Non-linear relationships between aflatoxin B₁ levels and the biological response of monkey kidney vero cells

Aflatoxin-producing fungi contaminate food and feed during pre-harvest, storage and processing periods. Once consumed, aflatoxins (AFs) accumulate in tissues, causing illnesses in animals and humans. Most human exposure to AF seems to be a result of consumption of contaminated plant and animal products. The policy of blending and dilution of grain containing higher levels of aflatoxins with uncontaminated grains for use in animal feed implicitly assumes that the deleterious effects of low levels of the toxins are linearly correlated to concentration. This assumption may not be justified, since it involves extrapolation of these nontoxic levels in feed, which are not of further concern. To develop a better understanding of the significance of low dose effects, in the present study, we developed quantitative methods for the detection of biologically active aflatoxin B₁ (AFB1) in Vero cells by two independent assays: the green fluorescent protein (GFP) assay, as a measure of protein synthesis by the cells, and the microculture tetrazolium (MTT) assay, as a measure of cell viability. The results demonstrate a non-linear dose-response relationship at the cellular level. AFB1 at low concentrations has an opposite biological effect to higher doses that inhibit protein synthesis. Additional studies showed that heat does not affect the stability of AFB1 in milk and that the Vero cell model can be used to determine the presence of bioactive AFB1 in spiked beef, lamb and turkey meat. The implication of the results for the cumulative effects of low amounts of AFB1 in numerous foods is discussed.

Low levels of aflatoxin B1, ricin, and milk enhance recombinant protein production in mammalian cells

Gene expression in transduced mammalian cells correlates with virus titer, but high doses of vector for gene therapy leads to toxicity in humans and in animals. Changing the optimal tissue culture medium by adding low levels of environmental stressors, such as 1 µM of the fungal toxin aflatoxin B1 (AFB1), 1 ng of the castor bean protein toxin ricin, or 1% reconstituted milk, enhances transcription and increases production of proteins in transduced mammalian cells as demonstrated by production of the following three recombinant proteins: firefly luciferase, β-galactosidase, and green fluorescent protein (GFP). Higher concentrations of the stress-producing substances damage the cells beyond recovery, resulting in inhibited gene expression and cell death. We also evaluated the effect of the stressor substances on the enhanced infectivity of virus. The presented findings extend methods for large-scale transient recombinant protein production in mammalian cells and suggest that it may be possible to reduce the cytotoxicity of the adenovirus by reducing the virus titer without adversely affecting gene expression levels.

Review of the inhibition of biological activities of food-related selected toxins by natural compounds

There is a need to develop food-compatible conditions to alter the structures of fungal, bacterial, and plant toxins, thus transforming toxins to nontoxic molecules. The term 'chemical genetics' has been used to describe this approach. This overview attempts to survey and consolidate the widely scattered literature on the inhibition by natural compounds and plant extracts of the biological (toxicological) activity of the following food-related toxins: aflatoxin B1, fumonisins, and ochratoxin A produced by fungi; cholera toxin produced by Vibrio cholerae bacteria; Shiga toxins produced by E. coli bacteria; staphylococcal enterotoxins produced by Staphylococcus aureus bacteria; ricin produced by seeds of the castor plant Ricinus communis; and the glycoalkaloid α-chaconine synthesized in potato tubers and leaves. The reduction of biological activity has been achieved by one or more of the following approaches: inhibition of the release of the toxin into the environment, especially food; an alteration of the structural integrity of the toxin molecules; changes in the optimum microenvironment, especially pH, for toxin activity; and protection against adverse effects of the toxins in cells, animals, and humans (chemoprevention). The results show that food-compatible and safe compounds with anti-toxin properties can be used to reduce the toxic potential of these toxins. Practical applications and research needs are suggested that may further facilitate reducing the toxic burden of the diet. Researchers are challenged to (a) apply the available methods without adversely affecting the nutritional quality, safety, and sensory attributes of animal feed and human food and (b) educate food producers and processors and the public about available approaches to mitigating the undesirable effects of natural toxins that may present in the diet.

CD154 as a potential early molecular biomarker for rapid quantification analysis of active Staphylococcus enterotoxin A

Staphylococcus aureus is a major bacterial pathogen producing a group of 21 enterotoxins (SEs). These enterotoxins have two separate but related biological activities. They cause gastroenteritis, and they function as superantigens that activate large numbers of T cells. In the current study, we demonstrate that short-term ex vivo exposure of primary naïve CD4(+) T-cells to SEA induces differential expression of the T cell surface receptor CD154 in a time- and dose-dependent manner. In addition, we show that SEA induces higher CD154 protein expression and higher splenocyte cell proliferation compared with SEB. We also demonstrate that expression of CD154 can be used for rapid detection of active SEA in milk.

The olive compound 4-hydroxytyrosol inactivates Staphylococcus aureus bacteria and Staphylococcal Enterotoxin A (SEA)

The foodborne pathogen Staphylococcus aureus produces the virulent staphylococcal enterotoxin A (SEA), a single chain protein which consists of 233 amino acid residues with a molecular weight of 27078 Da. SEA is a superantigen that is reported to contribute to animal (mastitis) and human (emesis, diarrhea, atopic dermatitis, arthritis, and toxic shock) syndromes. Changes in the native structural integrity may inactivate the toxin by preventing molecular interaction with cell membrane receptor sites of their host cells. In the present study, we evaluated the ability of the pure olive compound 4-hydroxytyrosol and a commercial olive powder called Hidrox-12, prepared by freeze-drying olive juice, to inhibit S. aureus bacteria and SEA's biological activity. Dilutions of both test substances inactivated the pathogens. Two independent cell assays (BrdU incorporation into newly synthesized DNA and glycyl-phenylalanyl-aminofluorocoumarin proteolysis) demonstrated that the olive compound 4-hydroxytyrosol also inactivated the biological activity of SEA at concentrations that were not toxic to the spleen cells. However, efforts to determine inhibition of the toxin by Hidrox-12 were not successful because the olive powder was cytotoxic to the spleen cells at concentrations found to be effective against the bacteria. The results suggest that food-compatible and safe antitoxin olive compounds can be used to inactivate both pathogens and toxins produced by the pathogens. Practical Application:  The results of this study suggest that food-compatible and safe antitoxin olive compounds can be used to reduce both pathogens and toxins produced by the pathogens in foods.

Milk inhibits the biological activity of ricin

Ricin is a highly toxic protein produced by the castor plant Ricinus communis. The toxin is relatively easy to isolate and can be used as a biological weapon. There is great interest in identifying effective inhibitors for ricin. In this study, we demonstrated by three independent assays that a component of reconstituted powdered milk has a high binding affinity to ricin. We discovered that milk can competitively bind to and reduce the amount of toxin available to asialofetuin type II, which is used as a model to study the binding of ricin to galactose cell-surface receptors. Milk also removes ricin bound to the microtiter plate. In parallel experiments, we demonstrated by activity assay and by immuno-PCR that milk can bind competitively to 1 ng/ml ricin, reducing the amount of toxin uptake by the cells, and thus inhibit the biological activity of ricin. The inhibitory effect of milk on ricin activity in Vero cells was at the same level as by anti-ricin antibodies. We also found that (a) milk did not inhibit ricin at concentrations of 10 or 100 ng/ml; (b) autoclaving 10 and 100 ng/ml ricin in DMEM at 121 °C for 30 min completely abolished activity; and (c) milk did not affect the activity of another ribosome inactivating protein, Shiga toxin type 2 (Stx2), produced by pathogenic Escherichia coli O157:H7. Unlike ricin, which is internalized into the cells via a galactose-binding site, Stx2 is internalized through the cell surface receptor glycolipid globotriasylceramides Gb3 and Gb4. These observations suggest that ricin toxicity may possibly be reduced at room temperature by a widely consumed natural liquid food.

Clostridium botulinum neurotoxin type B is heat-stable in milk and not inactivated by pasteurization

Foodborne botulism is caused by the ingestion of foods containing botulinum neurotoxins (BoNTs). To study the heat stability of Clostridium botulinum neurotoxins, we needed to measure and compare the activity of botulinum neurotoxins, serotypes A and B, under various pasteurization conditions. Currently, the only accepted assay to detect active C. botulinum neurotoxin is an in vivo mouse bioassay, which raises ethical concerns with regard to the use of experimental animals. In this study, noninvasive methods were used to simultaneously detect and distinguish between active BoNT serotypes A and B in one reaction and sample. We developed an enzymatic activity assay employing internally quenched fluorogenic peptides corresponding to SNAP-25, for BoNT-A, and VAMP2, for BoNT-B, as an alternative method to the mouse bioassay. Because each peptide is labeled with different fluorophores, we were able to distinguish between these two toxins. We used this method to analyze the heat stability of BoNT-A and BoNT-B. This study reports that conventional milk pasteurization (63 °C, 30 min) inactivated BoNT serotype A; however, serotype B is heat-stable in milk and not inactivated by pasteurization. Using this activity assay, we also showed that the commonly used food processes such as acidity and pasteurization, which are known to inhibit C. botulinum growth and toxin production, are more effective in inactivating BoNT serotype A than serotype B when conventional pasteurization (63 °C, 30 min) is used.

Ingested Shiga toxin 2 (Stx2) causes histopathological changes in kidney, spleen, and thymus tissues and mortality in mice

The Shiga toxin (Stx)-producing bacterial strain, Escherichia coli O157:H7, colonizes the distal small intestine and the colon, initiating serious illness, including hemolytic-uremic syndrome (HUS), characterized by microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. Although intravenous administration of purified Stx to primates has been able to reproduce the features of HUS, it has not been conclusively established as to whether ingestion of Stx alone without the bacterium poses a potential health risk. To help answer this question, in this study, we fed Shiga toxin 2 (Stx2) directly into the stomachs of mice via gavage. Our data show that ingestion of Stx2 at a concentration of 50 μg/mouse induces weight loss and kills the mice at 3-5 days post-gavage. Additional studies revealed that the toxin retains activity at low pH, that its activity is neutralized by treatment with toxin-specific antibody, and that about 1% of the fed toxin is absorbed into the blood circulation. Lethality by intraperitoneal (IP) injection of Stx2 occurred at much lower doses than by ingestion. Detailed histopathological evaluation of stained tissues by light microscopy revealed severe histopathological changes in kidneys, spleen, and thymus but not in the pancreas, lymph nodes, heart, lungs, trachea, esophagus, stomach, duodenum, jejunum, ileum, cecum, and colon. The pathological changes in the kidney appeared similar to those seen in humans with HUS. The cited data suggest that (a) most but not all of the toxin is inactivated in the digestive tract, (b) part of the oral-ingested toxin is absorbed from the digestive tract into the circulation, (c) enough active toxin reaches susceptible organs to induce damage, and (d) Stx2 in the absence of toxin-producing bacteria can be harmful to mice. The results are clinically relevant for food safety because we also found that heat treatments (pasteurization) that destroy bacteria did not inactivate the heat-resistant toxin produced and secreted by the bacteria.

Inhibition of biological activity of staphylococcal enterotoxin A (SEA) by apple juice and apple polyphenols

The foodborne pathogen Staphylococcus aureus produces the virulent staphylococcal enterotoxin A (SEA), a single-chain protein that consists of 233 amino acid residues with a molecular weight of 27 078 Da. SEA is a superantigen that is reported to contribute to animal (mastitis) and human (emesis, diarrhea, atopic dermatitis, arthritis, and toxic shock) syndromes. Changes of the native structural integrity may inactivate the toxin by preventing molecular interaction with cell membrane receptor sites of their host cells. In the present study, we evaluated the ability of one commercial and two freshly prepared apple juices and a commercial apple polyphenol preparation (Apple Poly) to inhibit the biological activity of SEA. Dilutions of freshly prepared apple juices and Apple Poly inhibited the biological activity of SEA without any significant cytotoxic effect on the spleen cells. Additional studies with antibody-coated immunomagnetic beads bearing specific antibodies against the toxin revealed that SEA added to apple juice appears to be largely irreversibly bound to the juice constituents. The results suggest that food-compatible and safe anti-toxin phenolic compounds can be used to inactivate SEA in vitro and possibly also in vivo, even after induction of T-cell proliferation by long-term exposure to SEA. The significance of the results for microbial food safety and human health is discussed.

In vitro cell-based assay for activity analysis of staphylococcal enterotoxin A in food

Staphylococcal enterotoxins (SEs) are a leading cause of food poisoning and have two separate biological activities; it causes gastroenteritis and functions as a superantigen that activates large numbers of T cells. In vivo monkey or kitten bioassays were developed for analysis of SEs emetic activity. To overcome the inherent limitations of such bioassays, this study describes an in vitro splenocyte proliferation assay based on SEs superantigen activity as an alternative method for measuring the activity of staphylococcal enterotoxin A (SEA). After incubation of splenocytes with SEA, cell proliferation was measured by labeling the proliferating cells' DNA with bromodeoxyuridine (5-bromo-2-deoxyuridine, BrdU) and quantifying the incorporated BrdU by immunohistochemistry. BrdU labeling is shown to be highly correlated with SEA concentration (R(2)=0.99) and can detect 20 pg mL(-1) of SEA, which is far more sensitive than most enzyme-linked immunosorbent assays. Our assay can also distinguish between active toxin and inactive forms of the toxin in milk. By applying immunomagnetic beads that capture and concentrate the toxin, our assay was able to overcome matrix interference. These results suggest that our in vitro cell-based assay is an advantageous practical alternative to the in vivo monkey or kitten bioassays for measuring SEA and possibly other SEs activity in food.

Effect of food matrices on the biological activity of ricin

A cell-free translation assay was applied for the quick detection of ricin in food samples. Three economically important foods-ground beef, low-fat milk, and liquid chicken egg--were tested. The results indicated that ground beef had very little matrix effect on the assay, whereas low-fat milk and liquid chicken egg showed clear interference on the protein translation. A simple dilution in phosphate-buffered saline (PBS) effectively eliminated the translational inhibition from these foods. The concentrations inhibiting 50% of luciferase translation derived from the current study were 0.01 nM for the pure ricin A chain, 0.02 nM for pure ricin, and 0.087 nM for crude ricin in PBS. In most cases, the half inhibitory concentration values for ricin in food matrices were significantly lower than for those in PBS buffer, suggesting that some components in these food matrices might potentiate the activity of ricin. Thermal stability tests indicated that the ricin A chain was the least stable among the three forms of ricin in all matrices measured. The thermal stability of pure and crude ricins varied depending on the matrices. The specific activities of ricin in PBS buffer were confirmed by a neutralization test with ricin-specific and nonspecific antibodies. This study demonstrates that the cell-free translation assay is a rapid and sensitive method for detection of biologically active ricin toxin in ground beef, low-fat milk, and liquid chicken egg and that food matrices can greatly affect the thermal stability of ricin.

Phloxine B, a versatile bacterial stain

The data presented suggest that Phloxine B, a color additive for food, drugs, and cosmetics has a potential use as a nontoxic, faster (<2 min), inexpensive (350 tests for <1 cent material) and simpler to use alternative to Gram staining. Using Phloxine B staining it was possible to differentiate among gram-negative and gram-positive bacteria by visual determination under normal room lighting, light microscopy, fluorescence microscopy and confocal microscopy. This work demonstrated that Phloxine B can be used as a differential versatile bacterial stain and establishes a correlation between the staining properties of the dye and its bactericidal effect.

Dietary trans 10, cis 12-conjugated linoleic acid reduces the expression of fatty acid oxidation and drug detoxification enzymes in mouse liver

Mice fed diets containing trans 10, cis 12 (t10, c12)-conjugated linoleic acid (CLA) develop fatty livers and the role of the hepatic fatty acid oxidation enzymes in this development is not well defined. We examined the effects of dietary cis 9, trans 11-CLA (c9, t11-CLA) and t10, c12-CLA on the expression of hepatic genes for fatty acid metabolism. Female mice, 8 weeks old, (six animals per group) were fed either a control diet or diets supplemented with 0.5% c9, t11- or c12-CLA for 8 weeks. DNA microarray analysis showed that t10, c12-CLA increased the expression of 278 hepatic genes and decreased those of 121 genes (>2 fold); c9, t11-CLA increased expression of twenty-two genes and decreased those of nine. Real-time PCR confirmed that t10, c12-CLA reduced by the expression of fatty acid oxidation genes including flavin monooxygenase (FMO)-3 95%, cytochrome P450 (cyt p450) 69%, carnitine palmitoyl transferase 1a 77%, acetyl CoA oxidase (ACOX) 50% and PPARalpha 65%: it increased the expression of fatty acid synthase by 3.5-fold (P<0.05 for all genes, except ACOX P=0.08). It also reduced the enzymatic activity of hepatic microsomal FMO by 40% and the FMO3 specific protein by 67%. c9, t11-CLA reduced FMO3 and cyt P450 expression by 61% (P=0.001) and 38% (P=0.06) and increased steoryl CoA desaturase transcription by 5.9-fold (P=0.07). Both decreased fatty acid oxidation and increased fatty acid synthesis seem to contribute to the CLA-induced fatty liver. Since FMO and cyt P450 are also involved in drug detoxification, suppression of the transcription of these genes by CLA may have other health consequences besides development of fatty liver.

Consumption of Bing sweet cherries lowers circulating concentrations of inflammation markers in healthy men and women

The purpose of this study was to determine the effects of consuming sweet cherries on plasma lipids and markers of inflammation in healthy humans. Healthy men and women (n = 18) supplemented their diets with Bing sweet cherries (280 g/d) for 28 d. After a 12-h fast, blood samples were taken before the start of cherry consumption (study d 0 and 7), 14 and 28 d after the start of cherry supplementation (study d 21 and 35), and 28 d after the discontinuation (study d 64) of cherry consumption. After cherries were consumed for 28 d, circulating concentrations of C-reactive protein (CRP), regulated upon activation, normal T-cell expressed, and secreted (RANTES), and NO decreased by 25 (P < 0.05), 21 (P < 0.05), and 18% (P = 0.07) respectively. After the discontinuation of cherry consumption for 28 d (d 64), concentrations of RANTES continued to decrease (P = 0.001), whereas those of CRP and NO did not differ from either d 7 (pre-cherries) or d 35 (post-cherries). Plasma concentrations of IL-6 and its soluble receptor, intercellular adhesion molecule-1, and tissue inhibitor of metalloproteinases-2 did not change during the study. Cherry consumption did not affect the plasma concentrations of total-, HDL-, LDL-, and VLDL- cholesterol, triglycerides, subfractions of HDL, LDL, VLDL, and their particle sizes and numbers. It also did not affect fasting blood glucose or insulin concentrations or a number of other chemical and hematological variables. Results of the present study suggest a selective modulatory effect of sweet cherries on CRP, NO, and RANTES. Such anti-inflammatory effects may be beneficial for the management and prevention of inflammatory diseases.

Retinoid x receptor agonists increase bcl2a1 expression and decrease apoptosis of naive T lymphocytes

Vitamin A affects many aspects of T lymphocyte development and function. The vitamin A metabolites all-trans- and 9-cis-retinoic acid regulate gene expression by binding to the retinoic acid receptor (RAR), while 9-cis-retinoic acid also binds to the retinoid X receptor (RXR). Naive DO11.10 T lymphocytes expressed mRNA and protein for RAR-alpha, RXR-alpha, and RXR-beta. DNA microarray analysis was used to identify RXR-responsive genes in naive DO11.10 T lymphocytes treated with the RXR agonist AGN194204. A total of 128 genes was differentially expressed, including 16 (15%) involved in cell growth or apoptosis. Among these was Bcl2a1, an antiapoptotic Bcl2 family member. Quantitative real-time PCR analysis confirmed this finding and demonstrated that Bcl2a1 mRNA expression was significantly greater in nonapoptotic than in apoptotic T lymphocytes. The RXR agonist 9-cis-retinoic acid also increased Bcl2a1 expression, although all-trans-retinoic acid and ligands for other RXR partner receptors did not. Treatment with AGN194204 and 9-cis-retinoic acid significantly decreased apoptosis measured by annexin V staining but did not affect expression of Bcl2 and Bcl-xL. Bcl2a1 promoter activity was examined using a luciferase promoter construct. Both AGN194204 and 9-cis-retinoic acid significantly increased luciferase activity. In summary, these data demonstrate that RXR agonists increase Bcl2a1 promoter activity and increase expression of Bcl2a1 in naive T lymphocytes but do not affect Bcl2 and Bcl-xL expression in naive T lymphocytes. Thus, this effect on Bcl2a1 expression may account for the decreased apoptosis seen in naive T lymphocytes treated with RXR agonists.

Expanding the bactericidal action of the food color additive phloxine B to gram-negative bacteria

Phloxine B (D&C red no. 28) is a color additive for food, drugs, and cosmetics. It has been previously shown to have anti-Staphylococcus aureus activities. In this work, the effect of Phloxine B on various gram-negative bacteria and other gram-positive bacteria including Bacillus cereus, Bacillus thuringiensis, Bacillus mycoides, Bacillus subtilis, Bacillus aureus, Salmonella, Escherichia coli and Shigella was studied, along with the mechanism of anti-microbial activity. In the presence of fluorescent light, the viable count for gram-positive bacteria, (Bacillus spp. and S. aureus) decreased in a dose and time dependent manner when incubated with Phloxine B. The viability of gram-positive bacteria was reduced by 99.99% in 40 min, while there was no effect on gram-negative bacteria (Salmonella choleraesuis, E. coli and Shigella flexneri). However, the use of ethylenediaminetetraacetic acid (EDTA) expands the spectrum of activity for Phloxine B to include gram-negative bacteria. EDTA increased membrane-permeability by releasing lipopolysaccharide. Overall, in an Agar diffusion test the light-dependent bactericidal activity of 1 microg of Phloxine B had a potency of 0.64 units of chloramphenicol and 0.5 units of tetracycline when tested on B. cereus, and had a potency of 0.7 units of chloramphenicol and 0.2 units of tetracycline when tested on S. aureus. The data suggest that the dye may have some potential anti-microbial applications.

Trypanosome microtubule-associated protein p15 as a vaccine for the prevention of African sleeping sickness

Trypanosomes cause African sleeping sickness, affecting millions of humans and animals. We tested trypanosome microtubule-associate protein (MAP p15) as a vaccine in mice, and show that p15 (native or recombinant) generated up to 100% protection from an otherwise lethal challenge of a heterologous strain of Trypanosoma brucei. We also tested the adenovirus as a vaccine delivery system and show that both adenoviral vector containing p15 gene or control adenovirus containing lacZ gene generated a protective response and exhibited strong CD8+ T-cell proliferation. These results suggest that the p15 protein itself is an effective vaccine and that the adenovirus may be used to mount a non-specific cellular immune response.

Ceramide accumulation precedes caspase-3 activation during apoptosis of A549 human lung adenocarcinoma cells

Ceramide, the basic structural unit of sphingolipids, controls the balance between cell growth and death by inducing apoptosis. We have previously shown that accumulation of ceramide, triggered by hydrogen peroxide (H(2)O(2)) or by short-chain ceramide analogs, induces apoptosis of lung epithelial cells. Here we elucidate the link between caspase-3 activation, at the execution phase, and ceramide accumulation, at the commitment phase of apoptosis in A549 human lung adenocarcinoma cells. The induction of ceramide accumulation by various triggers of ceramide generation, such as H(2)O(2), C(6)-ceramide, or UDP-glucose-ceramide glucosyltransferase inhibitor dl-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol, triggered the activation of caspase-3. This ceramide elevation also induced the cleavage of the death substrate poly(ADP-ribose) polymerase and was followed by apoptotic cell death. Ceramide-mediated apoptosis was blocked by a general caspase inhibitor, Boc-d-fluoromethylketone, and by overexpression of the antiapoptotic protein Bcl-2. Notably, overexpression of Bcl-2 reduced the basal cellular levels of ceramide and prevented the induction of ceramide generation by C(6)-ceramide, which implies ceramide generation as a possible target for the antiapoptotic effects of Bcl-2.

Structure of p15 trypanosome microtubule associated protein

Trypanosomes are protozoan parasites, containing subpellicular microtubules that are crosslinked to one another and to the plasma membrane by unique microtubule associate proteins (MAPs). One of these MAPs, purified from the subpellicular microtubules by a tubulin affinity column, is termed p15alpha. This protein was shown to be specific to the subpellicular microtubules by immunoelectron microscopy and was also shown to induce microtubule bundling. We show here that the gene coding for p15alpha is present in multiple copies. It is 258 bp long and contains 16 highly organized 15 bp long repetitive sequences. The deduced amino acid sequence indicates that p15alpha contains 16 highly organized, nearly identical tandem repeats, with a periodicity of five amino acids, which are rich in positively charged and in non-polar (hydrophobic) amino acids. The p15 family is unique to trypanosomes and has no significant sequence homology to other species. p15 can bind both tubulin and phospholipids in vitro and is therefore hypothesized to contribute to the stability of the trypanosome subpellicular microtubules.