Zoonotic and Animal Pathogens Research Laboratory, Medical Microbiology, Teviot Place, University of Edinburgh, Edinburgh EH8 9AG, UK1
Author for correspondence: David Gally. Tel: +44 131 651 1342. Fax: +44 131 650 6531. e-mail: d.gally{at}ed.ac.uk
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ABSTRACT |
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Keywords: Escherichia coli O157, VTEC, type 1 fimbriae, phase variation, adherence
Abbreviations: A/E, attaching and effacing; IHF, integration host factor; VTEC, verotoxigenic E. coli
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INTRODUCTION |
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In humans, the serious sequelae of haemorrhagic colitis and haemolytic uraemic syndrome stem from the activity of the shiga-like toxins, and include gastrointestinal tract colonization and damage involving intimate attachment and attaching and effacing (A/E) lesion formation (Frankel et al., 1998 ; Kaper, 1998
). Intimin is the key candidate initiating bacterial binding (DeVinney et al., 1999
; Kenny, 1999
) in the human host, possibly at the Peyers patches of the small intestine (Phillips & Frankel, 2000
).
There is a great deal of interest in understanding the successful dissemination of VTEC O157 among beef and dairy cattle and the reasons why it does not cause colitis and diarrheoa in this host. A logical starting point for such investigations is the carriage and expression of established bacterial adhesins, although to date there is no clear evidence that E. coli O157 colonizes the bovine gastrointestinal tract by binding to the epithelium. E. coli O157 does not carry genes for many of the established adhesins that could play a role in colonizing the bovine host, such as K99 and F41 (our unpublished data), but it does carry the genes for type 1 fimbriae. Type 1 fimbriae are the most common adhesin produced by E. coli and mediate adherence to mannose-containing glycoproteins found on the surfaces of many eukaryotic cells. Type 1 fimbriae have been shown to play an important role in the colonization of E. coli in the human urinary tract (Connell et al., 1996 ; Donnenberg & Welch, 1996
; Mulvey et al., 1998
) and, from their prevalence, it is assumed that they are important in the faecaloral cycle, for binding within the gastointestinal tract and/or for transfer between hosts (Orndorff & Bloch, 1990
). For cattle, it has been established that type 1 fimbriae bind well to both immature and mature (keratinized) rumen epithelium in vitro (Galfi et al., 1998
).
While the majority of VTEC O157 strains do not appear to be able to express type 1 fimbriae, other VTEC serotypes, notably O26 and O118, can express this adhesin (Enami et al., 1999 ). Initial adherence by factors such as type 1 fimbriae may make subsequent intimate adherence and A/E lesion formation more likely to occur (Mack et al., 1999
). Expression of type 1 fimbriae is phase variable, being dependent on an invertible DNA element (fim switch) that contains the promoter for fimA, the gene for the main fimbrial subunit (Abraham et al., 1985
; Klemm, 1984
; Olsen & Klemm, 1994
). In the off orientation, type 1 fimbriae cannot be produced. Inversion of the fim switch requires the activity of two recombinases, FimB and FimE, as well as numerous co-factors (Gally et al., 1996
; Klemm, 1986
; McClain et al., 1991
).
The research presented follows on from the findings of Li et al. (1997) , who identified a 16 bp deletion in E. coli O157 within the regulatory switch region that controls expression of type 1 fimbriae. They designed a primer that bridged this deletion as an effective probe for E. coli O157 strains. We have confirmed the presence of this deletion in 63 human and cattle E. coli O157 strains and have demonstrated that the phase switch controlling type 1 fimbrial expression is detectable only in the off orientation in VTEC O157. Repair of the 16 bp deletion restores phase-variable expression of fimA.
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METHODS |
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Switch-orientation assay.
Primers fim5' and fim3' (Table 1) were used to amplify a 604 bp region that incorporates the invertible fimA promoter element. The resulting PCR product was digested with HinfI, and the products separated using a 6% polyacrylamide Tris/borate EDTA gel run at 100 V for 2 h. The products were visualized after staining with ethidium bromide using Flowgen gel-analysis software and hardware.
Cloning of fim switch regions into pDG19 and allelic exchange.
The fim switch region from the strains was amplified using primers C1 and C2 (Table 1). Primer C1 incorporates a SphI site and C2 incorporates a BamHI site at its 5'-terminus, allowing the amplified fragment to be ligated into pDG19 (Table 1
). Allelic exchange of the cloned switch region into BGEC144 (Table 1
) was performed as described previously (Blomfield et al., 1991
; Leathart & Gally, 1998
).
Repair of the O157 fim switch.
The 16 bp of K-12 sequence absent in E. coli O157 ZAP1 was introduced into the plasmid carrying the ZAP1 fim switch region (pAJR1; Table 1) by using specific primers. Two primers were designed (rep1 and rep2; Table 1
), each of which had an 8 bp 5'-terminus that complemented the K-12 region absent from the sequence of ZAP1. In conjunction with primers C1 and C2 (Table 1
), amplification using Pwo polymerase (Roche) produced two blunt-ended products of 317 bp and 424 bp. These products were cleaned using a Qiagen PCR clean-up kit and ligated using standard conditions; 1 µl of this ligation reaction was then used as a template for PCR using primers C1 and C2 (Table 1
) to produce a fim switch region that was identical to that of ZAP1 (apart from the repair of the 16 bp deletion).
Switching-frequency assay.
Strains were plated on minimal glucose X-Gal agar at 37 °C. Using a sterile cork-borer, six off (white) colonies were excised from the plate, diluted in PBS and plated on minimal glucose X-Gal agar at 42 °C (3001000 c.f.u.). Plates were counted for white and blue colonies, and the frequency of off-to-on inversion was calculated as described previously (Gally et al., 1993 ).
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RESULTS |
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Genotypic and phenotypic analyses of type 1 fimbrial expression
To confirm that E. coli O157 does not produce type 1 fimbriae whereas E. coli O26 can, a simple slide agglutination test was performed on one human E. coli O157 outbreak strain and three E. coli O26 strains (two human and one bovine). The O157 strain was negative for agglutination, whereas the O26 strains varied in their ability to agglutinate, but all three were positive (Fig. 2, bottom panel).
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Cloning and analysis of fim switch regions
After differences in the phase state of the fim switch between VTEC isolates had been established, the contribution of the fim switch region alone to this behaviour was determined. To do this, isogenic fimAlacZYA strains containing the three O26 switch regions and one O157 switch region were created, allowing quantification of switching frequencies. This approach uses allelic exchange to replace the fim switch of E. coli K-12 MG1655 with the relevant VTEC switch region adjacent to a fimAlacZYA transcriptional fusion (Leathart & Gally, 1998 ; see Methods). This allows switching frequencies to be measured with the same recombinase and host factors present. SR1 resulted from the exchange of the E. coli O157 switch region into BGEC144 (fimB+E-). The frequency of FimB-promoted off-to-on switching in E. coli K-12 containing its wild-type switch was measured as 3x10-3 per cell per generation (Fig. 3
). However, SR1 gave no detectable switching (<1x10-5 per cell per generation; Fig. 3
). SR2-4 resulted from exchange of the three E. coli O26 regions into BGEC144 and all three strains had frequencies similar to that of E. coli K-12 (2x10-3 to 3·5x10-3; Fig. 3
). These data show that the E. coli O157 switch region is unable to invert in a permissive background, proving that sequence changes in this switch region prevent off-to-on inversion of the switch to allow expression of fimA. Interestingly, while there are clear differences in the degree of expression of type 1 fimbriae (and switch orientation) between the three O26 strains (Fig. 2
), these differences were not transferred with the respective switch regions and cannot be attributed to sequence variation in these regions.
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DISCUSSION |
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The four switch regions were sequenced and the 16 bp deletion was identified as the major difference between the O26 and O157 strains. This deletion was repaired in the E. coli K-12 fusion background and this repaired strain was then able to phase-vary fimA expression at frequencies equivalent to both E. coli K-12 and the O26 switch replacement strains. While other changes within the fim region of E. coli O157 may prevent type 1 fimbrial expression, this single deletion within the regulatory region is sufficient to nullify expression and is the likely reason why E. coli O157 cannot produce type 1 fimbriae. The reason why the 16 bp deletion prevents phase variation is unclear. The regulation of type 1 fimbrial phase variation has been studied in detail, with most binding sites for required co-factors being identified within the fim switch (Blomfield et al., 1997 ; Gally et al., 1994
, 1996
; Olsen et al., 1998
). The 16 bp deletion lies at the edge of a known integration host factor (IHF)-binding site (Fig. 4
) but does not involve the majority of bases considered important for IHF binding (Blomfield et al., 1997
). However, it is possible that the lack of fimA expression is caused by a failure of IHF to bind and bend the DNA at this site. Alternatively, the required DNA conformation for inversion may be strict and may be affected by the 16 bp deletion. A consistent distance of 314±1 bp between the 9 bp inverted repeats was observed in urinary tract E. coli isolates (Leathart & Gally, 1998
). For recombination the inverted repeats need to: (1) be brought together by DNA bending proteins (IHF and Lrp) and (2) be in phase to allow appropriate recombinase binding and activity. The 16 bp deletion could interfere with either or both of these requirements. In the course of repairing the 16 bp deletion, a 3 bp deletion was created at the same site (Fig. 4
); the strain containing this deletion was able to phase vary but at a 10-fold reduced rate compared to the full repair and E. coli K-12. This 3 bp deletion is not within the IHF-binding site and supports the premise that the 16 bp deletion prevents recombination by interfering with strict spatial requirements.
Clear differences were observed in the degree of expression of type 1 fimbriae (and switch orientation) between the three O26 strains tested. However, all three imported switch regions had similar off-to-on switching frequencies. Potentially, other genetic factors in the relevant host strain are responsible for the differences in expression levels. This could include differences in recombinase expression or activity. However, it is possible that, as shown for urinary tract isolates, regulators of other adhesins or virulence determinants may cross-talk with type 1 fimbriae to alter expression levels (Xia et al., 2000 ). Expression of type III secretion organelles and intimate attachment may not be compatible with the presence of fimbrial adhesions on the bacterial surface. The potential for coordinate expression between type 1 fimbriae and type III secretion in enterohaemorrhagic E. coli (EHEC) serotypes such as O26 remains to be investigated. Certainly, O157 EHEC infections do not seem to need type 1 fimbriae but, as with rabbit diarrhoeagenic E. coli (RDEC), O26 strains may be more likely to form A/E lesions once the bacterium is brought into close proximity to the host-cell epithelium by a fimbrial adhesin. Our preliminary data have demonstated that the level of type 1 expression does correlate with ability of E. coli to bind to bovine colonic epithelium. Therefore, expression of type 1 fimbriae by E. coli O26/O118 could enhance colonization in the bovine colon as well as that in the rumen (Galfi et al., 1998
). Obviously, host factors such as competitive flora and mucus secretion may also modulate this activity. Other factors that could explain VTEC O26/O118 virulence in the bovine host (in comparison to VTEC O157), including the level and activity of secreted proteins, are currently being investigated.
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ACKNOWLEDGEMENTS |
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Received 9 August 2000;
revised 20 September 2000;
accepted 25 September 2000.