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Applied and Environmental Microbiology, October 2003, p. 6316-6320, Vol. 69, No. 10
0099-2240/03/$08.00+0 DOI: 10.1128/AEM.69.10.6316-6320.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
Comparison of Campylobacter Isolates from Poultry and Humans: Association between In Vitro Virulence Properties, Biotypes, and Pulsed-Field Gel Electrophoresis Clusters
Éric Nadeau, Serge Messier, and Sylvain Quessy*
Faculty of Veterinary Medicine, Université de Montréal, St-Hyacinthe, Québec, Canada J2S 7C6
Received 6 February 2003/
Accepted 9 July 2003

ABSTRACT
The in vitro virulence properties of 197 temporally and geographically
related
Campylobacter isolates from chicken broilers and humans
were compared. Comparisons of the virulence properties associated
with genotypes and biotypes were made. All isolates adhered
to, and 63% invaded, INT-407 cells, whereas 13% were cytotoxic
for CHO cells. CHO cell-cytotoxic extracts were also cytotoxic
for INT-407 cells, but the sensitivity for Vero cells was variable.
The proportion of isolates demonstrating a high invasiveness
potential (>1,000 CFU ml
-1) or Vero cell cytotoxicity was
significantly higher for human than for poultry isolates. Invasiveness
was associated with
Campylobacter jejuni isolates of biotypes
1 and 2, whereas CHO and INT-407 cell cytotoxicity was associated
with
C. jejuni isolates of biotypes 3 and 4. Cytotoxic isolates
were also clustered according to pulsed-field gel electrophoresis
profiles.

INTRODUCTION
Campylobacter spp. are recognized as the most common cause of
food-borne bacterial gastroenteritis in humans (
2). Epidemiological
studies revealed that consumption of poultry products is an
important risk factor for sporadic cases of human campylobacteriosis
(
14,
15), and many studies have identified common types of
Campylobacter from poultry and humans (
5,
26,
28,
29). The extent to which
poultry consumption is responsible for human infections is nevertheless
not exactly known (
12). Asymptomatic infections, watery diarrhea,
and dysentery-type illnesses of humans have been reported (
36).
Campylobacter jejuni strains associated with dysentery-like
illness have been shown to be more invasive and cytotoxic than
other
Campylobacter strains in in vitro assays (
22). Adherence
to and invasion of host mucosal surfaces were proposed as essential
steps in the pathogenesis (
11). In addition, many cytotoxic
activities have been reported, but their significance in human
disease remains unclear (
37). Subgrouping
Campylobacter strains
with respect to their virulence factors would be an important
step in understanding the epidemiology and the pathogenesis
of the infection. pulsed-field gel electrophoresis (PFGE) is
one of the most powerful techniques for the genotyping of
Campylobacter (
27). Although the restriction endonuclease
SmaI has been widely
used for
Campylobacter PFGE analysis,
KpnI appears to be most
suitable for epidemiological studies of
Campylobacter (
24).
In a previous study, we reported that approximately 40% of chicken broilers and 60% of lots of slaughtered birds in the province of Quebec (Canada) were colonized by Campylobacter. PFGE and biotyping revealed that approximately 20% of human isolates were related to poultry isolates (26). To assess their pathogenic potential and to determine if clonally related isolates have common characteristics, temporally and geographically related Campylobacter isolates from chicken broilers and sporadic cases of human enteritis were compared for their in vitro virulence abilities.

Sampling and identification of Campylobacter isolates.
A total of 197
Campylobacter isolates, 173 from chicken broilers
and 24 from sporadic cases of human diarrhea, were used in this
study.
Campylobacter isolates were cultured during a 1-year
sampling (July 1998 to June 1999) as described previously (
26).
Samples of poultry cecal contents in two abattoirs in the region
of St-Hyacinthe, Québec, Canada, which originated from
52 slaughter lots of 33 farms, were obtained. The human fecal
samples originated from the only hospital of the area, located
in St-Hyacinthe. Fecal samples from all sporadic cases of diarrhea
(
n = 296) processed for bacteriology at the hospital were also
analyzed in our laboratory. Samples were directly inoculated
onto charcoal selective medium (CSM), and the plates were incubated
at 42°C under microaerobic conditions. All
Campylobacter isolates were identified by standard microbiological procedures
as previously described (
26).
C. jejuni accounted for 95.6 and
91.7% of poultry and human isolates, respectively, whereas the
remaining isolates were identified as
Campylobacter coli.

Adhesion and invasion assays of INT-407 cells.
For in vitro virulence properties, associations were determined
by using the Fisher exact test (

= 0.05). Adhesion and invasion
assays were performed as described by Grant et al. (
7), except
that 7.0
x 10
4 INT-407 cells were seeded into each well of 24-well
tissue culture plates (Nunc, Roskilde, Denmark) to obtain semiconfluent
monolayers after an incubation at 37°C for 24 h in 5% CO
2.
Briefly, each well was then inoculated with 5
x 10
7 CFU of
Campylobacter,
and the plates were centrifuged. For the adhesion assay, plates
were incubated for 30 min at 37°C, whereas for the invasion
assay they were incubated for 3 h and incubated for another
2 h in minimal essential medium (MEM) containing 1% fetal bovine
serum (FBS) and 250 µg of gentamicin ml
-1. Adherent and
intracellular bacteria were evaluated by plating serial dilutions
of the cell lysates on CSM agar plates without selective supplement
and by counting the colonies after incubation.
In accordance with the findings of other studies (11, 20), all C. jejuni and C. coli isolates adhered to INT-407 cells. The number of bacteria that adhered ranged from 1.0 x 103 to 1.8 x 106 CFU ml-1, representing 0.001 to 1.8% of the bacterial inoculum, in accordance with results observed in other studies (17, 18, 32). The ability of Campylobacter to invade epithelial cells in vitro is recognized as strain dependent (17). As observed by Lindblom and Kaijser (20), the proportions of poultry and human isolates that showed invasiveness were not statistically different (Pexact = 0.1547), being 63.6 and 58.3%, respectively. Lindblom and Kaijser (20) reported that 40% of human and hen isolates were invasive, whereas Tay et al. (34) reported invasiveness properties for 82% of human isolates. Adhesion to and invasion of epithelial cells by Campylobacter are affected by assay parameters, and considerable variations in in vitro techniques are described in the literature (10, 17, 30). Consequently, the comparison of in vitro adhesion and invasion abilities of Campylobacter isolates among studies is difficult. In the present study, the number of invasive bacteria ranged from 1 x 102 to 5 x 104 CFU ml-1, representing 0.0001 to 0.05% of the bacterial inoculum. Although 21.3% of the isolates showed intracellular numbers >1,000 CFU ml-1, the proportion of human isolates above this level was significantly higher than the proportion of poultry isolates (Pexact = 0.0277) (Table 1). However, a higher number of isolates from humans would be required to confirm this dissimilarity since the discrepancy between the numbers of human and poultry isolates may skew the invasion results in favor of human isolates.
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TABLE 1. Distribution of invasive and cytotoxic Campylobacter isolates among biotypes according to invasion ability for INT-407 cells and cytotoxic activity for CHO cells
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Cytotoxin assay of epithelial cells.
Campylobacter cells were harvested from a 24-h culture on a
Mueller-Hinton agar plate incubated at 42°C, washed with
phosphate-buffered saline, and resuspended at an optical density
at 650 nm (OD
650) of 0.75 in MEM containing 0.15% (wt/vol) polymyxin
B sulfate (Sigma). Suspensions were incubated at 37°C for
30 min and centrifuged at 2,500
x g for 20 min, and the supernatant
was filtered through a 0.45-µm-pore-size disposable filter
(Millipore). Toxin assays were carried out in 96-well tissue
culture plates (Nunclon). Polymyxin B extracts were added (100
µl) in triplicate, and doubling dilutions were performed
with MEM supplemented with 3% FBS. Freshly trypsinized Chinese
hamster ovary (CHO) cells (4
x 10
5 cells ml
-1) suspended in
MEM supplemented with 3% FBS were then added (100 µl).
Plates were incubated at 37°C for 48 h in 5% CO
2. Wells
were observed at 24-h intervals with an inverted microscope.
After 48 h, the MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium
bromide) dye reduction assay was performed as previously described
(
4) to minimize subjective errors. The cytotoxic titer was the
dilution of the well in which cell death (1 - [mean OD of test
wells/mean OD of negative-control wells]
x 100) was

25%.
This cutoff value was estimated by doubling the percentage of
intraplate variation of CHO cell growth. Titers <1/4 were
considered nonspecific. All CHO cell-cytotoxic isolates were
also tested with Vero and INT-407 cells by the same assay but
with a serum concentration of 5% for INT-407 cells and 1% for
Vero cells.
CHO cell cytotoxicity was observed for 26 (13.2%) of the Campylobacter isolates (Table 1). Cytotoxicity was associated with rounded and detached cells. For some isolates, this activity was observed microscopically as soon as 24 h after incubation. Titers varied from 1/16 to 1/128 for 23 of the 26 cytotoxic isolates, whereas they were less than 1/16 for the three others. Campylobacter cytotoxic activities have been classified into those of cytodistending toxin (CDT) and non-CDT toxins (31, 37), but comparison of cytotoxic activities among studies is difficult due to confusion concerning Campylobacter cytotoxins. Distension of cells by CDT can normally be observed after 48 h of incubation (9, 13). CDT activities have been frequently observed when sonication or supernatants of Campylobacter cultures were used (6, 13, 33). Non-CDT activities were observed in the present study. McFarland and Neill (23) also reported only non-CDT activity when CHO cells were added to Campylobacter polymyxin B extracts. Nevertheless, we cannot rule out the possibility that the cytotoxicity observed in the present study was associated with viable Campylobacter cells that may have passed through the 0-45-µm-pore-size filters. If this were the case, however, the viable bacteria would have reduced the MTT, consequently concealing the epithelial cell death caused by the cytotoxicity. Furthermore, no bacterial growth was observed in wells with the inverted microscope.
The proportion of human cytotoxic isolates was much lower than that reported in the literature (37). It is possible that titers considered nonspecific (<1/4) reflected a weak activity of cytotoxicity since non-CDT titers are generally low (31). Moreover, the cytotoxic cutoff value chosen might also contribute to concealing weak cytotoxic activities. Some authors reported more non-CDT cytotoxic activity in human isolates than in animal isolates (1, 19, 32, 33), whereas, as in the present study (Table 1), others reported no difference (20, 25).
All tested CHO cell-cytotoxic extracts were also cytotoxic for INT-407 cells (Table 2). Poultry isolates were generally not toxic for Vero cells or, if so, had weaker titers, whereas four of the five CHO cell cytotoxic human isolates were cytotoxic for Vero cells. A significantly higher proportion of human than of poultry CHO cell-cytotoxic extracts were cytotoxic for Vero cells (Pexact = 0.0372). Other authors reported a weaker reaction on Vero cells than on other cell lines (1, 16, 19) and conflicting results with regard to Vero cell sensitivity (31). Since more than one cytotoxin activity has been previously observed (31) and since the results obtained for the Vero cell assay were variable among CHO cell-cytotoxic isolates, we cannot eliminate the possibility of multiple cytotoxin activities for the tested isolates.
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TABLE 2. Comparison of cytotoxic activities of polymyxin B extracts of human and poultry Campylobacter isolates on different cell lines
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Subgrouping Campylobacter isolates with regard to their in vitro virulence abilities.
The isolates were biotyped according to the scheme of Lior (
21).
PFGE was done as described previously using the
KpnI enzyme
(
26). Genetic relationships of strains were also established
as described previously (
26,
35). In the dendrogram, genotypes
were delineated with a 90% similarity cutoff level. The 197
Campylobacter isolates were distributed in 57 different
KpnI
genotypes. Only poultry isolates (
n = 155) were clustered in
37 genotypes. Both human (
n = 5) and poultry isolates (
n = 18)
were found in 4 genotypes, and the remaining human isolates
(
n = 19) were clustered in 16 genotypes.
Nearly all invasive isolates with an intracellular numbers >1,000 CFU ml-1 belonged to biotypes 1 and 2 of C. jejuni, whereas CHO cell-cytotoxic isolates were associated with C. jejuni biotypes 3 and 4 (Table 1). The latter association has not been previously reported except for the observation of a higher cytotoxic activity for a polymyxin B extract of a C. jejuni biotype 4 isolate (8). Unlike Carvalho et al. (3), who identified a randomly amplified polymorphic DNA fingerprinting cluster comprising the more-invasive isolates, we observed that invasive isolates were distributed throughout the PFGE dendrogram. On the other hand, 18 of the 21 CHO cell-cytotoxic isolates from poultry were clustered in three genotypes (Table 2) and in two clusters in the dendrogram (Fig. 1). These results suggest that these cytotoxic isolates represent a subgroup of Campylobacter sharing particular characteristics. Nevertheless, we cannot rule out the possibility that the clusters are due to a geographical relationship since the exact locations of the farms and patients are not known.
Higher invasiveness has been reported among noncytotoxic isolates
(
20). In this study, CHO cell-cytotoxic isolates generally had
an intracellular numbers of 0 to 1,000 CFU ml
-1 except for two
C. jejuni biotype 3 cytotoxic isolates from poultry and two
C. jejuni biotype 1 cytotoxic isolates from humans that showed
intracellular numbers >1,000 CFU ml
-1. Since invasive isolates
belonged to different biotypes than cytotoxic isolates, it is
possible that distinct
Campylobacter populations carry these
two putative virulence factors. On the other hand, the cytotoxic
activity might affect the invasion process and the ability to
measure invasion.
Finally, isolates clustered in a particular KpnI genotype had similar invasion abilities, with few exceptions, and had the same CHO cell cytotoxicity status (cytotoxic or not), except for the poultry cytotoxic isolate 876, which clustered with the noncytotoxic isolate 867 (Fig. 1). This homogeneity of in vitro virulence properties within a particular KpnI genotype suggests that the phenotype may be stable for isolates clustered in a particular genotype. Surprisingly, all isolates included in the four genotypes comprising poultry and human cases were not invasive or were weakly invasive (
1,000 CFU ml-1) and not cytotoxic.
Overall, the data presented in this study revealed an association of in vitro virulence properties with biotype, genotype, and host of origin. Invasive isolates were associated with biotypes 1 or 2 of C. jejuni, whereas CHO cell- and INT-407 cell-cytotoxic isolates were associated with biotypes 3 and 4 of C. jejuni and were clustered with PFGE groups. Human isolates were more invasive and Vero cell cytotoxic than poultry isolates. These data also suggest that clonally related isolates have common in vitro virulence characteristics and that subgroups of Campylobacter, with respect to potential virulence abilities, exist. Further work is needed to determine if cytotoxic C. jejuni isolates associated with biotypes 3 and 4 are widely distributed among C. jejuni strains and to characterize the subgroup of biotype 3 and 4 C. jejuni cytotoxic isolates.

ACKNOWLEDGMENTS
We acknowledge Health Canada authorities at the Food and Veterinary
Hygiene Laboratory of Saint-Hyacinthe for their collaboration
and access to their facilities.

FOOTNOTES
* Corresponding author. Mailing address: Faculty of Veterinary Medicine, Université de Montréal, C.P. 5000, St-Hyacinthe, Québec, Canada J2S 7C6. Phone: (450) 773-8521. Fax: (450) 778-8113. E-mail:
sylvain.quessy{at}umontreal.ca.


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Applied and Environmental Microbiology, October 2003, p. 6316-6320, Vol. 69, No. 10
0099-2240/03/$08.00+0 DOI: 10.1128/AEM.69.10.6316-6320.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
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