacks thereof assembled in a low ionic strength crystallization buffer. Sodium tungstate was found to promote and stabilize the crystal formation, though in-plane crystallization was also possible in the absence of tungstate. All crystal forms exhibited a p4 symmetry with lattice parameters (a = b) ranging from 10.6 to 24.0 nm and with one or four octamers per unit cell in end-on orientation. In ice-embedded crystals, which showed a molecular packing different from that of negatively stained preparations, structural features of the Mi-CK octamer were observed at a resolution of 1 nm. The crystallization process took advantage of the electrostatic interaction between negatively charged lipid head groups of cardiolipin and positive charges located at the top/bottom faces of the Mi-CK octamer. In the absence of a cardiolipin support, Mi-CK formed linear filaments from a solution of phosphotungstate by association of octamers via their top/bottom faces. When tungstate was used instead of phosphotungstate, the filaments aligned themselves into large crystalline assemblies.

Purification of anticardiolipin and lupus anticoagulant activities by using cardiolipin immobilized on agarose beads

We describe a novel method for the purification of aPL, in which pure CL is immobilized on octyl-sepharose beads by hydrophobic interaction. No lipid contamination was present in eluates, and the system could be reutilized three times without loosing extracting capacity. Four patients with antiphospholipid syndrome were studied. A marked decrease in aCL and LA activities was found in all patient plasmas after the passage through a CL-octyl-sepharose column. Both activities were recovered in eluates which contained beta 2-GP-I and IgGs. beta 2-GP-I was also present in normal plasma eluates, which showed no aCL and slight LA activity. This method represents an improvement in the purification of aPL, and could be useful in explaining the mechanism of action of antibodies that are obtained using pure phospholipid as extracting matrix.

Cardiolipin liposomes sequester a reactivatable partially folded rhodanese intermediate

The interaction was studied between the mitochondrial enzyme thiosulfate sulfurtransferase and liposomes, in the form of large unilamellar vesicles (LUV), prepared from either cardiolipin (CL), PtdCho or PtdSer. At equivalent concentrations of lipid, more partially folded thiosulfate sulfurtransferase bound to CL/LUV than to PtdSer/LUV, and only traces were bound to PtdCho/LUV. Native thiosulfate sulfurtransferase did not bind to any of these LUV. We show that CL/LUV-sequestered thiosulfate sulfurtransferase is inactive but may be reactivated (approximately 56%) with the aid of detergents, thiosulfate, beta-mercaptoethanol and phosphate buffer. Reactivations in the presence of PtdSer/LUV or PtdCho/LUV was only 9% or 1%, respectively. Analysis of the complex by protease digestion and fluorescence spectroscopy indicated that thiosulfate sulfurtransferase was held by CL/LUV and PtdSer/LUV as a folding intermediate. Data presented here suggest that detergents may not interact directly with the protein, but, rather, their primary role in reactivation is to disrupt the LUV, allowing flexibility to the anchored thiosulfate sulfurtransferase molecule, thereby promoting folding. These studies complement other reports which imply a possible role for CL in protein translocation across the mitochondria, since we find that CL binds to thiosulfate sulfurtransferase and sequesters it in a translocation-competent prefolded conformation, which may readily lead to a correctly folded enzyme.

Otosyphilis: a diagnostic and therapeutic dilemma

Traditional treatment of otosyphilis with penicillin and corticosteroids has achieved hearing improvement; however, selecting which patients with a positive fluorescent treponemal antibody absorption (FTA-ABS) test will benefit from treatment remains a problem. In order to study this problem, 18 patients with cochleovestibular dysfunction of unknown etiology and positive syphilis serology were treated with intravenous penicillin and corticosteroids. In addition, lumbar puncture and human immunodeficiency virus (HIV) testing were performed on all patients. Hearing improved in 5 (31%) of 16 patients, tinnitus decreased in 11 (85%) of 13, and vertigo improved in 6 (86%) of 7. Factors associated with hearing improvement were hearing loss present less than 5 years, fluctuating hearing, and age less than 60. Improvement was unrelated to the severity of the loss or previous therapy. All patients with cerebrospinal fluid abnormalities, including two patients with HIV disease, had subjective improvements. A diagnostic and treatment protocol is presented.

Anticoagulant activity of beta 2-glycoprotein I is potentiated by a distinct subgroup of anticardiolipin antibodies

Plasmas of 16 patients positive for both IgG anticardiolipin (aCL) antibodies and lupus anticoagulant (LA) antibodies were subjected to adsorption with liposomes containing cardiolipin. In 5 of these plasmas both the anticardiolipin and the anticoagulant activities were co-sedimented with the liposomes in a dose-dependent manner, whereas in the remaining cases only the anticardiolipin activity could be removed by the liposomes, leaving the anticoagulant activity (LA) in the supernatant plasma. aCL antibodies purified from the first 5 plasmas were defined as aCL-type A, while the term aCL-type B was used for antibodies in the other 11 plasmas, from which 2 were selected for this study. Prolongation of the dRVVT was produced by affinity-purified aCL-type A antibodies in plasma of human as well as animal (bovine, rat and goat) origin. aCL-type B antibodies were found to be devoid of anticoagulant activity, while the corresponding supernatants containing LA IgG produced prolongation of the dRVVT only in human plasma. These anticoagulant activities of aCL-type A and of LA IgG's were subsequently evaluated in human plasma depleted of beta 2-glycoprotein I (beta 2-GPI), a protein which was previously shown to be essential in the binding of aCL antibodies to anionic phospholipids. Prolongation of the dRVVT by aCL-type A antibodies was abolished using beta 2-GPI deficient plasma, but could be restored upon addition of beta 2-GPI. In contrast, LA IgG caused prolongation of the dRVVT irrespective of the presence or absence of beta 2-GPI.(ABSTRACT TRUNCATED AT 250 WORDS)

[Effect of compounds having hypotensive activity on microviscosity of phospholipids membranes]

Vincapane, apressin, raunatin, reserpine and prazosin have been studied for their effect on the state of microviscosity of phospholipid bilayers from lecithin and cardiolipine. The method of fluorescent probing using pyrene and asymmetrical polymethine dye 4501 u have been used for this study. Character of changes of phospholipid bilayers microviscosity as affected by these compounds depends on their concentration in the solution. Vincapan lowers microviscosity of membranes from lecithin in the interval of the concentrations given. Advantages of fluorescent probing by means of asymmetric polymethine dye in comparison with pyrene were shown.

Blood spot screening and confirmatory tests for syphilis antibody

We developed a blood spot test for syphilis antibody using enzyme-linked immunosorbent assay (ELISA) technology. Dried blood was eluted by buffered saline or, for a supplementary confirmatory test, by treponemal-antibody test diluent. Eluates were diluted in an absorption buffer (Calypte Biomedical, Berkeley, Calif.) and added to plate wells coated with cardiolipin antigen (ADI Diagnostics, Toronto, Ontario, Canada). The wells were washed and treated sequentially with an immunoglobulin G conjugate, buffer washes, and enzyme substrate. Substrate conversion was measured photometrically, and specimen reactivity was determined by reference to nonreactive controls. The optimum test protocol was established by tests of serum and plasma. The serum ELISA specificity with normal specimens was 98.9%. The sensitivity with sera from patients with undefined syphilis was 97.4%, that with sera from patients with documented primary and secondary disease was 100%, and that with sera from patients with early and late latent disease was 95.7%. The specificity of the spot test with donor blood was 94.2%, and its specificity with newborn blood was 94.9%. The sensitivity with 25 spots spiked with reactive sera was 96%. The seroprevalence rates for parturient women in one hospital were 6.01% according to spot tests of sera from 599 newborns and 6.81% according to Rapid Plasma Reagin tests of 499 maternal serum specimens. Seventy percent of infants born to 50 seropositive women were reactive by either the newborn spot or the Rapid Plasma Reagin serum test. The results show that blood spots may be used in seroprevalence or serodiagnostic studies, especially to identify women who are infected or to identify possible cases of congenital infection. The test provides for studies of children and adults when routine venipuncture and serum handling and storage are problematic.

Comparison of the lipid regulation of yeast and rat CTP: phosphocholine cytidylyltransferase expressed in COS cells

The CTP: phosphocholine cytidylyltransferase (CT) gene from yeast and cDNA from rat liver were over-expressed 20-30-fold in COS cells. Most of the CT activities were found in the cytosolic fraction. The regulation of the yeast CT activity (Y-CT) by lipids was characterized for the first time in comparison with the regulation of the well-studied rat CT (R-CT). Sonicated vesicles composed of egg phosphatidylcholine (PC) or 1-stearoyl-2-oleoyl PC had no effect on Y-CT and only slightly stimulated R-CT activity. Both CTs were activated 10-50-fold by the anionic lipids cardiolipin, phosphatidyl-glycerol, phosphatidylinositol and oleic acid. The effects of varying the vesicle concentration and the mol% of anionic lipid in PC vesicles were tested. The concentration optima for the activation of Y-CT by oleic acid or anionic phospholipids were 5-10-fold lower than those for R-CT. For example, the stimulation of Y-CT activity by phosphatidylglycerol vesicles was optimal between 5 and 15 microM and declined at higher concentrations, but R-CT activation by these vesicles saturated at approximately 25 microM. The positively charged aminolipid sphingosine antagonized the stimulation by oleic acid of both Y-CT and R-CT. Y-CT activity was insensitive to PC vesicles containing the neutral lipids diacylglycerol, monoacylglycerol or oleyl alcohol. However, R-CT was stimulated 10-20-fold by vesicles containing these neutral lipids. Translocation of the CTs to microsomal membranes enriched with anionic or neutral lipids was compared. Oleic acid enrichment promoted translocation of Y-CT and R-CT, whereas diacylglycerol promoted only R-CT translocation. These data show that the activity of Y-CT is lipid-sensitive. Y-CT is affected only by charged lipids, whereas R-CT responds to charged and neutral lipid activators. The data are consistent with different modes of interaction of the two CTs with lipids.

Lipid modulation of nicotinic acetylcholine receptor function: the role of neutral and negatively charged lipids

The effects of negatively charged and neutral lipids on the function of the reconstituted nicotinic acetylcholine receptor from Torpedo californica were determined with two assays using acetylcholine receptor-containing vesicles: the ion flux response and the affinity-state transition. The receptor was reconstituted into three different lipid environments, with and without neutral lipids: (1) phosphatidylcholine/phosphatidylserine; (2) phosphatidylcholine/phosphatidic acid; and (3) phosphatidylcholine/cardiolipin. Analysis of the ion flux responses showed that: (1) all three negatively charged lipid environments gave fully functional acetylcholine receptor ion channels, provided neutral lipids were added; (2) in each lipid environment, the neutral lipids tested were functionally equivalent to cholesterol; and (3) the rate of receptor desensitization depends upon the type of neutral lipid and negatively charged phospholipid reconstituted with the receptor. The functional effects of neutral and negatively charged lipids on the acetylcholine receptor are discussed in terms of protein-lipid interactions and stabilization of protein structure by lipids.

[Effect of phosphatidylcholine cholesterol liposomes on the growth of bacterial cultures]

The in vitro studies showed that incubation of S. aureus, P. aeruginosa, E. coli and P. mirabilis in the presence of phosphatidyl choline cholesterol liposomal suspension was accompanied by inhibited microbial growth. The effect was time- and dose-dependent. The arranged structure of the liposomal membranes played an important role in genesis of the bactericidal action of the liposomes since the use of lipids of the same composition in a nonpolysomal form markedly lowered the bactericidal effect.

[Syphilis and blood transfusion]

Along 17 years (1973-1989), syphilis screening has been performed on 146,355 blood units in the author's blood bank. A total number of 143 positive results (confirmed by MHA-TP) was registered, which means an incidence of 0.097%. Of the total number of blood units, 31,529 came from professional donors, 51 of them (0.16%) being positive, while of the 114,826 blood units from voluntary donors 92 were positive (0.08%). With respect to voluntary donations, the highest incidence of positive reactions was found between 1980 and 1982, but this period registered also the highest number of blood units studied. Along this 17 year period 8 patients have received blood products with positive syphilis test. They were transfused on urgent request with fresh blood or platelet concentrates, the transfusion being performed before knowing the results of the screening for syphilis. No special measures were taken in 2 such cases, who died shortly after the transfusion on account of their disease. Two other were treated with penicillin at the time of transfusion. The remaining four patients received preventive penicillin. Even taking into account that positive screening tests are uncommon amongst blood donors, and that only under special circumstances the patients receiving contaminated blood may develop the illness, it seems advisable for every blood bank to perform the screening for syphilis on every blood donation.

Properties of the two terminal oxidases of Escherichia coli

Proton translocation coupled to oxidation of ubiquinol by O2 was studied in spheroplasts of two mutant strains of Escherichia coli, one of which expresses cytochrome d, but not cytochrome bo, and the other expressing only the latter. O2 pulse experiments revealed that cytochrome d catalyzes separation of the protons and electrons of ubiquinol oxidation but is not a proton pump. In contrast, cytochrome bo functions as a proton pump in addition to separating the charges of quinol oxidation. E. coli membranes and isolated cytochrome bo lack the CuA center typical of cytochrome c oxidase, and the isolated enzyme contains only 1Cu/2Fe. Optical spectra indicate that high-spin heme o contributes less than 10% to the reduced minus oxidized 560-nm band of the enzyme. Pyridine hemochrome spectra suggest that the hemes of cytochrome bo are not protohemes. Proteoliposomes with cytochrome bo exhibited good respiratory control, but H+/e- during quinol oxidation was only 0.3-0.7. This was attributed to an "inside out" orientation of a significant fraction of the enzyme. Possible metabolic benefits of expressing both cytochromes bo and d in E. coli are discussed.