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Infection and Immunity, January 2006, p. 781-785, Vol. 74, No. 1
0019-9567/06/$08.00+0 doi:10.1128/IAI.74.1.781-785.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
Molecular Characterization of the Escherichia coli Asymptomatic Bacteriuria Strain 83972: the Taming of a Pathogen
Per Klemm,1
Viktoria Roos,1
Glen C. Ulett,2
Catharina Svanborg,3 and
Mark A. Schembri2*
Microbial Adhesion Group, Centre for Biomedical Microbiology, BioCentrum-DTU, Technical University of Denmark, DK-2800 Lyngby, Denmark,1
School of Molecular and Microbial Sciences, University of Queensland, Brisbane QLD 4072, Australia,2
Institute of Laboratory Medicine, Section for Microbiology, Immunology and Glycobiology, Lund University, Sölvegatan 23, SE-223 62 Lund, Sweden3
Received 26 September 2005/
Accepted 5 October 2005

ABSTRACT
Escherichia coli 83972 is a clinical asymptomatia bacteriuric
isolate that is able to colonize the human urinary bladder without
inducing an immune response. Here we demonstrate that one of
the mechanisms by which this strain has become attenuated is
through the mutation of its genes encoding type 1 and P fimbriae.

TEXT
Urinary tract infections (UTI) are among the most common infectious
diseases of humans and a major cause of morbidity and mortality.
Acute pyelonephritis and asymptomatic bacteriuria (ABU) represent
the two extremes of UTI. Acute pyelonephritis is a severe systemic
infection caused by uropathogenic
Escherichia coli (UPEC) (
6,
7,
29,
31). ABU, on the other hand, is an asymptomatic carrier
state in which patients may carry >10
5 CFU/ml of a single
E. coli strain for years without provoking a host response.
In early studies, this was explained by a lack of virulence
genes; however, molecular epidemiology has shown that >60%
of ABU strains carry virulence genes but fail to express the
phenotype (
18,
19).
The ability of UPEC to cause symptomatic UTI is enhanced by adhesins, including type 1 and P fimbriae (11, 17). P fimbriae enhance the establishment of bacteriuria and activate the innate immune response in animal models and in the human urinary tract (2, 20, 21, 33, 35, 36). Binding is mediated by the PapG adhesin, which is located at the tips of the fimbriae and which recognizes the
-D-galatopyranosyl-(1-4)-ß-D-galactopyranoside receptor epitope in the globoseries of glycolipids (3, 12, 13). Type 1 fimbriae enhance colonization, induce host responses in the murine UTI model, and promote biofilm formation and invasion (4, 14, 16, 23). Receptor binding is also mediated by an adhesin located at the tips of the fimbriae (FimH) that binds to
-D-mannosylated proteins, such as uroplakins, which are abundant in the bladder (32). In this study, we characterized the type 1- and P-encoding fimbrial genes from the prototypical ABU strain E. coli 83972. The strain is a clinical isolate capable of long-term bladder colonization (1). It was isolated from a patient with ABU who had carried it for 3 years, and it has been used in colonization studies as a prophylactic agent to prevent UTI in humans (2, 8, 30, 34, 35).
E. coli 83972 does not express a detectable type 1 fimbrial phenotype when recovered from the urinary tract or after in vitro subculture (35). However, previous genetic analysis of the strain revealed that it contains the genes for type 1 fimbriae (9). To examine the E. coli 83972 type 1 fimbria-encoding genes in more detail, we performed a series of Southern hybridizations with probes spanning different regions within the MG1655 type 1 fimbrial gene cluster. A positive hybridization signal was obtained with a fimH gene probe but not with fimE or fimAIC probes (Fig. 1). Subsequent PCR amplification and sequencing of the fim cluster from E. coli 83972 revealed a 4.25-kb deletion affecting all genes except those encoding the minor components fimF, fimG, and fimH (Fig. 1). The chromosomally located fimH gene was expressed as a functional product, since the transformation of E. coli 83972 with plasmid pPKL114 (containing fimBEAICDFG) induced a mannose-sensitive agglutination of yeast cells (Table 1). Sequencing of the fimH gene from E. coli 83972 revealed the following changes relative to FimH from E. coli K-12: V48A, G87S, N91S, and S99N.
E. coli 83972 reportedly contains
pap gene sequences (
9) but
has never been shown to express P fimbriae. However, when we
grew 83972 on a solid medium and examined the cells by transmission
electron microscopy, we observed that the majority produced
fimbriae (Fig.
2A). Interestingly, very few cells produced fimbriae
when grown as liquid cultures. Purification and sodium dodecyl
sulfate-polyacrylamide gel electrophoresis analysis of the fimbriae
revealed a prominent 18-kDa protein that was confirmed by N-terminal
amino acid sequencing to be the PapA major subunit (Fig.
2B).
E. coli 83972 cells expressing these P fimbriae did not hemagglutinate
human red blood cells (RBCs), bind to human uroepithelial cells,
or bind to Gal

1-4Galß-containing glycolipids (data
not shown). Thus, strain 83972 produces P fimbriae that are
unable to bind to any known receptor targets.
The
pap gene cluster of
E. coli 83972 was amplified by PCR and
sequenced. A comparison of the amino acid sequence deduced from
each gene with the equivalent genes from UPEC CFT073 revealed
that the greatest divergence occurred in
papA,
papE, and
papG (Fig.
2E). The function of the 83972 PapG adhesin was assessed
by complementation with the following plasmids: (i) pDD3 (all
pap genes from UPEC J96 except
papG) and (ii) pDD4 (
papG from
UPEC J96). Only
E. coli 83972 (pDD4) cells readily agglutinated
RBCs and bound to human uroepithelial cells (Fig.
3A and B;
Table
1). Thus, the recognition of receptor targets by the P
fimbriae of
E. coli 83972 can be complemented in
trans by a
plasmid carrying a functional
papG gene. Western blot analysis
of fimbrial proteins using PapG-specific polyclonal antisera
revealed that PapG is expressed and suggests that it is incorporated
into the fimbrial structure (Fig.
2C). This interpretation is
supported by the fact that
E. coli 83972 produces fimbriae of
normal length and morphology (previous studies of fimbrial biogenesis
have demonstrated that disruption of the adhesin-encoding gene
results in the synthesis of organelles of aberrant length and
morphology) (
24). It is possible that the lack of function of
PapG may be associated with minor amino acid changes in the
protein sequence. In this respect, all but one of the residues
predicted to contribute to the PapG binding site (
5) are conserved
in the
E. coli 83972 PapG sequence (Fig.
4). By analogy with
the FimH adhesin (
22,
25), variations that alter the conformational
stability of the protein loops that carry the receptor-interacting
residues may also account for its lack of function.
E. coli 83972 was carried by a young girl for 3 years without
any symptoms. Whether the strain had already lost the ability
to express P and type 1 fimbriae previously during passage through
another host or did so in this particular girl is unclear. However,
several lines of evidence support the notion that the ancestor
of
E. coli 83972 was a pyelonephritic UPEC strain: (i) the FimH
allele of 83972 contains minor amino acid variations that are
consistent with those of previously characterized pyelonephritis
strains (
26-
28); (ii) the strain is able to express P fimbriae,
albeit an apparently nonadhesive type; (iii) multilocus sequence
typing of 83972 shows that it belongs to the B2 clonal group
(
http://www.mlst.net/) and this group contains
E. coli strains
associated with pyelonephritis and other extraintestinal invasive
clinical syndromes such as bacteremia, prostatitis, and meningitis;
and (iv) the strain possesses the F14 PapA allele, which has
been associated with other virulence factors, including S and
F1C fimbriae, hemolysin, and cytotoxic necrotizing factor 1
from
E. coli B2 strains (
10). Genes of nonfunctional products
tend to erode over time through accumulation of mutations, and
there are many instances where genome shrinkage has been associated
with bacterial lifestyle transition (
15). In
E. coli 83972,
the two primary adhesive organelles associated with uropathogenesis
have been inactivated by adaptive mutations as a trade-off with
the host.
The characterization of the fim- and pap-encoding genes in this study illustrates an important issue with regard to the current molecular knowledge of E. coli 83972. Previous studies demonstrating the presence of these genes in E. coli 83972 failed to correlate with its phenotypic characteristics (9, 33). Here we have shown for the first time that E. coli 83972 contains only some of the type 1 fimbrial genes and is not capable of producing these organelles. The finding that fimH is functional and constitutively expressed may explain a previous report that identified a clone capable of mannose-sensitive hemagglutination from a recombinant cosmid library derived from 83972 (9). In the case of P fimbriae, these organelles are expressed, but their function remains unknown since they do not bind to defined receptor targets. This study sheds new light on how E. coli 83972 has adapted to grow in the human bladder. The strain has lost the ability to express functional P and type 1 fimbriae and is thus able to persist in this environment without triggering a host immune response. In contrast to organisms that have acquired genes for pathogenesis, E. coli 83972 is an example of an organism that has adapted to commensalism through gene loss and mutation.

ACKNOWLEDGMENTS
We thank Birthe Jul Jorgensen, Göran Bergsten, Jannick
Jacobsen, and Rick Webb for expert technical assistance.
This work was supported by grants from the Australian National Health and Medical Research Council (401714), the University of Queensland, the Danish Medical Research Council (22-03-0462), and the Danish Research Agency (2052-03-0013).

FOOTNOTES
* Corresponding author. Mailing address: School of Molecular and Microbial Sciences, Building 76, University of Queensland, Brisbane QLD 4072, Australia. Phone: 617 33653306. Fax: 617 33654699. E-mail:
m.schembri{at}uq.edu.au.

Editor: V. J. DiRita

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Infection and Immunity, January 2006, p. 781-785, Vol. 74, No. 1
0019-9567/06/$08.00+0 doi:10.1128/IAI.74.1.781-785.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
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