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1 Baylor College of Medicine, Division of Infectious Diseases, Houston, TX, USA
2 University of Texas at Houston Health Science Center, Division of Infectious Diseases, Houston, TX, USA
3 University of Maryland School of Medicine, Baltimore, MD 21201, USA
4 University of Texas at Houston School of Public Health, Houston, TX, USA
5 St Luke's Episcopal Hospital, Houston, TX, USA
Correspondence
Pablo C. Okhuysen
Pablo.C.Okhuysen{at}uth.tmc.edu
Received 10 January 2007
Accepted 12 June 2007
Abbreviations: EAEC, enteroaggregative Escherichia coli; ETEC, enterotoxigenic E. coli.
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INTRODUCTION |
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 TOP INTRODUCTION METHODSRESULTSDISCUSSIONREFERENCES |
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The pathogenesis of EAEC diarrhoea is now being defined and is related to host- and pathogen-specific properties. A major obstacle in identifying the EAEC pathogenic mechanisms lies in the heterogeneity of gene profiles of EAEC strains. Although many putative virulence genes have been identified for EAEC, confusion regarding the role of EAEC in acute diarrhoeal illness derives from the lack of clearly implicated virulence factors and mechanisms. One study has suggested that infection with EAEC that contains virulence factors is associated with production of increased levels of faecal cytokines (Jiang et al., 2003). It is unknown whether other faecal inflammatory markers such as faecal leukocytes are associated with virulent strains of EAEC.
The prevailing model of the pathogenesis of EAEC infection includes adherence to the terminal ileum and colon by aggregative adherence fimbriae, production of cytotoxins such as the plasmid-encoded toxin (encoded by pet), E. coli heat-stable enterotoxin 1 (EAST1) and Shigella enterotoxin 1 (encoded by set1A), and probably a package of genes, including genes on a chromosomal island, that are regulated by AggR. The primary objective of this study was to determine the distribution of putative virulence genes in EAEC isolates identified from travellers with and without diarrhoea. An association of the studied genes with resultant inflammatory stool characteristics including leukocytes and gross mucus was also sought. We hypothesized that EAEC carrying certain virulence genes would be more likely to cause acute diarrhoea in travellers than EAEC not carrying these genes. Additionally, we hypothesized that stools from travellers with diarrhoea caused by EAEC carrying certain genes would be more likely to contain inflammatory markers than EAEC isolates not carrying these genes.
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METHODS |
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TOPINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Study population and collection of EAEC isolates. Stools samples were collected during 1999–2004 from travellers from industrialized countries with (n=49) and without (n=15) diarrhoea during short-term stays in Mexico (47 isolates), India (10 isolates) and Guatemala (seven isolates) (Jiang et al., 2002a). EAEC isolates from travellers with and without diarrhoea in Mexico were from stool samples from the same population. EAEC isolates from travellers with diarrhoea in India and Guatemala were from stool samples from a cohort similar to travellers to Mexico. Each EAEC isolate came from a different traveller, and only EAEC isolates identified as the sole pathogen in the stool sample of travellers were included in this study (Jiang et al., 2002a, b). Acute diarrhoea was defined as the passage of three or more unformed stools within a 24 h period plus one or more signs or symptoms of enteric infection (e.g. nausea, vomiting, abdominal pain or cramping, excessive gas, faecal urgency, bloody or mucus stool, or tenesmus) (DuPont & Ericsson, 1993). Seventeen E. coli isolates from travellers without diarrhoea in Mexico that did not demonstrate HEp-2 adherence and that did not carry genes for enterotoxigenic E. coli (ETEC) heat-labile toxin and/or heat-stable toxin production were used in the present study as non-diarrhoeagenic E. coli controls (Jiang et al., 2002a, b). Travellers who received an antibacterial agent with activity against enteric pathogens (e.g. fluoroquinolone, macrolide, azalide or trimethoprim-sulfamethoxazole) or who received anti-diarrhoeal medication (e.g. loperamide, bismuth subsalicylate or kaopectate) within the week prior to enrolment were excluded.
Microbiological studies. The presence of conventional pathogens was assessed prospectively in each case using published methods: stools were studied for the presence of Shigella spp., Salmonella spp., Vibrio spp., Campylobacter jejuni, Yersinia enterocolitica, Aeromonas spp. and Plesiomonas shigelloides (Jiang et al., 2002b). Entamoeba histolytica, Cryptosporidium spp. and Giardia lamblia were identified by enzyme immunoassays (Jiang et al., 2002b). Five lactose-positive colonies were retrieved from MacConkey agar plates from each stool sample and inoculated into individual peptone stabs that were transported to Houston, TX, for identification of known E. coli pathotypes. E. coli colonies negative for heat-labile and heat-stable enterotoxins of ETEC by oligonucleotide probes (Murray et al., 1987) were examined further for the aggregative phenotype diagnostic of EAEC using the HEp-2 cell adherence assay (Donnenberg & Nataro, 1995). E. coli isolates that did not demonstrate the aggregative phenotype and did not carry genes for ETEC heat-labile toxin and/or heat-stable toxin production were used as non-diarrhoeagenic E. coli controls. Enteropathogenic E. coli, enterohemorrhagic (or Shiga toxin-producing) E. coli and enteroinvasive E. coli were not screened for, based on the low prevalence of these pathogens among travellers with diarrhoea to the same regions in earlier studies (Vargas et al., 1998).
Identification of EAEC. E. coli in each stool sample were tested for the presence of EAEC on the basis of a characteristic pattern of adherence to cultured HEp-2 cells (Donnenberg & Nataro, 1995). Briefly, a chamber slide (Dynatek) was seeded with HEp-2 cells (ATCC) grown at 37 °C in 5 % CO2 in minimal essential medium (Gibco) supplemented with 10 % fetal calf serum. E. coli isolates were grown overnight at 37 °C without shaking in trypticase soy broth (BBL Microbiology Systems) with 1 % D-mannose. Bacterial culture (25 µl) was added to each chamber and incubated at 37 °C for 3 h. The chamber slide was washed five times with PBS, fixed with 100 % methanol, and stained with crystal violet (Difco). A positive-control EAEC strain (042) isolated in Peru from a child with diarrhoea and a negative-control non-adherent E. coli strain (HS) from a healthy subject were included in each assay. Each E. coli strain was tested twice in a blind fashion. A sample was interpreted as positive for EAEC if it showed the characteristic ‘stacked-brick’ aggregative appearance described by Donnenberg & Nataro (1995).
EAEC isolates found in the presence of conventional pathogens, as described above, were excluded from analysis.
Molecular studies.
DNA from E. coli isolates was isolated from an individual colony suspended in 25 µl PCR H2O subjected to 95 °C for 5 min. The oligonucleotide primers for the 11 genes assessed in this study are listed in Table 1
. Positive and negative controls for each gene were used in each PCR assay. Each PCR assay was performed in a final reaction volume of 10 µl containing 10 mM Tris/HCl (pH 8.3), 50 mM KCl, 2 mM MgCl2, 100 µM each of dATP, dCTP, dGTP and dTTP, 2.5 U AmpliTaq polymerase (Perkin-Elmer), 10 pmol each primer and 2 µl boiled bacterial cell lysate as the DNA template. The sequences of primers, the sizes of the amplicons and the reaction conditions are listed in Table 1. Amplifications were performed in a DNA thermal cycler (Perkin-Elmer). The amplified PCR products were analysed on a 1 % agarose gel.
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Statistical analysis.
Multiple comparisons of the distribution of genes carried by EAEC isolates and the stool characteristics of travellers among groups were performed using 
2 or Fisher's exact test for categorical data. Data were examined in a two-tailed fashion to estimate the P value.
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RESULTS |
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TOPINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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. The genes aggR, aap, astA and set1A were each identified individually significantly more frequently among the EAEC strains derived from the diarrhoea group compared with the non-diarrhoea group (P<0.05). Table 3 shows that many of the EAEC isolates in the diarrhoea group had two or three of these genes compared with the absence of these genes from the non-diarrhoea group (P<0.05). EAEC isolates negative for all four genes were identified more frequently from the non-diarrhoea group (P=0.002). EAEC strains with the combination aggR and set1A, with or without additional factors, were particularly strongly associated with diarrhoea; these genes were found in 18/49 travellers (37 %) with diarrhoea compared with 1/32 travellers (3 %) without diarrhoea (P<0.001).
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. Travellers with diarrhoea who shed EAEC isolates carrying aggR, aap, astA or set1A more commonly passed stools with gross mucus (57 %) and faecal leukocytes (40 %) compared with EAEC isolates negative for these genes (14 and 7 %, respectively) (P<0.05).
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DISCUSSION |
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TOPINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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The genes aggR, aap, astA and set1A have been described previously. aggR is a shared transcriptional activator for both aggA (AAF/I fimbrial subunit) and aafA (AAF/II fimbrial subunit). Other EAEC genes encoding possible virulence factors have also been shown to be under aggR control, such as aap, aat (CVD432), a chromosomal island of the pheU locus, and several enterotoxins (Dudley et al., 2006), and these genes may travel as a package of virulence genes (Jenkins et al., 2005). In our study, 27 % of EAEC isolates from travellers with diarrhoea had the dispersin gene (aap). Dispersin is a secreted low-molecular-mass protein (10.2 kDa) that coats the bacterial surface and promotes dispersal of EAEC on the intestinal mucosa. aap lies immediately upstream of aggR in EAEC strain 042 (a prototype EAEC isolate) (Grover et al., 2001; Sheikh et al., 2002). There is remarkable conservation of the aap-aggR locus among EAEC strains, as aap and aggR are separated by less than 1 kb (Czeczulin et al., 1999). In our study, 12/14 aap-positive strains (86 %) were positive for aggR. These findings are consistent with the findings of other studies, which show that EAEC isolates carrying aggR and aap may be phylogenetically or pathogenetically linked (Cerna et al., 2003; Lopez-Saucedo et al., 2003). astA encodes a heat-stable enterotoxin (EAST1). EAST1 is a 4.1 kDa protein originally discovered in EAEC but now known to be found in other diarrhoeagenic E. coli (Menard & Dubreuil, 2002; Vila et al., 1998; Yatsuyanagi et al., 2003; Zamboni et al., 2004; Zhou et al., 2002). This toxin is often compared with E. coli STa enterotoxin from ETEC, as they share physical and mechanistic similarities. set1A encodes Shigella enterotoxin 1. Similar to astA, set1A is also found in other E. coli pathotypes (Fasano et al., 1995).
Although this study did not test for gene products, the results identified genes of EAEC that appeared to predict virulent strains. EAEC isolates carrying two or three of the genes examined were frequently identified from travellers with diarrhoea and not from travellers without diarrhoea. We were particularly interested in the combination of aggR, a plasmid-borne master virulence regulator, and set1A, which is encoded on a chromosomal island partially under aggR control. Interestingly, Shigella enterotoxin 1, encoded by the set1A gene, is an oligomeric toxin that was first identified in Shigella flexneri 2a strains (Fasano et al., 1995). EAEC strains carrying specific genes under aggR control may be important alone or in combination with other virulence factors and, when present, may indicate pathogenic strains of EAEC. Recent data have shown that the AggR regulon controls both plasmid-borne virulence factors and chromosomal genes associated with clinical illness (Dudley et al., 2006).
Among travellers with diarrhoea, EAEC isolates carrying the genes of interest (aggR, aap, astA and set1A) were more likely to be associated with faecal mucus and faecal leukocytes than those from travellers with diarrhoea who shed EAEC isolates not carrying these genes. These findings are consistent with other studies, which suggest that EAEC can cause inflammatory enteritis (Bouckenooghe et al., 2000; Greenberg et al., 2002; Harrington et al., 2006; Wanke, 1995). These studies found that travellers who developed EAEC diarrhoea had higher stool levels of inflammatory markers, including interleukin (IL)-8, lactoferrin and leukocytes, than healthy volunteers with no identified enteric pathogen in their stool (Bouckenooghe et al., 2000; Greenberg et al., 2002; Huang et al., 2004b). IL-8 is thought to play an important role in EAEC pathogenesis. This chemokine recruits neutrophils to the intestinal epithelial mucosa, causing epithelial destruction and fluid secretion (Madara et al., 1993; Sansonetti et al., 1999). An intestinal faecal IL-8 response to EAEC infection may explain our findings of faecal leukocytes in many of these patients.
In summary, a limited number of genes (aggR, aap, astA and set1A) appeared to be commonly associated with EAEC diarrhoea. The identification of these genes, both qualitatively and quantitatively, along with characterization of stool inflammatory markers from travellers, may permit earlier diagnosis of this infection and improve the epidemiological understanding of this emerging enteric pathogen.
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ACKNOWLEDGEMENTS |
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REFERENCES |
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TOPINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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