Proteolytic activity of the Bacteroides fragilis enterotoxin causes fluid secretion and intestinal damage in vivo

Strains of Bacteroides fragilis that produce an enterotoxin have been implicated in diarrheal disease in farm animals and humans during the past decade. Our laboratory has purified and characterized this enterotoxin as a single polypeptide (M(r), approximately 20,000). Recently, we used PCR to clone and sequence the enterotoxin gene from B. fragilis and showed that it exhibits significant homology with extracellular metalloproteases. Further studies showed that the purified enterotoxin has protease activity. To further characterize the role of this enterotoxin in diarrheal disease, we studied the histological and pathological effects of highly purified B. fragilis enterotoxin in lamb, rabbit, and rat ligated intestinal loops. When the enterotoxin was injected into ligated ileal and colonic loops, there was significant tissue damage and subsequent fluid accumulation. The fluid response in the ileum was greater in lambs than in rabbits and rats, whereas the fluid response in the colon was greater in rabbits than in lambs and rats. Analysis of the intestinal fluid elicited by the enterotoxin revealed an accumulation of chloride and sodium as well as albumin and total protein. Histological examination revealed mild necrosis of epithelial cells, crypt elongation, villus attenuation, and hyperplasia. There was extensive detachment and rounding of surface epithelial cells and an infiltration of neutrophils. Enterotoxic activity was inhibited by the metal chelators EDTA and 1,10-phenanthroline; to some degree, the enterotoxic activity could be reconstituted by the addition of zinc to the chelated enterotoxin. Our results indicate that the enterotoxin elicits a significant fluid response subsequent to tissue damage in the small and large intestine. These data further support the idea that this enterotoxin is an important virulence factor in B. fragilis-associated diarrhea.

Enterotoxigenic Bacteroides fragilis

Enterotoxigenic Bacteroides fragilis (ETBF) has been recently associated with watery diarrheal disease in livestock and young children. These strains of B. fragilis secrete an extracellular heat-labile protein toxin with a mess of approximately 20 kD. This toxin has been shown to have two major biological activities: stimulation of secretion in ligated intestinal segments in lambs and calves and alteration of the morphology of intestinal epithelial cell lines in vitro. In vivo the pathology of animal intestine exposed to an ETBF strain is disruption of the colonic epithelium with rounded, swollen surface epithelial cells. Similarly, in vitro intestinal epithelial cells derived from human colonic carcinomas become rounded and detach from neighboring cells after treatment with the ETBF toxin. These results suggest that the ETBF toxin is a secretory cytoskeleton-altering toxin. However, the contribution of the ETBF toxin to the pathogenesis of ETBF-associated diarrheal disease remains to be determined.

The enterotoxin of Bacteroides fragilis is a metalloprotease

During the past decade, strains of Bacteroides fragilis that produce an enterotoxin have been implicated in diarrheal disease in animals and humans. The extracellular enterotoxin has been purified and characterized as a single polypeptide (M(r), approximately 20,000). Single specific primer-PCR was used to clone a portion of the B. fragilis enterotoxin gene. The recombinant protein expressed by the cloned gene fragment reacted with monospecific antibodies to B. fragilis enterotoxin by enzyme-linked immunosorbent assay and immunoblot analysis. The deduced amino acid sequence revealed a signature zinc-binding consensus motif (HEXXHXXGXXH/Met-turn) characteristic of metalloproteases termed metzincins. Sequence comparisons showed close identity to matrix metalloproteases (e.g., human fibroblast collagenase) within the zinc-binding and Met-turn region. Purified enterotoxin contained 1 g-atom of Zn2+ per molecule and hydrolyzed gelatin, azocoll, actin, tropomyosin, and fibrinogen. The enterotoxin also underwent autodigestion. The N-terminal amino acid sequences of two autodigestion products were identical to the deduced amino acid sequence of the recombinant enterotoxin and revealed cleavage at Cys-Leu and Ser-Leu peptide bonds. Gelatinase (type IV collagenase) activity comigrated with the toxin when analyzed by gel fractionation and zymography, indicating that protease activity is due to the enterotoxin and not to a contaminating protease(s). Optimal proteolytic activity occurred at 37 degrees C and pH 6.5. Primary proteolytic cleavage sites in actin were identified, revealing cleavage at Gly-Met and Thr-Leu peptide bonds. Enzymatic activity was inhibited by metal chelators but not by inhibitors of other classes of proteases. Additionally, cytotoxic activity of the enterotoxin on human carcinoma HT-29 cells was inhibited by acetoxymethyl ester EDTA. The metalloprotease activity of the enterotoxin suggests a possible mechanism for enterotoxicity and may have additional implications in the study of disease caused by B. fragilis.

Characterization of enterotoxigenic Bacteroides fragilis by a toxin-specific enzyme-linked immunosorbent assay

Within the past decade, certain strains of Bacteroides fragilis have been associated with diarrhea in humans and cytotoxic activity on certain colon carcinoma cell lines. An enzyme-linked immunosorbent assay (ELISA) for detecting the enterotoxin of B. fragilis in cultures and stools was developed by using high-titer monospecific goat and rabbit antitoxins in an indirect format. The lower limit of detection for purified toxin was approximately 0.05 micrograms/ml; the linear range was from 0.05 to 10 microgram/ml. Using the ELISA to screen cultures of toxigenic and nontoxigenic strains of B. fragilis, we observed 100% correlation with 16 known toxigenic strains which had various cytotoxic activities on HT-29 cells. In addition, we found 6 of 62 previously untested strains also to be positive in both assays. Stability studies revealed that although the cytotoxic activities of crude and purified toxin preparations incubated at elevated temperatures were rapidly lost, the ELISA responses were not significantly reduced. Sodium dodecyl sulfate(SDS)-polyacrylamide gel electrophoresis and SDS-capillary electrophoresis showed that the purified toxin autodigested to several stable peptides. Studies on partially purified membranes from the toxigenic strains revealed the presence of several membrane-associated components which were noncytotoxic but strongly immunoreactive in the ELISA. Preliminary studies with spiked feces indicated that the ELISA may be useful for screening not only cultures for the enterotoxigenic B. fragilis but also stool specimens. Ongoing studies are focusing on determining the nature of the toxin's apparent proteolytic capabilities and investigating the feasibility of using the ELISA on stool specimens from healthy and diarrheic humans.

Purification and characterization of alpha-toxin produced by Clostridium novyi type A

Our study describes the production, purification, and properties of alpha-toxin from Clostridium novyi type A 19402. The bacterium produced maximal amounts of alpha-toxin when grown at 37 degrees C for 72 h in dialysis flask cultures containing brain heart infusion supplemented with 0.75% Tween 80 and 2% glycerol. The alpha-toxin was purified by precipitation with polyethylene glycol 6000, followed by chromatography on Q-Sepharose, phenyl-agarose, and Mono-Q. By sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the toxin exhibited a single band with an M(r) of 200,000. The toxin also exhibited a single immunoprecipitin arc by crossed immunoelectrophoresis with antiserum against crude toxin. It was stable when stored at 4 degrees C and also following exposure to buffers with pHs in the range of 4 to 7. The toxin had a minimum lethal dose in mice of 5 to 10 ng, caused rounding of a variety of cells in tissue culture, and was negative in the rabbit ileal loop assay. The cytotoxic activity was inhibited by agents that affect receptor-mediated processes, and the toxin was less active on a CHO mutant cell line that is defective in endosomal acidification. The analysis of the amino acid composition revealed an unusually high proline content. The N-terminal sequence is Met-Leu-Ile-Thr-Arg-Glu-Gln-Leu-Met-Lys.

Purification and characterization of Clostridium difficile glutamate dehydrogenase

Recombinant Clostridium difficile glutamate dehydrogenase (L-glutamate:NAD oxidoreductase, EC 1.4.1.2) was purified 177-fold to electrophoretic homogeneity with a 62% recovery through a four-step procedure involving gel filtration and ion-exchange and dye affinity chromatography. The approximate molecular weights of the native enzyme by gel filtration and subunits by sodium dodecyl sulfate-polyacrylamide gel electrophoresis were consistent with a hexameric structure for the purified enzyme. The enzyme-catalyzed glutamate oxidation was an NAD-dependent sequential process in which NADP could not be substituted as coenzyme. Several dinucleotide analogs of NAD structurally altered in either the pyridine or the purine moiety were observed to function as coenzymes when substituted for NAD. Nicotinamide mononucleotide did not serve as a coenzyme for glutamate oxidation. Product inhibition by NADH was competitive with respect to NAD. In deadend inhibition studies, adenosine diphosphoribose was shown to be an effective coenzyme-competitive inhibitor.

Purification and characterization of an enterotoxin from Bacteroides fragilis

An enterotoxin produced by Bacteroides fragilis was purified to homogeneity and characterized as to its biological activity and basic molecular properties. Toxin preparations were prepared by growing B. fragilis VPI 13784 in brain heart infusion broth to early stationary phase, immediately precipitating the culture supernatant fluid with 70% ammonium sulfate, and stabilizing the precipitate with the protease inhibitor TPCK (tolylsulfonyl phenylalanyl chloromethyl ketone). The toxin was sequentially purified by anion-exchange chromatography on Q-Sepharose, hydrophobic interaction chromatography on phenyl-agarose, and high-resolution ion-exchange chromatography on Mono Q. The toxin appeared homogeneous as judged by polyacrylamide gel electrophoresis. The estimated molecular weight of the highly purified toxin as determined by gel filtration chromatography on Superose-12 and sodium dodecyl sulfate-polyacrylamide gel electrophoresis is 19,000. It has an isoelectric point of approximately 4.5 and is stable at pHs 5 to 10. The purified toxin is stable at -20 and 4 degrees C and upon freeze-drying, but it is unstable at temperatures above 55 degrees C. It is sensitive to proteinase K and Streptomyces protease but is resistant to trypsin and chymotrypsin. The activity of the purified toxin is neutralized by antiserum to a toxigenic strain of B. fragilis but not by antiserum to nontoxigenic strains. N-terminal amino acid analysis reveal an unambiguous sequence of Ala-Val-Pro-Ser-Glu-Pro-Lys-Thr-Val-Tyr-Val-Ile-Xxx-Leu-Arg-Glu-Asn-Gly- Ser-Thr . The highly purified toxin induced a strong fluid accumulation response in the lamb ileal-loop assay as well as a cytotoxic response (cell rounding) on HT-29 colon carcinoma cells. Thus, the purified toxin can cause both enterotoxic and cytotoxic activities.

Metabolism of levamisole, an anti-colon cancer drug, by human intestinal bacteria

1. Anaerobic incubation of levamisole with human intestinal flora resulted in the formation of three thiazole ring-opened metabolites, namely, levametabol-I, II and III. These new hydroxamic lactam-type metabolites were isolated and characterized by various spectroscopic methods. 2. Various pure cultures of human intestinal bacterial strains were shown, by quantitative h.p.l.c. analysis, to have ring-opening ability. Strong metabolizers include Bacteroides and Clostridium spp. Bacterial mixtures prepared from human faeces showed much greater ability to transform levamisole (74% in 48 h) than any pure strain culture. 3. Greatly decreased levamisole-transforming activity was observed with autoclaved bacterial cultures, and no activity was found with broth medium alone. This indicates that metabolism requires the presence of anaerobic bacteria and involves, at least in part, a non-enzymic process.

Metabolism of dietary genotoxins by the human colonic microflora; the fecapentaenes and heterocyclic amines

The microflora of the human colon is a complex ecosystem of anaerobic bacteria which have the capability of enzymatically transforming a variety of dietary (or biliary) compounds to genotoxic metabolites. In the past, most investigators studying the interplay between diet and colonic flora and its role in the etiology of cancers focused on the reductive and glycosidic potential of the bacterial enzymes--many of which reverse the oxidative and conjugative reactions performed by the liver. Recent work in our laboratory has focused on the metabolism of two relatively new classes of genotoxins, the fecapentaenes and the heterocyclic amines (pyrolysis carcinogens). The fecapentaenes (conjugated ether lipids) are produced in the colon by Bacteroides spp. from polyunsaturated ether phospholipids (plasmalogens) whose natural origin and function are unknown. The fecapentaenes are potent direct-acting genotoxins that are detected in the feces of most individuals on normal western diets. The heterocyclic amines, which originate from fried or broiled proteinaceous foods, normally require activation by the liver before being potent mutagens or carcinogens. However, the "IQ" subclass (e.g. IQ and MeIQ) can be activated in the colon by Eubacterium and Clostridium species to a 7-hydroxy form which is directly mutagenic in Salmonella. Although there is no direct evidence that the fecapentaenes or the 7-hydroxy "IQ" compounds influence risk for colon cancer, the potency and prevalence of these bacterial metabolites is cause for concern.

Carcinogenicity tests of fecapentaene-12 in mice and rats

Fecapentaenes, a class of direct-acting bacterial mutagens, have been isolated from the feces and intestinal tract of humans on a Western meat-containing diet. Two bioassays to test pure fecapentaene-12 (FP-12) for carcinogenicity were performed. FP-12 in dimethylsulfoxide (DMSO) solution was injected i.p. into newborn ICR/MA mice on days 1, 3, 7, 10, 14 and 21. The mice killed after 21 months had neoplasms in liver, lung, glandular stomach and subcutaneous fibrosarcoma. Intrarectal (i.r.) infusion of FP-12 in an aqueous vehicle into male F344 rats for 71 weeks, and killing the rats after 21 weeks more, displayed no evidence of neoplasia associated with FP-12 exposure. The positive control, N-nitrosomethylurea (NMU), given i.r. as 4 2-mg doses in 2 weeks, as expected, yielded multiple colonic neoplasms in less than 11 months. Fecapentaene may exert its effect in bacteria and in newborn mice through the generation of hydroxy radicals. However, adult rodent and human colon may have adequate biochemical defense mechanisms against low level, even continuous exposures to chemicals like FP-12, and thus be at low risk of neoplasia, as was found.

Case-control study of colorectal cancer and fecal mutagenicity

Fecal mutagenicity was measured in 68 patients with colorectal cancer and in 114 controls, using Salmonella tester strains TA98 and TA100 with and without S9 activation. Samples were also tested for fecapentaenes by high-performance liquid chromatography, to permit the separation of fecapentaene and non-fecapentaene mutagenicity. Overall, no significant case-control differences in fecal mutagenicity were observed. However, when samples containing high concentrations of fecapentaenes were excluded, non-fecapentaene TA98 mutagenicity was observed in eight cases (12%) and only four controls (4%), resulting in an estimated relative risk of 4.4 (95% confidence interval = 1.0-21.1). The association of colorectal cancer risk with non-fecapentaene TA98 mutagenicity could not be explained as an artifact of diagnostic workup or gastrointestinal bleeding among the cases. Smoking could also be excluded as a source of the TA98 mutagenicity seen, but possible dietary origins are still being explored.

The precursors of fecapentaenes: purification and properties of a novel plasmalogen

Fecapentaene-12 and fecapentaene-14 are genotoxic poly-unsaturated ether-lipids produced by the colonic microflora in humans and pigs. Although the fecapentaenes have been extensively characterized, little is known about the nature of the precursors from which they are produced. We purified one form of these precursors from feces of an individual who excreted high levels of fecapentaene-12 and its precursors. Purification was carried out by a series of extractions and precipitation in organic solvents followed by silica and amine high performance liquid chromatography. The purified precursor had identical UV spectral characteristics as the fecapentaenes indicating that it contained the same ether-linked pentaenyl functional group. However, it was not mutagenic. The precursor was amphiphilic in nature, behaving like a synthetic "model" ether-phospholipid on silica and C18 thin layer chromatography. When incorporated into phosphatidylcholine micelles it could be hydrolyzed in vitro by a combination of lipase and phospholipase C to fecapentaene-12. Our findings indicate that the general structure of this precursor is that of a phospholipid, specifically a plasmalogen--the exact structures of which remain to be determined.

Fecapentaene concentration and mutagenicity in 718 North American stool samples

The fecapentaenes (FP) are the predominant fecal mutagens identified to date, but they have not been shown to be carcinogenic. Epidemiologists looking for other fecal mutagens that may be related to colorectal cancer must disentangle from their investigations the pervasive mutagenic effect of the fecapentaenes. As a first step to studying the epidemiology of fecal mutagenicity independent of fecapentaenes, we compared FP measurements and Salmonella mutagenicity assay results for 718 acetone-extracted stool samples collected from a variety of subjects in the Washington DC metropolitan areas. In this large group, 50% of mutagenic samples contained elevated fecapentaenes. Specifically, three-quarters of the samples mutagenic in TA100 contained high FP levels. In contrast, mutagenicity in TA98 was not generally explainable by fecapentaenes, suggesting that non-fecapentaene TA98 mutagenicity should be one focus of future efforts to uncover colorectal carcinogens of etiologic importance.

Case-control study of colorectal cancer and fecapentaene excretion

The fecapentaenes are potent mutagens found in high concentrations in the stools of some individuals. These compounds are produced in vivo by common species of the colonic microflora, from precursors of unknown origin. The fecapentaenes have been postulated to increase the risk of colorectal cancer. To test this hypothesis, we measured fecapentaene excretion in 69 patients with adenocarcinoma of the colon or rectum, newly diagnosed at three Washington, DC area hospitals. The cases were compared with 114 surgical controls, frequency matched to the cases on age, sex, and hospital. We attempted to measure fecapentaene excretion 4 times for each subject: before surgery; and at 1 mo; 3 mo; and 6 mo following surgery. Contrary to our study hypothesis, we found fecapentaene excretion during the four study periods to be similar or even lower in cases compared to controls. An indirect measurement of fecapentaene precursors also tended to be lower in cases. The case-control differences could not be explained as effects of bleeding or of the colorectal diagnostic workup, which was assessed in a separate group of 86 patients. We conclude from these data that the excretion of fecapentaenes does not increase the risk of colorectal cancer, at least when measured near the time of diagnosis.

Conversion of IQ, a dietary pyrolysis carcinogen to a direct-acting mutagen by normal intestinal bacteria of humans

The dietary carcinogen, 2-amino-3-methyl-3H-imidazo[4,5-f]quinoline (IQ) is mutagenic in the Salmonella/microsomal mutagenicity assay when activated by microsomal enzymes. IQ is found in many cooked foods, notably fried beef and pork. In laboratory rodents IQ is carcinogenic. We showed that mixed and pure cultures of human intestinal anaerobes, notably Eubacterium spp., metabolized IQ to 2-amino-3,6-dihydro-3-methyl-7H-imidazo[4,5-f]quinoline-7-one (HOIQ). Unlike IQ, both the synthetic and bacterially produced HOIQ were direct-acting mutagens, i.e. active without microsomal activation. This new direct-acting mutagen, from the bacterial metabolism of a dietary pyrolysis carcinogen, raises new concerns about the possible role of this class of genotoxins in the etiology of human cancer.

Fecapentaenes and their precursors throughout the bowel--results of an autopsy study

The fecapentaenes are potent mutagens found in the stool of some humans and pigs. These compounds are produced by Bacteroides species in the gut from an uncharacterized family of precursor compounds, and have been postulated to pose a risk of human colorectal cancer. To better understand fecapentaene production in vivo, and to determine if excreted levels measured in epidemiologic studies are representative of the entire colon, fecapentaenes were assayed from multiple sites in the bowel in an autopsy study of 16 humans and 2 pigs. An indirect measurement of fecapentaene precursors was also made. Colonic concentrations of fecapentaenes and precursors varied widely between individuals, but were consistent for each individual throughout the colon. In addition, the measurements of rectal contents, assumed to approximate values in excreted stool, were equivalent to measurements from the colon.

Characterization of a mutagenic bacterial product in human feces

Mutagens detectable with the Ames assay have been found in the feces of apparently healthy individuals and the incidence of this mutagenic activity was found to be greater in a population at high risk for colon cancer than in a population at low risk. A compound accounting for the mutagenic activity has been isolated by high performance liquid chromatography. Two closely related forms which behave identically chemically could be resolved. The compound was active on Salmonella typhimurium TA98 and TA100, had a characteristic ultraviolet absorption spectrum with maxima at about 320, 340, and 365 nm, fluoresced green in long wavelength ultraviolet light, and had the same mobility on the thin-layer chromatography as the mutagenic activity in a direct ether extract of feces. The compound was unstable in air but could be stabilized in the presence of butylated hydroxytoluene. Upon oxidation the compound lost its mutagenicity and its ultraviolet absorption spectrum underwent a blue shift so that the absorption maxima were at 295, 310, and 325 nm. Determination of the structure of the mutagen has been difficult since the compound was not volatile and production of a volatile derivative has not been successful. On thin-layer chromatography plates the compound reacted with reagents that detect chlorinated compounds. By thermal energy analysis it did not appear to contain a nitroso group. The compound increased in concentration upon anaerobic incubation of feces at 37 C and this increase was prevented by cold, air, and antimicrobial agents. This suggests to us that the fecal flora produces the compound.

Production of Clostridium difficile antitoxin

We have produced antitoxin to the toxin of Clostridium difficile in rabbits and in goats. Antitoxin dilutions of 1/8,000 and 1/5,120 were capable of neutralizing lethal doses of the toxin in mice and in tissue culture, respectively.

Bacteriophages of Bacteroides

Sixty-eight bacteriophages specific for nine species (DNA homology groups) of Bacteroides were isolated from sewage. Four distinct morphological types were isolated, three of which had not previously been described. Attempts to use these phages to transduce Bacteroides fragilis were unsuccessful. A total of 91 phage-susceptible strains of Bacteroides were used with these phages in a study of the feasibility of developing a scheme for identification of Bacteroides species. Blind trials were performed with 10 B. fragilis-specific phages and a collection of over 200 bacterial strains, including 144 strains of B. fragilis. Of the strains of B. fragilis, 78% were identified correctly within 24 hr. A phage-carrier state, or pseudolysogeny, was observed with most of the phage-host systems, and this state was studied in detail with B. fragilis phage Bf-1. The presence of a thick capsule around some cells in a pure culture of a host strain appears to render these cells resistant to phage infection, thus perpetuating the carrier state. It is suggested that such capsules may play a role in the virulence of strains of Bacteroides.

Isolation of auxotrophs of Bacteroides fragilis

Techniques employing ethane methanesulfonate and modified penicillin enrichment were developed whereby auxotrophs of Bacteroides fragilis could be readily isolated. Several auxotrophic phenotypes are described.