Repression of heat-stable enterotoxin synthesis in enterotoxigenic Escherichia coli

The expression of Longus type 4 pilus of enterotoxigenic Escherichia coli is regulated by LngR and LngS and by H-NS, CpxR and CRP global regulators

Enterotoxigenic Escherichia coli produces a long type 4 pilus called Longus. The regulatory elements and the environmental signals controlling the expression of Longus-encoding genes are unknown. We identified two genes lngR and lngS in the Longus operon, whose predicted products share homology with transcriptional regulators. Isogenic lngR and lngS mutants were considerably affected in transcription of lngA pilin gene. The expression of lngA, lngR and lngS genes was optimally expressed at 37°C at pH 7.5. The presence of glucose and sodium chloride had a positive effect on Longus expression. The presence of divalent ions, particularly calcium, appears to be an important stimulus for Longus production. In addition, we studied H-NS, CpxR and CRP global regulators, on Longus expression. The response regulator CpxR appears to function as a positive regulator of lng genes as the cpxR mutant showed reduced levels of lngRSA expression. In contrast, H-NS and CRP function as negative regulators since expression of lngA was up-regulated in isogenic hns and crp mutants. H-NS and CRP were required for salt- and glucose-mediated regulation of Longus. Our data suggest the existence of a complex regulatory network controlling Longus expression, involving both local and global regulators in response to different environmental signals.

Animal Enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is the most common cause of E. coli diarrhea in farm animals. ETEC are characterized by the ability to produce two types of virulence factors: adhesins that promote binding to specific enterocyte receptors for intestinal colonization and enterotoxins responsible for fluid secretion. The best-characterized adhesins are expressed in the context of fimbriae, such as the F4 (also designated K88), F5 (K99), F6 (987P), F17, and F18 fimbriae. Once established in the animal small intestine, ETEC produce enterotoxin(s) that lead to diarrhea. The enterotoxins belong to two major classes: heat-labile toxins that consist of one active and five binding subunits (LT), and heat-stable toxins that are small polypeptides (STa, STb, and EAST1). This review describes the disease and pathogenesis of animal ETEC, the corresponding virulence genes and protein products of these bacteria, their regulation and targets in animal hosts, as well as mechanisms of action. Furthermore, vaccines, inhibitors, probiotics, and the identification of potential new targets by genomics are presented in the context of animal ETEC.

The tib adherence locus of enterotoxigenic Escherichia coli is regulated by cyclic AMP receptor protein

Enterotoxigenic Escherichia coli (ETEC) is a Gram-negative enteric pathogen that causes profuse watery diarrhea through the elaboration of heat-labile and/or heat-stable toxins. Virulence is also dependent upon the expression of adhesive pili and afimbrial adhesins that allow the pathogen to adhere to the intestinal epithelium or mucosa. Both types of enterotoxins are regulated at the level of transcription by cyclic AMP (cAMP) receptor protein (CRP). To further our understanding of virulence gene regulation, an in silico approach was used to identify putative CRP binding sites in the genome of H10407 (O78:H11), an ETEC strain that was originally isolated from the stool of a Bangledeshi patient with cholera-like symptoms circa 1971. One of the predicted binding sites was located within an intergenic region upstream of tibDBCA. TibA is an autotransporter and afimbrial adhesin that is glycosylated by TibC. Expression of the TibA glycoprotein was abolished in an H10407 crp mutant and restored when crp was provided in trans. TibA-dependent aggregation was also abolished in a cyaA::kan strain and restored by addition of exogenous cAMP to the growth medium. DNase I footprinting confirmed that the predicted site upstream of tibDBCA is bound by CRP. Point mutations within the CRP binding site were found to abolish or significantly impair CRP-dependent activation of the tibDB promoter. Thus, these studies demonstrate that CRP positively regulates the expression of the glycosylated afimbrial adhesin TibA through occupancy of a binding site within tibDBp.

Bacterial heat-stable enterotoxins: translation of pathogenic peptides into novel targeted diagnostics and therapeutics

Heat-stable toxins (STs) produced by enterotoxigenic bacteria cause endemic and traveler’s diarrhea by binding to and activating the intestinal receptor guanylyl cyclase C (GC-C). Advances in understanding the biology of GC-C have extended ST from a diarrheagenic peptide to a novel therapeutic agent. Here, we summarize the physiological and pathophysiological role of GC-C in fluid-electrolyte regulation and intestinal crypt-villus homeostasis, as well as describe translational opportunities offered by STs, reflecting the unique characteristics of GC-C, in treating irritable bowel syndrome and chronic constipation, and in preventing and treating colorectal cancer.

Cyclic AMP receptor protein-dependent repression of heat-labile enterotoxin

Enterotoxigenic Escherichia coli is a major cause of acute diarrheal illness worldwide and is responsible for high infant and child mortality rates in developing nations. Two types of enterotoxins, one heat labile and the other heat stable, are known to cause diarrhea. The expression of soluble heat-labile toxin is subject to catabolite (glucose) activation, and three binding sites for cAMP receptor protein (CRP or CAP) were identified upstream and within the toxin promoter by DNase I footprinting. One CRP operator is centered at -31.5, thus encompassing the promoter's -35 hexamer. Potassium permanganate footprinting revealed that the occupancy of this operator prevents RNA polymerase from forming an open complex in vitro. However, the operator centered at -31.5 is not sufficient for full repression in vivo because the deletion of the other two CRP binding sites partially relieved the CRP-dependent repression of the heat-labile toxin promoter. In contrast to heat-labile toxin, CRP positively regulates the expression of heat-stable toxin. Thus, the conditions for the optimal expression of one enterotoxin limit the expression of the other. Since glucose inhibits the activity of CRP by suppressing the pathogen's synthesis of cyclic AMP (cAMP), the concentration of glucose in the lumen of the small intestine may determine which enterotoxin is maximally expressed. In addition, our results suggest that the host may also modulate enterotoxin expression because cells intoxicated with heat-labile toxin overproduce and release cAMP.

In vivo expression of the heat stable (estA) and heat labile (eltB) toxin genes of enterotoxigenic Escherichia coli (ETEC)

Enterotoxigenic Escherichia coli (ETEC) colonize the intestine and adhere to the epithelium by means of different host specific colonization factors (CFs). Colonizing ETEC produce one or both of two enterotoxins; the heat stable (ST) and heat labile (LT) toxins which are both able to cause diarrhoea. The regulation of virulence genes in ETEC during infection of the human intestine is mainly unknown. In this study we analysed the level of mRNA expression of estA, coding for ST, and eltB, coding for the B subunit of LT, during human infection. The expressions of the toxins in ETEC strains expressing both ST and LT were investigated in bacteria isolated directly from patient stool without sub-culturing, (in vivo) and compared to the expression pattern of the corresponding ST/LT strains grown in liquid broth (in vitro) by quantitative competitive RT-PCR using fluorescent primers. We found that estA and eltB are expressed in the in vivo samples but no significant up-or down regulation of the expression levels of either estA or eltB could be determined in vivo as compared to in vitro.

Fermentable-sugar-level-dependent regulation of leukotoxin synthesis in a variably toxic strain of Actinobacillus actinomycetemcomitans

Actinobacillus actinomycetemcomitans, a Gram-negative periodontopathic bacterium, produces a leukotoxin belonging to the RTX family. The production of leukotoxin varies greatly among different strains of this species and under different culture conditions. A toxin-production-variable strain, 301-b, stably produces significant amounts of leukotoxin in anaerobic fructose-limited chemostat cultures, but does not do so in the presence of excess fructose. This communication describes the cloning and sequencing of the leukotoxin promoter region from 301-b, showing that this strain has a promoter region similar to that from strain 652, a moderately toxic strain. Northern blot analysis using a leukotoxin gene probe demonstrated that change in toxin production in response to the level of external fructose was due to alteration in the transcriptional level of the leukotoxin gene. Pulsing of fructose into the fructose-limited chemostat culture remarkably reduced the intracellular cAMP level from 40 pmol (mg dry wt cells)(-1) to 3.1 pmol (mg dry wt cells)(-1), which was restored when the culture was returned to fructose-limited conditions. Further, it was found that addition of external cAMP to the culture with excess fructose resulted in an apparent recovery of leukotoxin production. Taken together, these findings indicate that a cAMP-dependent mechanism, possibly a catabolite-repression-like system, may be involved in the regulation of leukotoxin production in this bacterium.

Effects of carbon and nitrogen sources, sodium chloride and culture conditions on cytotoxin production by Salmonella choleraesuis

The effects of carbon and nitrogen sources, incubation temperature, shaking speed, initial pH of culture broth as well as various concentration of NaCl on the production of cytotoxin by Salmonella choleraesuis SC-5 were evaluated in the present study. Results reveal that the optimal temperature, initial medium pH and shaking speed for cytotoxin production was 37 degrees C, pH 6.0-8.0 and 100 rpm, respectively. Tryptone was the best of the eight nitrogen sources tested for toxin production by S. choleraesuis. Among the nine carbon sources tested, S. choleraesuis produced a higher amount of cytotoxin in media containing glucose, fructose, galactose, sorbitol or mannitol as the carbon source. No toxin was detected in broths containing 4.0% or more sodium chloride in Tryptic soy broth (TSB). Cultures of S. choleraesuis in the medium containing 2.0% tryptone, 0.5% NaCl, 0.25% K2HPO4 and 0.25% of the best carbon source under the optimal conditions for 14 h resulted in the highest cytotoxin production. The Vero cell CD50 of S. choleraesuis lysate of cells grown under these optimal conditions was a titer of 589-758 per mg of lysate protein.

The regulatory effect of fermentable sugar levels on the production of leukotoxin by Actinobacillus actinomycetemcomitans

The relationship between sugar availability and RTX (repeats in toxin) cytotoxin (leukotoxin) production in the periodontopathic bacterium, Actinobacillus actinomycetemcomitans, was investigated using a chemostat. A actinomycetemcomitans 301-b produced significant amounts of leukotoxin in anaerobic fructose-limited chemostat cultures at a dilution rate of 0.15 h-1 and at pH 7.0. When the growth limitation was relieved by pulsing the cultures with 50 or 150 mM fructose (final concentrations), leukotoxin production immediately stopped and the amount of cellular leukotoxin decreased until the culture was returned to fructose-limited conditions. Leukotoxin synthesis was also repressed in the chemostat cultures by pulsing with glucose but not with the non-fermentable sugar analog, alpha-methyl-D-glucoside. Leukotoxin production was also repressed by fructose in chemostat cultures of ATCC 33384, which is generally recognized as a non-leukotoxin-producing or minimally leukotoxic strain.

Catabolite repression of the adhesion of Vero cytotoxin-producing Escherichia coli of serogroups O157 and O111

The virulence traits that mediate Vero cytotoxin-producing E. coli (VTEC) adherence are unclear. Many VTEC strains possess the eaeA gene which is involved in the attaching and effacing effects of enteropathogenic E. coli (EPEC). Most eae-positive VTEC adhered to HEp-2 cells in a localized manner; however some strains did not adhere. Thus we investigated the adhesion of poorly adherent strains, especially those of serogroups O111 and O157. To establish a model, the adherence to HEp-2, INT407 and Caco-2 cells of 12 O157 VTEC and six O111 VTEC isolated from cases of human infection were studied after growth of the bacteria under different conditions. For adhesion tests mannose is usually added during prior broth culture of the bacteria, and during the period of attachment, so that any adhesion due to mannose-sensitive type 1 pili is inhibited. Bacteria cultured in peptone water in the absence of mannose adhered to all three lines; there were localized clusters of bacteria on 1%-82% cells, whether mannose was present during the attachment period or not. Bacteria grown in the presence of D-mannose, or any other sugar that was metabolized, showed little adherence (range 0-9%). alpha-Methyl-glucoside also caused marked inhibition of adhesion. It was concluded that inhibition of adhesion was due to catabolite repression.

Nutritional requirements for synthesis of heat-stable enterotoxin by Yersinia enterocolitica

A defined medium that supported the growth of and synthesis of heat-stable enterotoxin (YST) by clinical isolates of Yersinia enterocolitica at levels equivalent to those observed in a complex Trypticase soy broth-0.6% yeast extract medium was developed. The defined medium contained four amino acids (L-methionine, L-glutamic acid, glycine, and L-histidine), inorganic salts, N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) buffer, and potassium gluconate as the carbon source. Methionine was required for growth by most strains of Y. enterocolitica used in this study; thus, it was not possible to determine whether it was also required for the synthesis of YST. The other 17 amino acids commonly found in proteins did not stimulate the synthesis of YST when added to the defined medium. The yield of YST observed with other carbon sources fermented by Y. enterocolitica ranged from 4- to 26-fold lower than that obtained with potassium gluconate. The divalent cations Ca2+ and Mn2+ had no effect on the synthesis of YST; however, concentrations of Fe2+ above 10 microM inhibited the synthesis of the enterotoxin. The addition of a mixture of pyrimidines containing thymine, cytosine, and uracil, each at a concentration of 2.0 mM, stimulated the synthesis of YST by 10 to 15%, whereas a mixture of adenine and guanine, each at a similar concentration, inhibited the synthesis of YST. Vitamins had no effect on the amounts of YST produced by Y. enterocolitica strains grown in the defined medium.(ABSTRACT TRUNCATED AT 250 WORDS)

Construction of a bifunctional Escherichia coli heat-stable enterotoxin (STb)-alkaline phosphatase fusion protein

A fusion between the genes encoding the Escherichia coli STb heat-stable enterotoxin (estB) and alkaline phosphatase (phoA) was constructed, and the expressed protein product was characterized. The STb-alkaline phosphatase protein (STb-PhoA) had an apparent molecular mass of 50,000 daltons and was detected with both monoclonal anti-alkaline phosphatase and polyclonal anti-STb antibodies. Expression of the gene fusion resulted in high-level production of alkaline phosphatase activity, indicating that STb-PhoA was processed and exported into the periplasm of the E. coli host strain. Amino acid sequence analysis of the hybrid protein yielded the sequence Ser-Thr-Gln-Ser-Asn-Lys-Lys, indicating that STb-PhoA was processed during export in a fashion identical to that of native STb (Y. M. Kupersztoch, K. Tachias, C. R. Moomaw, L. A. Dreyfus, R. G. Urban, C. Slaughter, and S. Whipp, J. Bacteriol. 172: 2427-2432, 1990). STb-PhoA was purified from an expressed bacterial lysate by preparative isoelectric focusing. In a rat ligated intestinal loop model, purified STb-PhoA induced highly significant (P less than 0.002) fluid secretion. In addition, the specific activity of STb-PhoA was nearly identical to that of purified STb. Thus, the STb-PhoA hybrid protein represents a readily obtainable source of biologically active (STb) enterotoxin that may prove useful in studies to determine the mode of toxin action.

Cloning, sequencing, and expression in Ficoll-generated minicells of an Escherichia coli heat-stable enterotoxin gene

The gene encoding a heat-stable enterotoxin of Escherichia coli was cloned as a 960-bp fragment from a plasmid isolated from a Mexican strain of human origin. Deoxyribonucleotide sequencing unveiled a 216-bp open reading frame similar to that of a previously sequenced ST-toxin gene. The gene is preceded by a proposed binding site for the cAMP-mediated positive regulator (CAP) that is part of a 23-bp inverted repeat. The proposed CAP site is followed by a 6A, 1T, and 6A deoxyribonucleotides. Minicells containing the toxin gene, which were isolated from Ficoll gradients, shown to preserve the localization of intracellular and periplasmic enzymes, allowed the detection of a biosynthetically radiolabeled polypeptide with an apparent Mr 8400. The data suggest that the enterotoxin genes estA2, estA3, and estA4 are very similar, even in clinical strains isolated from distinct geographical locations; that the transcription of heat-stable enterotoxin genes is controlled by the cAMP-mediated positive regulatory system, and that the heat-stable enterotoxins are initially synthesized as 72 amino acid precursors to yield the extracellular active 18-19 amino acid polypeptides.

Suppression of heat-stable enterotoxin production by Yersinia spp. in milk

One of 16 raw milk isolates of Yersinia enterocolitica and Y. intermedia produced heat-stable enterotoxin (ST) in milk at 25 degrees C but not at 4 degrees C after 21 days of incubation. A catabolite repression of ST synthesis by the lactose-fermenting strain of Y. enterocolitica was observed when 4.6% lactose was added to trypticase soy broth. However, the lactose-fermenting strain was killed by acid produced by lactose fermentation in milk and did not produce ST in milk with the pH adjusted to neutrality. This study suggested that lactose and fat in milk are not the fundamental inhibitors of ST synthesis by Y. enterocolitica and that repression of ST synthesis may be related to other components.

Effect of heat-stable enterotoxin of Escherichia coli and theophylline on ion transport in porcine small intestine

The effect of a heat-stable enterotoxin of Escherichia coli was compared with that of theophylline on ion transport in the pig jejunum, using both in vivo and in vitro techniques. The maximal electrical response to heat-stable enterotoxin was only one-half that of theophylline even though the magnitude of the net secretory response was similar. A net, active secretion of HCO3 was partially responsible for the secretory response induced by heat-stable enterotoxin, whereas theophylline induced an active secretion of chlorine which could account for the entire secretory response. Heat-stable enterotoxin elevated tissue cyclic guanosine monophosphate levels, whereas theophylline elevated both cyclic adenosine monophosphate and cyclic guanosine monophosphate. Cyclic guanosine monophosphate levels induced by heat-stable enterotoxin were markedly potentiated by theophylline. Results suggest that HCO3 secretion in the pig jejunum may be controlled by the cyclic guanosine monophosphate system and this system also activates a neutral secretory process which at high heat-stable enterotoxin doses accounts for the bulk of the net secretion observed. Conversely, the chlorine secretion elicited by theophylline is entirely electrogenic and is consistent with results obtained in other species.

Cultural methods for the production of heat-stable enterotoxin by porcine strains of Escherichia coli and its detection by the infant mouse test

Casamino acids-yeast extract medium (CAY) and a tryptone-yeast extract medium (TY-1) were evaluated in testing for production of heat-stable enterotoxin (ST) by porcine strains of Escherichia coli using the infant mouse assay. More strains were ST-positive when grown in CAY medium than in TY-1 medium. Questionably or indeterminately ST positive strains were investigated in detail to determine whether or not they were weak ST producers. Growth in four different media and in different batches of CAY medium, inactivation of culture supernatant fluids at a lower temperature, addition of mitomycin C to growing cultures and preparation of periplasm-cytoplasm fractions of bacteria by sonication, all failed to yield ST positive samples. ST value limits (i.e. ratios of intestinal weight to remaining body weight of challenged mice), which clearly differentiated positive or negative strains for ST production, were set for CAY medium. A minimal salts-amino acids medium (MSA) was devised. Both in shake flask and fermenter cultures MSA medium gave better ST yields than CAY and a previously described defined medium.

Catabolite repression of Escherichia coli heat-stable enterotoxin activity

The Escherichia coli heat-stable enterotoxin (ST) coded for by plasmid pYK007 (Apr ST+) showed a dependence for cyclic adenosine 3',5'-monophosphate (cAMP) to express ST activity in an adenyl cyclase (cya) deletion mutant; no ST activity was detected in the presence of cAMP in a cAMP receptor protein (crp) deletion mutant or in a double deletion mutant (delta cya delta crp). The cya-crp effect on ST activity could not be accounted for by a modification of the copy number of plasmid deoxyribonucleic acid per chromosome equivalent or by an alteration in the secretion of an active intracellular enterotoxin.

Enterotoxins from gram-negative bacteria relevant for veterinary medicine

The chemistry, mechanism of action, assay methods, pharmacology, and prevention and treatment of diarrhoea due to toxins of gram-negative microbes are discussed. Other virulence factors are mentioned briefly. Special emphasis is placed on non-specific treatment by oral rehydration.

Escherichia coli heat-stable enterotoxin: rapid method of purification and some characteristics of the toxin

In this study we used Escherichia coli strain F11(P155) of porcine origin. The heat-stable enterotoxin (ST) was produced with a batch fermentor under agitation (500 rpm) and forced aeration (5 liters/min) in Casamino Acids yeast extract medium containing 0.2% glucose. The pH varied from 7.2 to 7.8. The maximum amount of ST was obtained after 7 h of growth. ST was purified by ammonium sulfate precipitation, ultrafiltration on Amicon membranes, and chromatography in Bio-Gel P-4. The enterotoxin, which was purified approximately 1,000 times, was active at nanogram levels. On 20% polyacrylamide gel electrophoresis, ST exhibited an Rf of 0.6 ST was recovered from the gel slices by eluting in buffer and testing the activity in suckling mice, since no band appeared on the gel after staining with Coomasie brilliant blue R, Schiff reagent, or red oil. ST was resistant at pH 2 to 10 and at 100 degrees C for 15 min, but it was inactivated at 121 degrees C; it did not lose biological activity after treatment with pronase, lipase, or amylase. In suckling mice antiserum obtained from rabbits or goats immunized with ST neutralized the enterotoxin activity of a cell-free supernatant of purified ST.

Heat-stable enterotoxin produced by Yersinia enterocolitica isolated from patients

Twenty-three strains of Yersinia enterocolitica were isolated from children with gastrointestinal illness and examined for the production of enterotoxins by using both suckling mouse and Chinese hamster ovary (CHO) cell assay systems. Six strains were found to be enterotoxigenic in the suckling mouse assay, but all strains were negative in the CHO cell assay. Enterotoxin was detected in the culture supernatant when organisms were grown at 25 C but not at 37 C. Enterotoxin in a 15-fold concentrated culture supernatant was precipitated by adding absolute ethanol to a concentration of 90%. However, after being dialyzed against distilled water in Spectra/por 6 membrane tubing, it was soluble in 80% acetone. One unit dose of partially purified enterotoxin was 5.0 micrograms of protein/mouse in the suckling mouse assay. The molecular weight of enterotoxin was between 10,000 and 50,000 daltons as determined by ultrafiltration. It was stable to heat (121 C X 20 min or 100 C X 60 min). These observations indicated that Y. enterocolitica isolated in Japan also produce an enterotoxin similar to the heat-stable enterotoxin of Escherichia coli. However its physiochemical properties seem to be different from those of E. coli.

Factors affecting expression of the Escherichia coli pilus K99

An enzyme-linked antibody centrifuge assay for the detection of Escherichia coli pilus K99 was developed and shown to be a specific and quantitative assay for the detection of cell-bound K99. The data presented demonstrate the usefulness of the assay as a diagnostic tool. Using the assay, several factors that affect expression of K99 were investigated. Expression of K99 was dependent upon the degree of aeration provided: nonaerated bacteria produced little or no K99, whereas aerated bacteria produced large amounts of K99. K99 also appeared to be produced only by logarithmically growing cells, whereas there was a demonstrable decline in the amount of K99 per cell during stationary phase. Glucose was shown to repress K99 expression. At 0.5% glucose, K99 expression was highly repressed. Glucose-mediated repression could be overcome by the addition of cyclic adenosine 3',5'-monophosphate. Several other carbon sources also inhibited K99 expression, including pyruvate, arabinose, and lactose; glycerol was stimulatory.

Production of heat-stable, methanol-soluble enterotoxin by Yersinia enterocolitica

Seven isolates of Yersinia enterocolitica serotype 0:8, recovered during an outbreak of gastrointestinal illness, were examined for enterotoxin production. All seven strains were enterotoxigenic in the suckling mouse model, and three of five isolates tested produced keratoconjunctivitis in the guinea pig eye model (Sereny test). Enterotoxin was detected in broth supernatant fluid after 12 h of incubation at 25 degrees C. The toxin was not inactivated by exposure to 121 degrees C for 30 min or by storage at 4 or -40 degrees C for at least 5 months. The enterotoxin was also acid stable and methanol soluble. Methanol extraction did not affect its heat stability or enterotoxic activity in suckling mice. These physical characteristics plus limited ultrafiltration studies suggest that the enterotoxin is a low-molecular-weight substance. Y. enterocolitica enterotoxin resembles Escherichia coli heat-stable enterotoxin in heat and pH stability, methanol solubility, and enterotoxic activity in suckling mice. However, its role in the pathogenesis of human diarrhea has not been established.

Evidence for two heat-stable enterotoxins produced by enterotoxigenic Escherichia coli

Heat-stable enterotoxins from a bovine and porcine strain of Escherichia coli were isolated and showed significant differences in amino acid composition and heat stability.

Nutritional requirements for synthesis of heat-labile enterotoxin by enterotoxigenic strains of Escherichia coli

Optimal growth conditions have been established for production of heat-labile enterotoxin (LT) by both porcine and human strains of enterotoxigenic (ENT(+)) Escherichia coli. There were no unusual growth factor requirements, and some strains produced fairly high levels of LT in a basal salts medium containing 0.5% glucose if the pH was carefully controlled. Several amino acids markedly stimulated LT synthesis when added to the basal salts-glucose medium. Methionine and lysine were the most stimulatory for both human and porcine strains. Either aspartic acid or glutamic acid further enhanced LT synthesis in the presence of methionine and lysine, with aspartic acid being more stimulatory for porcine strains and glutamic acid more stimulatory for human strains. There were no apparent vitamin requirements and no unusual cations needed for toxin synthesis except that Fe(3+) was slightly stimulatory for porcine strains. The stimulation by Fe(3+) was observed only in the presence of the three amino acids, suggesting that the effect was indirect rather than on toxin synthesis. The carbon source also influenced the yield of LT. Glucose supported maximal synthesis, but other carbon sources which exhibit a high degree of catabolite repression also supported high levels of synthesis. Little or no LT was released below pH 7.0; therefore, because the pH drops during growth from 7.5 to 6.8, even in highly buffered media, it was necessary to adjust the pH to 8.0 to effect complete release of cell-associated toxin. The defined medium containing three amino acids reduced the amount of UV-absorbing material in culture supernatants about fivefold and increased LT activity for various strains from two- to fivefold over a complex Casamino Acids-yeast extract medium. Conditions found to be optimal for synthesis of LT were inhibitory for the heat-stable enterotoxin.

Production and partial purification of Salmonella enterotoxin

By using a strain of Salmonella typhimurium, we detected the presence of an enterotoxin, as determined by the rabbit ileal loop assay, in various complex and defined media. The enterotoxin was concentrated by ultrafiltration of culture supernatant fluids and eluted in and adjacent to the void volume of a Sephadex G-100 column. This suggested that the enterotoxic factor was of a relatively high molecular weight, and additional evidence indicated it was heterogeneous in size. Further chromatography, using a diethylaminoethyl-cellulose anion exchanger, facilitated at least a 50-fold purification of the Salmonella enterotoxin.

Heat-stable enterotoxin from Escherichia coli: factors involved in growth and toxin production

Six enterotoxigenic strains of Escherichia coli produced variable levels of heat-stable enterotoxin (ST) when grown under pH control at 8.5 in a simple synthetic medium containing neither amino acids nor vitamins. Bacterial growth and ST production were at levels as high as or higher than those observed in complex media. ST elaboration was detectable in the early logarithmic phase of growth and appeared to be related to disappearance of glucose in the growth medium. The results of this study did not suggest pH-dependent release of ST. Imposition of pH control in complex media resulted in increased growth rates, earlier detectable ST synthesis, and elevated levels of ST. In synthetic medium, attainment of the stationary growth phase was followed by a significant decrease in culture density and a concomitant increase in ST. Cellular autolysis experiments revealed that as much as 20% of the total ST activity was present in a cell-associated form.

Purification and chemical characterization of the heat-stable enterotoxin produced by porcine strains of enterotoxigenic Escherichia coli

Heat-stable enterotoxin (ST) produced by porcine strains of enterotoxigenic (ENT+) Escherichia coli has been purified to apparent homogeneity by sequential ultrafiltration, acetone fractionation, preparative gel electrophoresis, diethylaminoethyl Bio-Gel A ion-exchange chromatography, and Bio-Gel P-10 gel filtration. The enterotoxin, purified more than 1,500-fold, exhibited a molecular weight of 4,400, as determined by both sodium dodecyl sulfate-gel electrophoresis and gel filtration. A molecular weight of 5,100, representing 47 residues, was calculated from amino acid analysis data. The amino acid content was distinctive, with an unusually high proportion of cystines and few hydrophobic amino acids. A single amino-terminal residue, glycine, was observed. Purified ST was stable to heating (100 degrees C, 30 min) and did not lose biological activity after treatment with Pronase, trypsin, proteinase K, deoxyribonuclease, ribonuclease, and phospholipase C. Periodic acid oxidation and several organic solvents (acetone, phenol, chloroform, and methanol) had no effect on the biological activity of ST. Further, purified ST was stable to acid treatment at pH 1.0 but lost biological activity at pH values greater than 9.0. Neither lipopolysaccharide nor lipid contamination was evident in purified preparations. A characteristic absorption spectrum was observed during the course of the purification, which shifted from a maximum at 260 nm in crude preparations to 270 nm for the purified toxin. Antiserum obtained from rabbits immunized with ST or ST coupled to bovine serum albumin neutralized the action of the enterotoxin in suckling mice; however, passive hemagglutination and hemolysis titer assays suggested that ST is a poor antigen.