Download Expression of and Cytokine Activation by Eschevichia coli Curi

602
Expression of and Cytokine Activation by Escherichia coli Curli Fibers
in Human Sepsis
Zhao Bian,1 Annelie Brauner,2 Yinghua Li,2
and Staffan Normark1
1
Microbiology and Tumorbiology Center and 2Department of Clinical
Microbiology, Karolinska Hospital, Stockholm, Sweden
Curli organelles are expressed by commensal Escherichia coli K12 and by Salmonella typhimurium at temperatures !377C, which bind serum proteins and activate the contact-phase
system in vitro. This study demonstrates, by means of an anti-CsgA (curli major subunit)
antibody, that a significant fraction of E. coli isolates (24 of 46) from human blood cultures
produce curli at 377C in vitro. Serum samples from 12 convalescent patients with sepsis, but
not serum from healthy controls, contained antibodies against CsgA (n = 12 ). This study
further demonstrates that a curli-expressing E. coli strain and a noncurliated mutant secreting
soluble CsgA induce significantly (P ! .05 ) higher levels of proinflammatory cytokines (tumor
necrosis factor–a, interleukin [IL]–6, and IL-8) in human macrophages differentiated from
THP-1 cells. These data, therefore, provide direct evidence that curli are expressed in vivo in
human sepsis and suggest a possible role for curli and CsgA in the induction of proinflammatory cytokines during E. coli sepsis.
“Septic shock” is defined as a systemic response to infection
[1], characterized as an ill-regulated host inflammatory reaction.
Mediator systems, such as the complement system [2] and the
contact activation system [3, 4], are thought to play a key role
in sepsis, via generation of small peptides that can cause an
increased vasopermeability and vasodilatation. Attention has
also been directed to the role of proinflammatory cytokines in
septic shock. Relatively small amounts of these cytokines, produced locally in tissue, benefit the host by activating antimicrobial pathways and stimulating tissue repair. However, a
massive, uncontrolled release of cytokines and the resultant
mediator cascade signal the onset of tissue injury and lethal
shock. In this context, regulation of the local inflammatory
response, particularly by the proinflammatory cytokines interleukin (IL)–1b and tumor necrosis factor (TNF)–a [5], in response to bacterial invasion is important.
Although much is known about the role of lipopolysaccharides (LPSs) in septic shock [5, 6], little is known about the
role of other bacterial components. Several clinical studies have
shown that no significant survival advantage results from blockReceived 14 May 1999; revised 24 September 1999; electronically published 7 February 2000.
Financial support: Grants from the Swedish Medical Research Council
(16x-10843), the Swedish Natural Science Research Council (3373-309),
the Göran Gustafssons Foundation of Natural and Medical Science, and
Karolinska Institute Magnus Bergvall Foundation and an unrestricted grant
from Bristol-Myers Squibbs to S.N. Y.L. is also supported as a graduate
student from the Division of Neonatology, Astrid Lindgren Children’s Hospital, Karolinska Institute.
Reprints or correspondence: Dr. Zhao Bian, Microbiology and Tumorbiology Center, Karolinska Institute, Box 280, 171 77 Stockholm, Sweden
([email protected]).
The Journal of Infectious Diseases 2000; 181:602–12
q 2000 by the Infectious Diseases Society of America. All rights reserved.
0022-1899/2000/18102-0025$02.00
age of LPSs in patients with sepsis [7– 9], suggesting that other
components of gram-negative bacteria may play a role in the
induction and pathology of septic shock.
Curli are a novel class of bacterial surface structures, expressed in both Escherichia coli and Salmonella, that are characterized by their ability to bind serum protein fibronectin [10,
11]. Colonies of curli-expressing strains typically stain positive
for Congo red, allowing for an initial rapid screen of curli
production [12, 13]. Considerable work has been done to elucidate the nature and biological role of curli in both E. coli and
Salmonella infection. It is now well appreciated that curli are
assembled, following a unique pathway, by bacterial secretion
of soluble CsgA, the major subunit protein of curli [14], triggered by a surface-located nucleator protein, CsgB, and are
polymerized into curli fibers at the bacterial cell surface [15].
Curli are able to bind different human proteins of the fibrinolytic and coagulation cascades [16, 17] and can also interact
with major histocompatibility complex class I molecules [18].
More recently, it has been reported that curli are capable of
activating the contact-phase system in human plasma in vitro,
allowing an anticoagulation effect and generation of the cellular
mediator bradykinin [19]. However, because expression of curli
in vitro is temperature regulated and curli fibers normally are
not expressed in E. coli K12 and in Salmonella typhimurium at
377C, it could be argued that these bacterial fibers do not play
any role in the infection of warm-blooded animals and humans.
In the present investigation, E. coli isolates from blood cultures of patients with sepsis were analyzed for curli expression
at 377C by use of a specific anti-CsgA antibody. By using immunoblotting assays, we obtained serum samples from patients
with sepsis and from healthy controls and analyzed them for
antibodies directed against CsgA, the curli major subunit protein. To address a possible role of curli in sepsis, we measured
JID 2000;181 (February)
Cytokine-Activating Curli by Septic E. coli
the induction of the proinflammatory cytokines TNF-a, IL-6,
and IL-8 in human macrophages after exposure to a curliexpressing E. coli K12 strain and its corresponding csgA and/
or csgB mutants.
Materials and Methods
Patients. Twelve patients with E. coli bacteremia (6 men and
6 women, with an age range of 22–88 years) were treated at Karolinska Hospital, Stockholm, Sweden. All patients presented with
fever 1397C and had general signs of septicemia. The portal of
entry was the urinary tract in 7 patients, all of whom had positive
urine cultures. Five patients were treated with combinations of
gentamicin, 4 patients with a single therapy of trimethoprim-sulfamethoxazole, 1 patient with cefuroxime, and 1 patient with gentamicin, and 1 patient did not receive any antimicrobial therapy.
Two patients died after ∼1 month, but in neither of these cases
could death be referred to the E. coli septicemia.
Patient sera. Convalescent serum samples were obtained from
12 patients with E. coli bacteremia, on average, 16 days (range,
9–29 days) after admission to the hospital. Serum samples from 12
healthy volunteers were used as controls.
Bacterial strains. Forty-six E. coli strains (no. 1–no. 50) from
blood cultures of septicemic patients, including the 12 patients mentioned earlier, were collected at Karolinska Hospital. These included MC4100, a curli-expressing strain that is a derivative of E.
coli K12 [20], and MHR222, MHR204, and MHR261, isogenic
curli-deficient mutant strains generated by transposon insertional
mutagenesis of the csgA and csgB genes in MC4100 [13]. Bacteria
were grown on colonization factor agent (CFA) agar [21] at 287C
for 48 h or at 377C for 24 h. Media supplemented with Congo red
(40 mg/mL) and Coomassie brilliant blue (20 mg/mL) were used for
curli indicator plates to judge colony morphology and color (figure
1). Scores were given as (1) 2 (negative result); (2) 1, 11, or
111 (Congo red staining positive with light to dark red color);
or (3) 1111 (score of 111 plus an rdar morphotype [22]. Bacterial cells collected from the agar plate were used for immunoblotting, fibronectin-binding, and cytokine induction experiments in
macrophages.
Polymerase chain reaction (PCR).
Oligonucleotide primers
used to amplify a portion of the csgA gene were 50-GGCGGAAATGGTTCAGATGTTG-30 and 50-CGTATTCATAAGCTTCTCCCGA-30. PCR was done in a 100-mL reaction containing 50 pmol
of each primer and 250 mM of each dNTP in Taq reaction buffer
(10 mM Tris-HCl at pH 8.3, 1.5 mM MgCl2, and 50 mM KCl).
Crude chromosomal DNA isolated from E. coli cells was used as
a template. Amplification was done on an automatic thermocycler
(Perkin-Elmer, Santa Clara, CA). Each reaction was electrophoresed through a 3% agarose gel to verify production of the appropriate fragments.
Immunoblot. Bacteria were grown on CFA agar plates at 287C
for 48 h or at 377C for 24 h. Bacterial cells were treated with 90%
formic acid prior to electrophoresis and were resuspended in protein
sample buffer. The constituent proteins were separated on a 15%
SDS polyacrylamide slab gel [23]. Immunoblot experiments were
performed essentially as described elsewhere [24]: The proteins were
transferred onto a nitrocellulose membrane with a pore size of 0.45
603
mm (Immobilon P; Millipore, Sundbyberg, Sweden) at 100 V for
60 min. The membrane was soaked in 5% nonfat milk in Trisbuffered saline (TBS; 10 mM Tris-HCl and 150 mM NaCl at pH
7.5) for at least 30 min. For the analysis of clinical serum samples,
the membrane was cut into 5-mm-wide slices, and each slice was
put into one of the small individual tanks of an AutoBLOT 36
(GeneLabs Diagnostics, Singapore). The membrane slices were incubated with septic sera or nonseptic sera for 60 min at room
temperature with 1 : 400 dilution in TBS containing 0.5% bovine
serum albumin (BSA). Alkaline phosphatase–conjugated goat antihuman IgG (Dakopatts AB, Stockholm) at 1 : 4000 dilution was
used as the second antibody. Colored alkaline phosphatase products on the nitrocellulose sheet were developed at room temperature
with nitroblue tetrazolium/5-bromo-4-chloro-indolyl phosphate in
alkaline phosphatase buffer (0.1 M Tris-HCl, 0.1 M NaCl, and 5
mM MgCl2 at pH 9.5). For the analysis of clinical E. coli isolates,
the membranes were blocked and incubated with a 1 : 4000 dilution
of anti-CsgA antibody ZB-aA; a secondary goat antibody against
rabbit IgG conjugated with horseradish peroxidase was used for
detection, according to the protocol of the manufacturer (Roche,
Bromma, Sweden).
Electron microscopy.
Electron microscopy (EM) was performed with an EM109 electron microscope (Phillips Zeiss, Eindhoven, The Netherlands) at 60–80 kV with copper or nickel grids
(200- or 300-mesh, respectively) coated with thin films of 2% Formvar (Sigma, St. Louis). After a 24-h incubation at 377C, the bacterial colonies were overlaid with PBS (pH 7.4), and cell suspension
was allowed to sediment for 2 min on a grid. After washing with
PBS and then distilled water, the specimen was negatively stained
with 0.2% uranyl acetate and was air-dried before the EM study
was performed. For the immunoelectron microscopy, specimens
were blocked with 1% BSA/PBS and then incubated with ZB-aA
(1 : 200 dilution in 1% BSA/PBS) for 60 min at 377C. After washing
with PBS, the specimen was incubated with gold particle–
labeled protein A (10 nm colloidal gold particles) for 30 min at
377C, rinsed with PBS and then distilled water, and negatively
stained.
Assay of fibronectin binding. Quantitation of nonradiolabeled
fibronectin binding to E. coli bacteria was modified according to
the method described by Flock et al. [25]. Bacteria were grown on
CFA agar plates at 287C for 48 h or at 377C for 24 h. Microtiter
wells (Dynatech Laboratories, Chantilly, VA) were coated with 100
mL of bovine fibronectin (Sigma), ranging from 0.08 to 10 mg/mL,
at 47C overnight and at room temperature for an additional 2 h.
After blocking with 2% BSA in PBST (PBS with 0.05% Tween 20)
for 2 h at room temperature, 200 mL of PBST with 4 3 10 83 cells/
mL bacteria (OD600, 0.94–0.95) was added to the wells and incubated at room temperature for 3 h. The wells were washed, and
the amount of 1003 light absorbance of the adherent bacteria was
determined by use of a microplate reader (A405). Each value was
obtained from the mean 5 SD of 4 independent assays.
The THP-1 cell line and its differentiation into macrophages.
The human monocytic cell line THP-1, originally derived from a
child with acute monocytic leukemia [26], was obtained from the
American Type Culture Collection (Rockville, MD). Cells were
maintained in RPMI 1640 medium (Sigma) containing 10% heatinactivated myoclone (GibcoBRL, Paisley, UK), 2 mM L-glutamine (Sigma), 10 mM HEPES (GibcoBRL), and 50 mM 2-mer-
Figure 1. A, Congo red–staining phenotype of representative sepsis Escherichia coli isolates at 377C in vitro. Bacteria were grown at 377C for 24 h on colonization factor agent (CFA) agar plates
containing Congo red dye. Levels of Congo red staining are indicated as 1 to 1111 (Materials and Methods) on the left, and numbers of representative sepsis E. coli isolates are indicated on the
right of the bacterial colonies. B, Western immunoblots of curli fibers and CsgA protein produced by sepsis E. coli isolates at 377C in vitro. Sepsis E. coli isolates were grown on CFA agar plates at
377C for 24 h, and MC4100 (a), MHR261 (b), and MHR204 (c) were grown at 287C for 48 h. Bacterial cells were treated with 90% formic acid before electrophoresis to depolymerize the curli fibers.
Proteins of bacterial lysate were separated with 15% SDS-PAGE, transferred onto blotting membrane, and analyzed with the CsgA-specific antibody ZB-aA. The locations of slot and molecular
markers (in kDa) are indicated on the right. P, CsgA polymers (curli fibers); D, CsgA dimers; M, CsgA monomers.
JID 2000;181 (February)
Cytokine-Activating Curli by Septic E. coli
captoethanol. The cells were grown at 377C in a humidified 5%
CO2, 95% air atmosphere for 48 h in a cell culture flask (Nunc,
Roskilde, Denmark). The cells were thereafter differentiated with
10 nM phorbol myristic acid (Sigma) at 5 3 10 5 cells/mL for another 48 h in a Teflon-coated bottle. The expression of surface
antigens (CD14, CD45, and HLA-DR) was analyzed with flow
cytometry, showing that 196% of the cells had turned into
macrophages.
Stimulation of macrophages. The differentiated THP-1 cells
were seeded at 106 cells in 1-mL volumes of the complete medium
in 24-well flat-bottom tissue culture plates (Costar, Cambridge,
MA). The cultures were prepared in duplicate and stimulated with
either E. coli MC4100 or the mutants MHR222, MHR204, and
MHR263 at the concentration of 107 bacteria/mL (bacterium-tocell ratio, 10 : 1). Supernatants were removed from the wells at 0
(data not shown), 1 (data not shown), 2, 3, 4, 5, 6, and 7 h (data
not shown) poststimulation.
ELISA for cytokine production. Supernatant samples of cell
cultures were stored at 2207C before the assays. The amounts of
TNF-a, IL-6, and IL-8 in cell culture supernatants were quantified
by use of ELISA (BioSite AB, Stockholm) according to the manufacturer. The detection limits were 25 pg/mL for TNF-a, 6.25 pg/
mL for IL-6, and 62.5 pg/mL for IL-8, respectively.
Statistical analysis. Comparison of apical versus basal cytokine activity was done by use of the paired Student’s t test. Comparison between groups was done by multiple analysis of variance
with the post hoc Tukey-Kramer multiple comparison test. All data
are reported in absolute values, where indicated, 5 SD. Statistical
significance was considered to be achieved at P ! .05.
Results
Pathogenic E. coli isolates from human blood cultures express
curli at 377C in vitro. E. coli K12 expresses curli only when
bacteria are growing at a low temperature [12, 13], making it
doubtful that these highly interactive fibers could play a role
in human disease. To further address this question, we analyzed
46 clinical E. coli isolates from blood cultures of patients with
sepsis by use of a nonradiolabel assay of fibronectin-binding
and an immunoblotting assay with a CsgA-specific antibody,
ZB-aA [15].
Among the 46 E. coli isolates, 31 colonies were stained
with Congo red (see Materials and Methods) when grown at
287C, and 24 colonies stained positive at 377C, ranging from 1
to 1111 (figure 1A). Colonies of E. coli isolates 1, 10, 30,
33, and 36 were stained only when grown at 287C (11 to
111), but not at 377C (2), thereby showing thermoregulation,
as in E. coli K12. Colonies from 2 isolates, 7 and 31, were more
heavily stained with Congo red when grown at 377C than at
287C, indicating that more curli fibers were produced by these
isolates at the higher temperature, which suggests a reversed
thermoregulation for these 2 isolates.
Curliated E. coli typically bind soluble fibronectin [10, 12].
Twenty-two of the 46 E. coli isolates, including 4 Congo
red–binding negative isolates grown at 287C or 377C, were
605
therefore tested for fibronectin binding by use of a nonradiolabeled assay (table 1). The colonies of isolates that stained
differently with Congo red (from 2 to 1111) bound fibronectin, ranging from 5 to 103 (98 for MC4100 and 12 for
MHR204 when grown at 287C), meaning that most of the isolates that stained positive with Congo red also were capable of
binding human soluble fibronectin.
Table 1. Forty-six sepsis Escherichia coli isolates analyzed with immunoblotting (WB [Western blot]), fibronectin (FN) binding, and
Congo red–staining assays (CRS).
E. coli strains
(clinical isolate no.)
1
2
3
4
5
6
7
8
9
10
11
12
13
17
18
19
20
21
22
23
24
25
26
27
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
Lab strain
MC4100
MHR204
377C
287C
a
FN binding
56 5 4
21 5 5
44 5 9
39 5 5
53 5 15
46
59
6
18
13
5
5
5
5
5
5
5
8
3
1
3
2
3
951
13 5 1
25 5 5
63 5 5
28 5 5
65 5 4
29 5 3
73 5 8
103 5 8
65 5 9
46 5 10
98 5 5
12 5 7
CRS/WB
11/1
111/1
2/2
2/2
1/1
1111/1
11/1
5/2
2/2
111/1
1111/1
1111/1
2/2
111/1
111/1
2/2
111/1
5/2
2/2
2/2
1/1
2/2
1/1
11/1
111/1
111/1
111/1
111/1
111/1
2/2
2/2
11/1
11/1
2/2
111/1
1111/1
1111/1
1111/1
1111/1
11/1
1111/1
1111/1
1/1
111/1
1111/1
2/2
111/1
2/2
a
FN binding
44 5 9
11 5 4
38 5 8
43 5 3
73 5 6
19
11
11
38
11
18
5
5
5
5
5
5
4
7
8
12
4
5
14 5 5
1456
21 5 3
33 5 4
17 5 7
46 5 7
46 5 10
51 5 1
37 5 5
34 5 5
48 5 7
12 5 6
11 5 4
CRS/WB
2/2
11/1
2/2
2/2
5/2
1111/1
111/1
5/2
2/2
2/2
1111/1
1111/1
2/2
111/1
1/1
2/2
11/1
2/2
1/1
2/2
5/2
2/2
5/2
1/1
11/1
5/2
1111/1
111/1
2/2
2/2
2/2
2/2
1/1
2/2
11/1
1111/1
1111/1
1111/1
111/1
1/1
1111/1
1111/1
2/2
11/1
1111/1
2/2
2/2
2/2
NOTE. 2, negative for both CRS and WB assays; 1 to 111, CRS positive
with light to dark red color; 1111, CRS with 111 plus an rdar morphotype.
a
Each FN-binding value is the amount of 1003 light absorbance (mean 5
SD).
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Bian et al.
In addition, all 46 E. coli isolates were analyzed by immunoblotting with the specific anticurli subunit CsgA antibody,
ZB-aA (figure 1B). Bacterial cells were treated with 90% formic
acid before electrophoresis to depolymerize curli fibers into
CsgA monomers. Specific signals of CsgA monomers, CsgA
dimers, and CsgA polymers (curli fibers) were detected from
the isolates that also stained positive on Congo red plate and
bound fibronectin at 287C or 377C. From these data, we conclude that a significant fraction (24 of 46) of human E. coli
sepsis isolates are capable of producing curli at 377C in vitro.
We also used PCR to test 4 sepsis isolates unable to produce
curli in vitro at either 377C or 287C for the presence of the csgA
gene. As shown in figure 2, all 4 sepsis strains yielded a PCR
fragment of the same size as in MC4100. No such DNA fragment was amplified in E. coli MHR206 [13], because disruption
(csgD1 : : Tn105) of the csgD gene completely abolishes transcription of the csgBA operon. Hence, likely most, if not all,
of the E. coli sepsis isolates contain the genetic information for
curli expression.
Different expressing levels of curli fibers among sepsis E. coli
isolates. The level of CsgA polypeptide differed among isolates according to the results of the Western blot assay (figure
1B) and, in general, corresponded to the different degree of
Congo red staining and fibronectin binding (table 1). Some of
the sepsis E. coli isolates, including 22 and 32, were examined
by immunoelectron microscopy. As exemplified in figure 3, E.
coli 22 (panel A) expressed considerably fewer curli fibers (1)
at 377C than did E. coli 32 (panel B). Thus, the different levels
of Congo red staining and fibronectin binding seen at 377C
probably reflect different expression levels of curli fibers among
different isolates.
Septic sera, but not sera from healthy controls, contain antibodies against the curli major subunit CsgA protein. As shown
JID 2000;181 (February)
in figure 4, a protein band (lanes 1, MC4100 lysate) corresponding to the major subunit protein CsgA reacted specifically
with anti-CsgA antibody, ZB-aA (lane A1, positive control),
and all the septic sera of convalescence (lanes 171 to 461) but
not with the sera from healthy controls (figure 4II, lanes
1H–12H). Membranes transferred with proteins of MHR204
lysate (lanes 2, lacking CsgA) did not react with ZB-aA (lane
A2, negative control) or the septic sera (figure 4I), indicating
that the septic sera contain antibodies directed against the curli
subunit protein CsgA. It is therefore suggested that E. coli associated with sepsis express curli fibers (polymerized CsgA protein) in vivo.
Curli-expressing and noncurliated, but CsgA-secreting, E. coli
strains induce higher levels of TNF-a, IL-6, and IL-8 than do
CsgA-negative mutants. Figure 5 illustrates the kinetics of
cytokine production in human macrophages differentiated from
monocytic THP-1 cells. The macrophages were stimulated by
infecting them with the curli-expressing wild-type strain
MC4100, the noncurliated but CsgA-secreting strain MHR261,
the csgA mutant MHR204, and the csgBA double mutant
MHR222. All 4 E. coli strains induced a time-dependent increase in TNF-a, IL-6, and IL-8 production in macrophages.
Nonstimulated cells produced very low levels of TNF-a and
IL-8 and an undetectable level of IL-6. At 3 h after infection,
the levels of TNF-a, IL-6, and IL-8 stimulated by MHR222
and MHR204 were significant but were lower than those seen
after MC4100 and MHR261 stimulation. However, at 5 and 6
h after stimulation, the levels of all 3 cytokines were significantly
higher (P ! .05) when cells were induced by MC4100 (except
for TNF-a at 5 h) and MHR261 than when cells were induced
by MHR222 and MHR204, demonstrating a striking difference
depending on the presence or absence of the CsgA protein. The
most dramatic difference between the strains was seen for IL-
Figure 2. Amplification of csgA gene by polymerase chain reaction in sepsis Escherichia coli isolates. A portion (295 bp) of DNA fragment
was amplified from crude chromosomal DNA and was isolated from sepsis E. coli 19, 21, 24, and 25 (grown at 377C for 24 h) and the wild-type
curli-expressing E. coli MC4100, as well as the csgD mutant MHR206 (287C for 48 h), respectively. The size of DNA markers (Mr) is indicated
on the left in base pairs.
JID 2000;181 (February)
Cytokine-Activating Curli by Septic E. coli
607
Figure 3. Electron micrographs of 2 immunogold-labeled sepsis Escherichia coli, expressing different levels of curli fibers at 377C. Bacteria
were grown on colonization factor agar plates for 24 h at 377C. Bacterial cells were probed with CsgA-specific antibody ZB-aA and labeled with
protein A–gold particles (10 nm in diameter). Magnification: A (22), 3100,000; B (32), 360,000.
6. Both the curli-producing strain MC4100 and the curli-deficient but CsgA-secreting mutant MHR261 gave rise to a 5-fold
higher level of IL-6 at 5 h after infection and a 4-fold higher
level at 6 h after infection than those of the noncurliated mutants lacking CsgA. The activity of the cytokines reached a
maximal level at 6 h and then decreased rapidly (data not
shown), which likely reflects the nutrient consumption by proliferation of bacteria and the decreasing number of viable
macrophages.
Dose-dependent induction of TNF-a by curli-expressing E.
coli. It is impossible so far to purify native curli fibers, because all of the protocols applied for curli purification involve
a denaturation process with detergent and formic acid. To see
if cytokine (TNF-a) induction is dose dependent, macrophages
were infected for 5 h with different amounts (104–108 cfu) of
the curli-expressing strain MC4100 and the csgBA double mutant MHR222, respectively (figure 6). Both MC4100 and
MHR222 induced a dose-dependent increase in TNF-a production in macrophages. However, the levels of TNF-a were
significantly higher (P ! .05) when macrophages were infected
with 105–108 cfu of MC4100 versus the same concentrations of
MHR222, strongly suggesting that curli (CsgA/CsgB) contribute significantly to TNF-a production by intact bacteria.
A sepsis E. coli isolate expressing curli induces higher TNFa production in vitro than does sepsis isolate lacking curli. Two
sepsis E. coli isolates with the same serotype (O1K1), 32 (curli
positive at 377C) and 30 (curli negative at 377C), were compared
for their ability to induce TNF-a production in human macrophages (figure 7). Macrophages were infected with 106 cfu of
sepsis E. coli 32 and 30, respectively. At 5 h after infection, a
significantly higher (P ! .0 5) level of TNF-a was induced by 32
than by 30. A similar level of TNF-a was induced by 106 cfu
of MC4100 at 5 h after infection. There was no statistical difference in the cytokine production between 30 and MHR222.
Discussion
E. coli and Salmonella frequently express thin, highly aggregated surface organelles, curli, when grown at temperatures
!377C [10–13]. It has therefore been questioned whether these
curli fibers play any role in diseases of warm-blooded animals.
In the present study, we demonstrate that 150% (24 of 46) of
clinical E. coli isolates from blood cultures of patients with
sepsis were capable of producing curli at 377C in vitro, as indicated by Congo red staining and fibronectin binding and
verified by immunoblotting assays (table 1, figure 1).
Congo red binding has been used as an indicator of surface
hydrophobicity for certain pathogens [27]. By making use of a
Congo red staining assay [12, 13], we were able to screen quickly
for in vitro expression of curli fibers among many E. coli isolates
at laboratory conditions. We found that 46 clinical E. coli isolates express a variable Congo red–binding pattern when grown
at either 287C or 377C, giving scores from 1 to 1111 (see
Materials and Methods and figure 1A). In most cases, when
the colonies stained with the dye, bacteria were expressing curli
fibers, as judged by immunoblotting with specific antibodies to
the major curli subunit protein CsgA (figure 1B). However, the
degree of staining and morphology of bacterial colonies varied
on curli indicator plates when viewed by use of EM, depending
on the expression level of curli fibers (figure 2). Colonies from
4 bacteremic isolates (5, 8, 26, and 30) stained slightly (5) with
Congo red when grown at 377C, but no curli signal was detected
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Bian et al.
JID 2000;181 (February)
Figure 4. Western blot to probe for antibodies to the curli major subunit CsgA in serum samples from sepsis patients. Curli-expressing wildtype Escherichia coli strain MC4100 and csgBA double mutant MHR222 were grown on colonization factor agar plates for 48 h at 287C. Bacterial
cells were treated with 90% formic acid before electrophoresis. Proteins of bacterial lysate from MC4100 or MHR222 were transferred onto 2
blotting membranes, which were cut into 5-mm-wide slices. Each membrane slice was put into a small tank in AutoBLOT 36 (GeneLabs Diagnostics,
Singapore) and incubated 1 : 400 with the septic serum or healthy serum for 60 min at room temperature. I, lanes 1: MC4100; lanes 2: MHR222;
lane A: probed with ZB-aA; lanes 17–46: analyzed with the convalescent sera from different septic patients. II, lanes 1: MC4100; lanes 2:
MHR222; lanes H1–H12: analyzed with sera from healthy controls. Locations of CsgA monomers and molecular weight are indicated on the left
in kDa.
by immunoblotting. However, in no case did we detect a CsgA
signal by immunoblotting from colonies negative for Congo
red staining. Consequently, Congo red staining in general is a
quick and efficient assay to screen for curli production in E.
coli but is not absolutely accurate.
Fibronectin binding is one of the unique features of curliexpressing bacteria of both E. coli and Salmonella [10, 11]. We
have shown in the present study, by using a nonradiolabeled
assay, that clinical E. coli sepsis isolates bind to soluble fibronectin when they are expressing curli fibers (table 1). Thus, curliexpressing clinical isolates growing either at 287C or 377C
(judged by immunoblotting assay) gave different values of light
absorbance ranging from 11 to 103. Colonies with higher scores
on curli indicator plates showed the highest level of fibronectin
binding, with a light absorbance similar to that of curli-expressing MC4100. The isolates that were negative by immunoblotting uniformly showed a low light absorbance, similar
to that seen for the csgA mutant MHR204. A few sepsis E.
coli isolates—for example, 41 and 46—gave a high score
(1111) by means of Congo red staining but showed a relatively low fibronectin-binding activity. Bacteria from such colonies are highly adhesive, forming aggregates at the bottom of
the wells. Such aggregates may sometimes cause problems in
measuring the light absorbance, which may explain the discrepancies found in the fibronectin-binding assay.
The most direct evidence of curli expression makes use of a
CsgA-specific antibody. A significant fraction (24 of 46) of human E. coli sepsis isolates were capable of producing curli fibers
at 377C in vitro, and 1 isolate did not express curli at 287C.
Only 7 isolates in the collection expressed curli only at 287C
but not at 377C, such as E. coli K12. We do not know yet
whether isolates expressing curli at 377C contain adaptory mutations in the csgD promoter affecting sigma-factor recognition,
as has recently been shown for S. typhimurium producing curli
at 377C [22]. Curli production is under a highly complex regulation involving not only the transcriptional regulator CsgD
[13] but also OmpR, a regulator belonging to a 2-component
signal transduction system. Mutations in the ompR gene were
recently shown to affect curli production in E. coli, even though
the underlying mechanisms are not clear [28]. In S. typhimurium, the expression of curli can be induced at the higher temperature (377C) by iron limitation [22]. It may therefore be that
an even higher percentage of E. coli are capable of producing
curli at 377C when growing in the bloodstream, a compartment
low in available iron.
E. coli is the most common gram-negative organism isolated
from patients with sepsis [29]. Recently, it was demonstrated
that curli-expressing E. coli and Salmonella are capable of activating the contact-phase system in human plasma, allowing
for an anticoagulation effect and generation of the cellular mediator bradykinin [19]. It has therefore been speculated that
curli expression in bacteria may contribute to the coagulation
JID 2000;181 (February)
Cytokine-Activating Curli by Septic E. coli
609
Figure 5. Kinetics of cytokine induction in human macrophages exposed to Escherichia coli. Macrophages differentiated from monocytic THP1 cells were stimulated with curli-expressing wild-type strain MC4100, CsgA-secreting strain MHR261, csgA mutant MHR204, and csgBA double
mutant MHR222, respectively, at 10 organisms per cell. Cytokine levels were quantified at 2, 3, 4, 5, and 6 h after infection by a capture ELISA.
*P ! .05 and **P ! .001 (paired t test). The detection limits of tumor necrosis factor (TNF)–a, interleukin (IL)–6, and IL-8 were 25, 6.25, and
62.5 pg/mL, respectively. The values are expressed as the mean 5 SD of 4 independent experiments.
abnormalities and blood pressure fall seen during sepsis. However, this important study did not reveal whether curli are expressed during sepsis.
In the present study, we analyzed serum samples from 12
convalescent patients with E. coli sepsis and serum samples
from 12 healthy individuals. We found antibodies to the curli
subunit CsgA only in the former group, and even sepsis patients
with the isolated E. coli strains 19, 21, 24, and 25 did not
produce curli fibers in vitro (table 1, figure 1). Together, these
data provide evidence that curli fibers are expressed in vivo in
human sepsis. It is conceivable that only strains producing very
high levels of curli during infection could produce organelles
in such quantities that they could affect the coagulation status and generate sufficient levels of bradykinin to induce
symptoms.
As an initial phase in the infectious process, most pathogenic
bacteria must be able to attach to epithelial surfaces, which is
frequently due to the expression of adhesive surface organelles,
such as fimbriae that recognize eukaryotic cell surface receptors.
Attachment may trigger a signal transduction cascade that results in the upregulation of proinflammatory cytokines [30].
Attached bacteria may also deliver effector proteins into the
cytosol or nucleus of host cells or tissue, affecting the transcription of genes for different cytokines if the organism contains a type III secretion system [31]. However, adhesins might
also allow host cells to better recognize those patterns on microbes, such as LPSs and other bacterial products that are
known to activate innate immune responses [32], an issue that
needs to be addressed in light of the results presented here.
We demonstrate here that curli are responsible to a significant
degree for the induction of TNF-a, IL-6, and IL-8 when human
macrophages are infected with E. coli strain MC4100 that is
grown on plates but is not expressing type 1 fimbriae or any
other known adhesive structure. Strikingly, the noncurliated E.
610
Bian et al.
coli mutant MHR261, which secretes soluble CsgA, also acts
as a potent cytokine inducer, suggesting that it is not curlimediated attachment to macrophages that is responsible for the
massive induction of cytokine, but rather it is the CsgA protein
itself, irrespective of whether CsgA is present as a curli-CsgA
polymer on the bacterial surface or as a secreted soluble CsgA
monomer. To our knowledge, this is the first time that a fimbrial
subunit monomer is shown to have a role in cytokine induction.
At present we do not know whether secreted CsgA acts directly
to activate innate immune responses or whether it contains a
sufficient amount of bound LPSs to trigger cytokine induction
through the LPS pathway, thereby acting as a signaling bullet.
Interestingly, a partially purified Salmonella flagellin, FliC, the
main subunit of the flagellar filament, has recently been shown
to cause TNF-a induction in human promonocytic cells [33],
suggesting that the assembly of flagellin into filaments is not
required to achieve a TNF-a response. However, the possibility
cannot be ruled out that other bacterial components may be
present in the crude flagellar preparation and may contribute
to TNF-a induction.
It has been shown elsewhere that type 1 and P-fimbriae–
positive E. coli strains stimulated the induction of IL-1a, IL1b, TNF-a , IL-6, and IL-8 in peripheral blood monocytes; IL6, IL-8, and IL-1a were induced in a bladder epithelial cell line;
and only IL-6 and IL-8 were induced in a kidney cell line [34].
Activation of the ceramide-signaling pathway contributes to
the cytokine induction by P-fimbriated E. coli in epithelial cells
[35]. Subsequently, fimbriae-associated adhesins of S. typhimurium were shown to induce increased levels of mRNA for
Figure 6. Dose-dependent tumor necrosis factor (TNF)–a production in human macrophages in response to Escherichia coli. Macrophages (106 cells/mL) differentiated from THP-1 cells were stimulated
with the curli-expressing wild-type E. coli MC4100 and the csgBA
double mutant MHR222, respectively. TNF-a level was quantified at
5 h after infection by a capture ELISA. The detection limit of TNFa was 25 pg/mL. The data are expressed as mean 5 SD of 4–6 independent experiments. cfu, colony-forming units.
JID 2000;181 (February)
Figure 7. Curli-expressing sepsis Escherichia coli isolate induces tumor necrosis factor (TNF)–a production in vitro. Sepsis E. coli 32
(curli positive) and 30 (curli negative) were grown at 377C for 24 h on
colonization factor agar plates. Macrophages (106 cells/mL) differentiated from T helper type-1 cells were stimulated with 106 colony-forming units of bacteria of 32 and 30, respectively. TNF-a level was quantified at 5 h after infection by a capture ELISA. *P ! .05 (paired t test).
The detection limit of TNF-a was 25 pg/mL. The values are expressed
as mean 5 SD of 4–6 independent experiments.
IL-1b, IL-6, granulocyte-macrophage colony-stimulating factor, chemokines, and melanocyte-growth stimulatory activity
in murine macrophages [36]. However, some virulent strains of
intracellular bacteria do not induce TNF-a. Hence, Brucella
suis releases an inhibitor for the release of TNF-a from E.
coli–infected macrophages [37], and Yersinia enterocolitica encodes a suppressor, YopP (YopJ in Y. pseudotuberculosis), which
impairs activation of NF-kB and thereby suppresses TNF-a
production [38–40]. Thus, bacteria have evolved different
means of inducing and manipulating the host cytokine network.
TNF-a is considered a central mediator in septic shock, and
it plays an important role in host defense against intracellular
pathogens [41, 42]. Together with IL-1, TNF-a is necessary to
keep infections localized and prevent the development of sepsis,
but once an infection has become systemic, the effects of IL-1
and TNF-a are detrimental, and sepsis ensues. IL-6 is another
cytokine recognized to play a role in sepsis that frequently is
found in high concentrations in sepsis patients; circulating levels
of IL-6 correlate with clinical outcome [43, 44]. A variety of
cells, including macrophages and monocytes, produce IL-8,
which shows a remarkable specificity for neutrophils [45]. IL8 is resistant to chemical or proteolytic inactivation, and its
biological activity in inflamed tissues may last for several hours
[46]. However, the precise role of IL-6 and IL-8 in the pathogenesis of sepsis is still not well established.
Although LPSs are very important mediators of sepsis, other
factors must also be involved [47], because LPS-insensitive mice
remain vulnerable to gram-negative sepsis [48, 49]. In fact, there
is significant evidence that other components of gram-negative
bacteria also play an important role in the pathologic changes
JID 2000;181 (February)
Cytokine-Activating Curli by Septic E. coli
that are associated with infections mediated by these organisms
[50]. The case illustrated herein shows that bacterial curli fibers
and/or secreted curli subunit monomers affect proinflammatory
cytokine induction. It may be beneficial for the septic host if
the biological effects of curli and its CsgA subunit could be
neutralized by blocking antibodies or by other means, even
though the precise role of these effects during infection remains
to be determined.
Acknowledgments
We are grateful to Dr. M. Rhen for helpful discussions and comments
on the manuscript. We appreciate Dr. M. Granström (Karolinska Hospital) for allowing us to use AutoBLOT 36 and Ms. C. Bengtsson for
helping to set up the instrument. We thank Dr. M. Hammar for providing the E. coli MHR strains.
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