Strain RDEC-1 Research Articles

Effect of oligomannoside-type glycopeptides in diarrheal disease of rabbits induced by Escherichia coli strain RDEC-1.

We examined the effects of a soluble receptor-like complex carbohydrate (an oligomannoside-type glycopeptide) on adherence of a type 1 fimbriated rabbit enteropathogen, Escherichia coli strain RDEC-1. Oligomannoside-type glycopeptide, but not a non-glycosylated peptide mixture used as control, inhibited adherence of type 1 fimbriated RDEC-1 to both rabbit ileal brush borders and guinea pig erythrocytes in vitro. In contrast, during RDEC-1 infection of rabbits the receptor-like oligomannoside did not affect the presence and severity of diarrhea, fecal shedding of organisms, luminal colonization by RDEC-1, or enteroadherence of organisms.

Antigen absorption in bacterial diarrhea: in vivo intestinal transport of beta-lactoglobulin in rabbits infected with the entero-adherent Escherichia coli strain RDEC-1.

We studied the absorption of both antigenic and degraded beta-lactoglobulin (beta-LG) from the ileum to the portal blood, in rabbits infected at weaning with the entero-adherent Escherichia coli strain RDEC-1. The infection was characterized by high bacterial excretion from days 7 to 18 postinfection (pi), acute diarrhea for 10 days, and considerable growth retardation. Intestinal absorption of beta-LG was measured at four stages of the infection: early (day 3 pi), peak (day 10 pi), late (day 18 pi), and recovery (day 30 pi). During the 30-day period of infection, age-matched control rabbits exhibited a significant decrease in antigenic and degraded beta-LG absorption. In both control and infected animals, more than 90% of the beta-LG was absorbed by a degrading pathway and the remainder in antigenic form by a minor pathway. RDEC-1 infection significantly raised antigenic beta-LG absorption from days 10 to 30 pi, which delayed the decrease that normally occurs with age. Degraded beta-LG absorption was not modified by the infection, except for a slight increase observed at the recovery stage (day 30 pi). These results suggest that RDEC-1 diarrhea increases absorption of food antigens. The subsequent local or systemic immune responses are not known, but pathologic consequences are possible in susceptible individuals.

INCREASED ANTICEN ABSOBPTIN INDUCED BY INFECTION WITH THE ADHERENT ENTEROPATHOCENIC E.COLI STRAIN RDEC-1 IN RABBIT ILEUH AND PEYER'S PATCHES

Intestinal absorption of antigenic (intact) and degraded β-lacto-globulin (β-lg) and horseradish perioxidase (HRP) was studied in rabbits experimentally infected at weaning with the rabbit-specific Escherichia coli strain RDEC-1 (015: NM). Traneptithelial fluxes of both proteins were measured at four stages post infection: early, peak, late and recovery, on segments of ileum and Peyer's patches mounted in Ussing chambers. During this period of observation, absorption of antigenic β-lg and intact HRP decreased significantly in the ileum and Peyer's patches of age-matched controls probably reflecting gut closure to the food antigen. No significant age-related decrease was observed in the controls for the transport of these proteins in degraded form. During the early phase of RDEC-1 infection, absorption of egraded protein fluxes rose across Peyer's patches only. During the peak phase of infection, absorption of antigenic β-lg (74.0 ng/h.cm2) and intact HRP (21.8 ng/h cm2) rose fivefold compared to controls (15.8 and 4.4 ng/h.cm2 respectively). These rises, which were also found in Peyer's patches, delayed the physiologic decrease in protein absorption which occurs with age. These results indicate that increased antigen absorption is observed during RDEC-1 bacterial diarrhea, and that it might interfere with the immune responses of the host.

Production of Diarrhea in the Rabbit by a Mutant of Escherichia coli (RDEC-1) That Does Not Express Adherence (AF/R1) Pili

Escherichia coli (RDEC-1) adheres to Peyer's patch and absorptive epithelium in the rabbit in the closely adhering manner characteristic of enteropathogenic and enterohemorrhagic E. coli. An adherence pilus (AF/R1) is important for adherence to Peyer's patch M cells in vivo and ileal brush borders in vitro. A nonpiliated mutant (42-2-37-8) of the RDEC-1 strain colonized the gut lumen less readily than the parent strain. The mutant adhered infrequently to Peyer's patch lymphoid follicle epithelium (22% vs. 84%, P less than .0001). Ileal close adherence was less frequent at 3 d, but by 9 d after inoculation had increased, approaching that of the parent RDEC-1 strain. However, adherence was focal, and fewer bacteria were present at each adherence site compared with the parent RDEC-1 strain. The result was a lower frequency of diarrhea (34% vs. 65%, P = .003) and mortality (9.4% vs. 27%, P = .035) with the 42-2-37-8 strain. Loss of AF/R1 pili compromised the ability of the RDEC-1 strain to adhere to Peyer's patch and absorptive epithelium and to produce diarrhea.

Electrolyte transport in rabbit cecum. I. Effect of RDEC-1 infection

To investigate the characteristics of intestinal ion and fluid secretion induced by the adherent, effacing enteropathogenic Escherichia coli strain RDEC-1, we infected weanling rabbits with 10(7)-10(8) RDEC-1 organisms and then studied cecal ion transport under short-circuit conditions in Ussing chambers. Results in tissues with confluent adherent organisms were compared with those in uninfected ceca and in ceca stimulated with dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP). The short-circuited cecum normally absorbed Na and Cl, secreted bicarbonate (as represented by the residual ion flux), and displayed a high rate of nondiffusional Na and Cl transport. RDEC-1 infection did not alter the short-circuit current (Isc), but it increased the conductance (Gt), decreased the potential difference (PD), abolished net Na absorption, and reversed Cl absorption to secretion. The changes in Na and Cl net fluxes may be explained by inhibition of a Na-Cl linked absorptive process. In contrast, DBcAMP significantly increased the Isc, PD, and Gt, decreased net Na flux, and abolished net Cl absorption by stimulating electrogenic Cl secretion. These results suggest that RDEC-1-induced changes in cecal ion transport are not mediated by cAMP. The reduction in Na-Cl linked absorption is consistent with anatomic changes in the apical surfaces of absorptive epithelial characteristic of effacing enteroadherence, whereas the increased conductance is consistent with tight junction disruption seen with RDEC-1 infection.

Hydrophobic properties of the rabbit enteric pathogen Escherichia Coli strain RDEC-1

Escherichia coli strain RDEC-1 (serotype 0.15: H-) is a rabbit pathogen which attaches to the intestinal mucosa in a manner that is morphologically identical to the binding of enteropathogenic E. coli in the intestine of human infants. The organisms express mannose-resistant bacterial lectins previously defined as attachment factor, rabbit-1 (AF/R1) pili. We examined the effect of AF/R1 pilus expression on the hydrophobicity of RDEC-1. Hydrophobicity was determined by four complementary methods: the salt aggregation test, hydrophobic interaction chromatography, adherence to polystyrene, and contact angle with water measurements. The organisms were more hydrophobic when AF/R1 pili were expressed compared with bacteria grown to suppress pilus expression. Binding of bacteria to polystyrene surfaces was dependent on AF/R1 pilus expression. Attachment of piliated RDEC-1 to more hydrophilic surfaces such as sulphonated polystyrene was minimal. Adherence of piliated E. coli to polystyrene was reduced by 80% when propanol was added to reduce the surface tension of the suspending buffer. Conversely, in the presence of propanol adherence of piliated organisms to sulphonated polystyrene increased 12-fold. The data indicate that AF/R1 pilus expression increased the cell surface hydrophobicity of E. coli RDEC-1. In addition, expression of the hydrophobic adhesins mediated bacterial attachment to hydrophobic surfaces.

Role of type 1 somatic pili (fimbriae) in mucosal attachment of the enteroadherent Escherichia coli, strain RDEC-1, in rabbits.

We have previously shown that attachment of the rabbit enteroadherent Escherichia coli, strain RDEC-1, to ileal brush borders in vitro is mediated by both mannose-resistant, AF/R1 pili and mannose-sensitive, type 1 pili. Because the role of type 1, somatic pili as adhesins that mediate bacterial enteroadherence in vivo remains controversial, we examined adherence of RDEC-1 expressing either type 1 pili or AF/R1 pili to rabbit ileum in ligated loops and after oral infection of rabbits. A rabbit fecal commensal E. coli, 640, which also expressed type 1 pili was used as a control. After oral infection of rabbits we evaluated: (a) diarrhea, (b) fecal shedding of organisms, (c) luminal colonization of jejunum and ileum, and (d) mucosal adherence of bacteria to jejunum, ileum, and colon. RDEC-1 expressing either type 1 or AF/R1 pili adhered to enterocytes both in ileal ligated loops and in the distal ileum, cecum, and proximal colon of infected rabbits. In contrast, enteroadherence of 640 was not observed. Diarrhea developed in rabbits challenged with either type 1 or AF/R1 piliated RDEC-1, but not in rabbits fed 640. Seven days after infection of rabbits with RDEC-1 bearing type 1 pili, luminal colonization of jejunum (4.22 +/- 0.55 CFU/g, X +/- SE) and ileum (6.34 +/- 0.55) was the same as after infection with AF/R1 piliated RDEC-1 (jejunum 4.47 +/- 0.20, ileum 5.81 +/- 0.70) and significantly greater than luminal colonization by type 1 piliated 640 (jejunum less than 2.22 +/- 0.14, ileum less than 2.06 +/- 0.15).(ABSTRACT TRUNCATED AT 250 WORDS)

Pilus-mediated interactions of the Escherichia coli strain RDEC-1 with mucosal glycoproteins in the small intestine of rabbits

Escherichia coli strain RDEC-1 (serotype 015:NM) is an effacing adherent enteropathogen that binds to the intestine of rabbits in a manner morphologically identical to the binding of human enteropathogenic E. coli strains to human intestine. The rabbit enteropathogen adheres to mucosal enterocytes in vivo and to microvillus membranes in vitro. Binding of RDEC-1 to ileal brush borders and to M cells overlying Peyer's patches is mediated by pili (fimbriae) expressed on the cell surface of bacteria. To examine whether similar binding occurs to glycoproteins present in the intestinal lumen, RDEC-1 was fed to rabbits and the intestinal luminal contents were examined for in vivo colonization by RDEC-1. In addition, preparations of rabbit luminal glycoproteins were tested for their ability to agglutinate RDEC-1 in vitro. After the infection of rabbits, RDEC-1 organisms were found in the intestinal lumen associated with glycoproteins, as defined by positive histochemical staining of luminal material by periodic acid-Schiff and Alcian blue. In vitro aggregation of RDEC-1 by luminal glycoproteins of rabbit intestine, a luminal glycoprotein fraction purified on column chromatography and rabbit ileal microvillus membranes, was determined using a microtiter plate assay and an aggregometer. Nonpiliated RDEC-1 did not interact with either of the intestinal glycoprotein preparations or microvillus membranes. RDEC-1 expressing mannose-resistant AF/R1 pili or mannose-sensitive type 1 pili coaggregated with both rabbit luminal glycoprotein preparations and rabbit ileal microvillus membranes at equivalent protein concentrations. These studies demonstrate that RDEC-1 are associated with luminal glycoproteins during in vivo infection of rabbits, and that piliated RDEC-1, but not nonpiliated, coaggregate with rabbit glycoprotein preparations in vitro. Luminal glycoproteins contained within the mucus layer may serve as a site for replication and colonization of bacteria before bacterial enteroadherence.

Regional differences in attachment of enteroadherent Escherichia coli strain RDEC-1 to rabbit intestine: luminal colonization but lack of mucosal adherence in jejunal self-filling blind loops.

RDEC-1 (serotype 015:NM) is an enteropathogenic Escherichia coli that adheres to mucosal epithelial cells in the ileum, cecum, and colon, but not in the jejunum, of rabbits during diarrheal illness. As RDEC-1 can adhere to jejunal brush borders in vitro, rapid distal clearing of the enteropathogen from the upper gastrointestinal tract may explain the absence of in vivo adherence of RDEC-1 to rabbit jejunum. To test this hypothesis, self-filling blind loops (SFBL) were created in the jejunum of rabbits that were then inoculated with RDEC-1 to determine if stasis would promote increased luminal colonization and increased mucosal attachment of RDEC-1 in the proximal small intestine. Eight rabbits with SFBL and seven nonoperated littermates were inoculated with RDEC-1 4 weeks after surgery. Semiquantitative estimation of fecal shedding of RDEC-1 and the presence and severity of diarrhea were monitored for 7 days. Rabbits were then killed, and colonization of jejunum and ileum by anaerobic bacteria and luminal colonization of jejunum, ileum, and cecum by RDEC-1 was quantitated. Mucosal adherence of RDEC-1 to jejunum, ileum, and cecum was determined by Giemsa staining of intestinal sections obtained from rabbits with and without SFBL. In addition, brush border membranes (BBM) were prepared from blind loop jejunum, nonoperated jejunum, and ileum. In vitro adherence of RDEC-1 to brush borders was quantitated under phase contrast microscopy. Maximal fecal shedding of RDEC-1 began 3-4 days after infection in all rabbits, but diarrhea was more severe in rabbits with SFBL.(ABSTRACT TRUNCATED AT 250 WORDS)

Colonization, Virulence, and Mucosal Interaction of an Enteropathogenic Escherichia coli (Strain RDEC-1) Expressing Shigella Somatic Antigen in the Rabbit Intestine

The group factor 3,4 somatic antigen was transferred by recombination from a donor strain of Shigella flexneri type 2a to the Escherichia coli O15 rabbit pathogen strain RDEC-1. A hybrid clone (7482-1-1) that expressed only the S. flexneri 3,4 somatic antigen and a second clone (7482-1-7) from the same mating mixture that expressed only the O15 somatic antigen were compared for virulence in rabbits. The 7482-1-1 strain produced diarrhea in 10% of rabbits versus 84% for the 7482-1-7 strain (P less than .001). In fluorescent antibody-stained, frozen, sectioned tissues, fewer 7482-1-1 bacteria adhered to ileum (P = .013) and cecum (P = .044). Light and electron microscopic studies demonstrated that the recombinant strain, which adhered to membranous (M) cells to the same degree as the 7482-1-7 strain, was found beneath the epithelium, where its presence resulted in a marked acute inflammatory response. Both the 7482-1-1 and 7482-1-7 strains exhibited characteristic close adherence to M cells and absorptive ileal and cecal mucosa.

Functional heterogeneity of intestinal Escherichia coli strains expressing type 1 somatic pili (fimbriae): assessment of bacterial adherence to intestinal membranes and surface hydrophobicity.

Although the role of host-specific, nonmannose-sensitive pilus adhesins in the intestinal adherence of pathogenic Escherichia coli is well established, a similar role for mannose-sensitive type 1 or common pili is less clear, since these structures can be expressed by most E. coli, even nonpathogens. We first examined whether type 1 pili, expressed by the rabbit-effacing, adherent, enteropathogenic E. coli strain RDEC-1, mediated interactions with intestinal membranes of several species and compared these interactions with those mediated by the nonmannose-sensitive adhesin of RDEC-1. We next grew a series of E. coli intestinal strains in static broth to promote type 1 pilus expression and determined whether E. coli expressing type 1 pili differed in their affinity for intestinal membranes (as measured by phase-contrast microscopy and aggregometry), hydrophobic surface properties, net negative surface charge (as measured by hydrophobic interaction chromatography and salt aggregation), and hemagglutination patterns. In contrast to the species-specific attachment to rabbit brush borders of RDEC-1 expressing its nonmannose-sensitive adhesin, type 1 pili on RDEC-1 mediated mannose-sensitive attachment to intestinal membranes of all four species tested. Expression of type 1 pili on other E. coli strains resulted in varying degrees of nonspecies-specific, mannose-sensitive attachment to intestinal membranes. This attachment correlated with increasing surface hydrophobicity rather than with hemagglutination patterns. These results indicate that various E. coli strains expressing type 1 pili are functionally heterogeneous and suggest that some type 1 pili might contribute to in vivo enteroadherence.

Rabbit mucosal receptors for an enteropathogenic Escherichia coli strain: Appearance of bacterial receptor activity at weaning

Adherence studies using the rabbit enteropathogenic Escherichia coli strain RDEC-1 indicate that this strain attaches in a species-specific manner to receptors located on its host intestinal epithelial cells. Because intestinal epithelial cells undergo marked developmental changes within the first few weeks of life, we designed a study to determine whether the presence of bacterial receptors on rabbit brush borders changed during this time. The adherence of RDEC-1 to rabbit brush borders isolated from rabbits aged 2–35 days and from adult rabbits was examined. No adherence of RDEC-1 was detected to rabbit brush borders isolated from rabbits 15 days or younger. Receptors for RDEC-1 were first detected on rabbit brush borders from 21-day-old rabbits and by 35 days, RDEC-1 receptor activity on rabbit brush borders had reached adult levels. Piliated, enteroadherent, human Escherichia coli pathogens did not adhere to rabbit brush borders of any age group indicating that nonspecific adherence did not occur.

Attachment of human and pig (K88) enterotoxigenic Escherichia coli strains to either human or porcine small intestinal cells.

Enterotoxigenic Escherichia coli strains pathogenic for humans and enterotoxigenic E. coli strains pathogenic for pigs producing the K88 pili antigen both bound to isolated small intestinal cells from either humans or pigs. Neither the K99 enterotoxigenic E. coli (from lambs and calves) nor the rabbit pathogenic strain RDEC-1 bound to human or pig small intestinal cells under the same conditions.

Peyer's patch lymphoid follicle epithelial adherence of a rabbit enteropathogenic Escherichia coli (strain RDEC-1). Role of plasmid-mediated pili in initial adherence.

Escherichia coli (strain RDEC-1) adheres to M cells of rabbit Peyer's patch lymphoid follicle epithelium. The RDEC-1 strain contains an 85 X 10(6) D plasmid that codes for pili, which, when purified, adhere to gut absorptive epithelium. This study compared the in vivo lymphoid follicle adherence of the RDEC-1 strain with that of a Shigella flexneri (ShD15) that contained the 85 X 10(6) D plasmid and expressed the RDEC-1 pili, a control E. coli, and a control S. flexneri (ShD12). The bacteria were given in a dose of 10(10) to 0.7-1.1 kg rabbits. The rabbits were sacrificed at 2, 4, 6, and 12 h postinoculation. Peyer's patch tissue was examined by electronmicroscopy and direct fluorescence microscopy. The piliated ShD15 and RDEC-1 bacteria adhered in large numbers at 2 and 4 h postinoculation, but only the RDEC-1 strain persisted and increased in numbers past that time. Control strains did not adhere. The ShD15 strain adhered to and was rapidly taken into M cells, precipitating an acute inflammatory reaction within the follicle and adjacent lumen. Initial lymphoid follicle M cell adherence of the ShD15 strain was associated with the possession of the adherence pilus plasmid. The failure of the ShD15 strain to survive and colonize the lymph follicle epithelium contrasts with the success of the RDEC-1 strain and indicates that the RDEC-1 strain possesses virulence factors in addition to pili.

Genetic transfer of a mucosal adherence factor (R1) from an enteropathogenic Escherichia coli strain into a Shigella flexneri strain and the phenotypic suppression of this adherence factor.

Escherichia coli strain RDEC-1 avidly adheres to rabbit ileal brush borders. Two separate experiments were designed to determine whether pili promote this adherence. (1) Adherence of strain RDEC-1 was phenotypically suppressed by changing the culture medium. Loss of adherence was correlated with the absence of pili. Thus, growth of strain RDEC-1 in Penassay broth (Difco Laboratories, Detroit, Mich.) promoted both adherence and expression of pili on greater than or equal to 90% of organisms, whereas growth in brain-heart infusion medium suppressed adherence and reduced the percentage of piliated organisms to less than or equal to 13%. (2) The adherence ability of strain RDEC-1 was genetically transferred to previously nonadherent and nonpiliated Shigella flexneri. The Shigella exconjugants that inherited the adherence ability were uniformly piliated, while all nonadherent Shigella exconjugants were nonpiliated. Finally, the pili on both RDEC-1 and the Shigella exconjugant strains were shown to be distinct from type 1 pili. Therefore, unique pili confer upon strain RDEC-1 the ability to adhere to rabbit intestinal brush borders.

Specific adherence of Escherichia coli (strain RDEC-1) to membranous (M) cells of the Peyer's patch in Escherichia coli diarrhea in the rabbit.

The RDEC-1 strain Escherichia coli is an enteroadherent bacterium that produces diarrhea in the rabbit. A histopathologically similar disease has been described in humans. The RDEC-1 bacterium adheres to the epithelium of lymphoid follicles in rabbit ileal Peyer's patches by 4 h postinoculation, 3-4 d before its adherence to absorptive epithelium. The purpose of this study was to determine whether the RDEC-1 bacterium adheres to a specific cell type in the lymphoid follicle epithelium. RDEC-1 bacteria were given in a dose of 2 X 10(6) by the orogastric route to postweanling rabbits. The distal ileal Peyer's patch, taken from 5 control rabbits and 43 rabbits at intervals in the first 24 h postinoculation, was examined by routine and high-voltage electron microscopy. The RDEC-1 bacterium adhered specifically to M (membranous) rather than absorptive epithelial cells of the lymphoid follicle epithelium. Further understanding of how the bacterium attaches to M cells, which transport antigens to intraepithelial lymphocytes, could be useful in designing vaccines to protect mucosal surfaces.

Diarrhea due to Escherichia coli strain RDEC-1 in the rabbit: the peyer's patch as the initial site of attachment and colonization.

A strain of Escherichia coli (RDEC-1) has been described that in rabbits colonizes the intestine; adheres to mucosal epithelial cells of the ileum, cecum, and colon; and causes diarrhea by an unknown mechanism. This study attempted to determine the location of the bacteria in the rabbit intestine during the unexplained six- to seven-day interval between bacterial inoculation and onset of diarrhea. Specimens of ileum, cecum, colon, ileal Peyer's patches, sacculus rotundus, and appendix from control rabbits and from rabbits killed at intervals after inoculation with RDEC-1 bacteria were examined by light and direct fluorescence microscopy. Bacteria in large numbers attach to the tips of the Peyer's patch lymphoid follicles by 24 hr, but they did not attached to ileal, cecal, or colonic mucosa until three days after inoculation. The lag time between inoculation and onset of diarrhea was probably due to the need for the bacteria first to attach to and then colonize the Peyer's patch lymphoid follicles. The intestinal mucosa was probably colonized by bacteria shed from the Peyer's patches.

Attachment of bacteria to intestinal epithelial cells in diarrhea caused by Escherichia coli strain RDEC-1 in the rabbit: stages and role of capsule.

RDEC-1 is a strain of Escherichia coli that, in rabbits, attaches to intestinal mucosal epithelial cells bereft of microvillar borders and causes diarrhea by an unknown mechanism. The stages of attachment of RDEC-1 bacteria to mucosal epithelial cells were examined using high-voltage electron microscopy of thick (0.5-micrometers) sections of ileum and cecum of rabbits with diarrhea. The tissues were stained with ruthenium red or antisera to strain RDEC-1 OK antigens. Micrographs, including stereopairs, demonstrated several stages of bacterial attachment. Bacteria were attached to the glycocalyxes of epithelial cell microvilli and to pedestal-like extrusions of the surfaces of epithelial cells lacking microvilli. Structures consistent with bacterial pili were rarely visualized. Attachment to microvilli appeared to be a result of the interaction of polysaccharides of the microvillar glycocalyx and the K antigen of the bacterial capsule.

Characterization of colonization of the rabbit gastrointestinal tract by Escherichia coli RDEC-1

We have reported previously the isolation of an Escherichia coli strain (RDEC-1) that in rabbits is able to colonize the gut, adhere to mucosal epithelial cells of the ileum, cecum, and colon, and cause diarrhea by a novel mechanism. The purpose of the present study was to characterize more fully the colonization of the rabbit gut by strain RDEC-1. Colonization reached a maximum 3 to 4 days post-inoculation with strain RDEC-1 and did not decrease until 15 days post-inoculation. Diarrhea occurred 3 to 4 days after colonization reached its maximum intensity. Semiquantitative rectal swab cultures were found to be correlated with counts of colony-forming units of strain RDEC-1 per gram of ileum, cecum, and colon and their contents and were used to chart the course of colonization of the rabbit gut. The actual number of colony-forming units per gram was dependent on the stage of colonization and ranged from 4.0 X 10(3) to 2.4 X 20(6) in the ileum to 3.1 X 10(5) to 3.6 X 10(7) in the cecum. The number of colony-forming units per gram was not affected by the presence of diarrhea. E. coli RDEC-1 colonizes the ileum, cecum, and colon of rabbits heavily for a relatively long period of time.

Diarrhea Due to Escherichia coli in the Rabbit: A Novel Mechanism

A strain of Escherichia coli O15 (RDEC-1) isolated from several rabbits with diarrhea was examined to determine (1) whether the strain could produce diarrhea when administered by the orogastric route to other rabbits and (2) whether this strain was invasive or enterotoxigenic. Strain RDEC-1 produced diarrhea in 48 of 62 rabbits when given by the orogastric route in doses that ranged from 1.5 X 10(2) to 4 X 10(10) bacteria. The organism did not give a positive result in the 18-hr ileal loop or Serény tests and did not invade HeLa cells. Culture supernatants of strain RDEC-1 did not give positive results in the 6- or 18-hr rabbit ileal loop, suckling mouse, Y-1 adrenal cell, or Chinese hamster ovary cell assays. Fluorescent antibody staining of sections of intestine prepared in a cryostat revealed great numbers of E. coli strain RDEC-1 that adhered to the epithelial surface. It is evident that E. coli can produce diarrhea without being able to invade the mucosa or synthesize enterotoxin. Strains of E. coli similar to RDEC-1 may account for some of the E. coli-associated diarrhea that occurs in humans.