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Previous Article | Next Article 
Applied and Environmental Microbiology, May 1999, p. 2272-2275, Vol. 65, No. 5
Department of Food and Environmental
Hygiene1 and Department of Bacteriology
and Immunology,
Received 19 October 1998/Accepted 3 March 1999
The genomic stability of 12 Campylobacter jejuni
strains consisting of two groups of human and chicken isolates was
studied by analysis of their PFGE (pulsed-field gel electrophoresis)
patterns after passage through newly hatched chicks' intestines. The
patterns of SmaI, SalI, and SacII
digests remained stable after intestinal passage, except for those of
two strains. One originally human strain, FB 6371, changed its genotype
from II/A (SmaI/SacII) to I/B. Another strain,
BTI, originally isolated from a chicken, changed its genotype from I/B
to a new genotype. The genomic instability of the strains was further
confirmed by SalI digestion and ribotyping of the
HaeIII digests. In addition, heat-stable serotype 57 of strain FB 6371 changed to serotype 27 in all isolates with new genotypes but remained unchanged in an isolate with the original genotype. Serotype 27 of strain BTI remained stable. Our study suggests
that during intestinal colonization, genomic rearrangement, as
demonstrated by changed PFGE and ribopatterns, may occur.
Typing has been widely used in the
characterization of Campylobacter jejuni isolates from
different sources, such as human diarrheal stools, animal fecal
samples, and food and water samples (14). The conventional
typing systems available are serotyping (11, 17), biotyping
(12), and phage typing (19). Molecular typing
methods, including ribotyping, RFLP (restriction fragment length
polymorphism) of the fla gene, and PFGE (pulsed-field gel electrophoresis), have been used during the 1990s. Because C. jejuni has only three copies of the ribosomal genes, ribotyping is
not a very good method for distinguishing them and has not achieved
wide use (6). The flagellin locus has been sequenced and
analyzed, showing that intragenetic and intergenetic recombination in
this region of the genome is rather common and may lead to variability
in genotypes and produce difficulties in the interpretation of
genotyping results (9). Genetic recombination of flagellar genes detected by PCR-RFLP is not correlated with the serotype conversions occasionally seen in chicken flocks (1, 3). Several studies have revealed PFGE to be a very good method for distinguishing subtypes within serotypes (4, 23) and is also useful for typing of strains that are untypeable with antisera (4). The discriminatory power of PFGE typing can be
increased if two enzymes, for example, SmaI and
KpnI (4) or SacII (7) are
used in combination. The stability of PFGE patterns is unknown, although some evidence exists that patterns might change in stressful environments (26).
During our longitudinal studies on the epidemiology of human
campylobacter infections in Finland in 1995 and 1996, we identified two
groups of C. jejuni strains with identical or highly related SmaI patterns and differing SacII patterns which
shared many fragments (7). Our hypothesis was that under
various environmental conditions, these patterns might be transformed.
In the present study, the genomic stability of these two C. jejuni PFGE genotype groups was studied after passage through
chick intestines. The first group included seven strains with identical
(pattern I) or related (pattern II) SmaI patterns and with
SacII patterns which differed by 4 to 11 fragments and
SalI patterns with 2 to 4 differing fragments. The second
group consisted of five strains with SmaI PFGE pattern XI,
SacII patterns differing by 7 to 11 fragments, and
SalI patterns differing by 1 to 4 fragments. Combined
SmaI/SacII/SalI PFGE patterns were
designated genotypes. Since their isolation, the strains were stored
deep-frozen at Brucella broth cultures were inoculated by oral gavage into the crops
of newly hatched chicks (inoculum size, 108 to
109 CFU per animal). Each strain was inoculated into seven
animals. Ten control animals were used to verify that chicks did not
have campylobacters before the inoculation experiment by culturing of
cecal samples. Each group of chicks was reared in a separate solid-bottom cardboard box at the Laboratory for Animal Activities for
6 days. After 6 days, the animals were euthanized with CO2 and campylobacter CFUs in cecal samples diluted in 0.1% peptone water
were counted and 0.1 volumes of 10 For PFGE analysis, the isolates were grown on brucella blood agar for 2 days at 37°C in a microaerobic atmosphere. The bacterial cells were
harvested and treated with formaldehyde to inactivate endogenous
nuclease (5). Otherwise, DNA was prepared by the method of
Maslow et al. (13) as described earlier (7). The DNA fragments were separated with Gene Navigator (Pharmacia LKB Biotechnology AB, Uppsala, Sweden) in 1% agarose gel in 0.5× TBE (45 mmol of Tris, 45 mmol of boric acid, 1 mmol of EDTA) at 200 V. SmaI and SalI fragments were separated with a
ramped pulse of 0.5 to 25 s for 20 h, and SacII
fragments were separated with a ramped pulse of 0.3 to 20 s for
20 h.
All 12 strains and the isolates of FB 6371 and BTI with changed PFGE
patterns after intestinal passage were ribotyped after digestion of
their DNAs with HaeIII. HaeIII digests were made both from the respective PFGE plugs and from DNA isolated from bacterial cells after microaerobic growth in brucella broth at 37°C
for 44 h. DNA was isolated and ribotyping was performed as described earlier (6). Heat-stable Penner's serotyping of
the 12 strains and of the selected isolates after passage was performed as described earlier (18).
The selected C. jejuni strains and their sources of
isolation, serotypes, PFGE patterns, and ribotypes are shown in Table 1. Figure
1a, b, and c shows the SmaI,
SalI, and SacII patterns of certain of the
SmaI pattern type I strains (strain FB 7052 in lane 3 and
strain 48A in lane 5 were not included in the stability studies),
respectively. SalI patterns show some variation (Fig. 1b,
lanes 1, 2, 7, 8, and 10). SacII, which cuts the DNA most frequently, generated differing patterns with many shared fragments (Fig. 1c). Patterns of SmaI pattern type XI strains are not
shown.
None of the control chicks was colonized with campylobacters. All
chicks were from the same breeder never known to have campylobacters. All animals were colonized with all of the selected PFGE genotypes. Campylobacter counts were 108 to 1010 CFU/g.
PFGE pattern analysis was performed with 14 colonies of each strain
isolated from seven chicks, two from each. All genotypes of the
SmaI pattern XI group remained unchanged after passage (results not shown). Similarly, all genotypes of the SmaI
pattern type I strains remained unchanged, except that genotype II/A
(strain FB 6371) changed into genotype I/B in all seven chicks; only
one of the 14 colonies studied had the original pattern, II/A. Figure 2 shows the SmaI (lanes 2 to
6), SalI (lanes 7 to 11), and SacII (lanes 12 to
16) patterns of four isolates representing the original genotype (lanes
2, 7, and 12), the stable pattern (lanes 5, 10, and 15, and the changed
pattern (lanes 3, 4, 6, 8, 9, 11, 13, 14, and 16). Six fragments
changed in the SacII pattern. One of the 14 colonies (strain
BTI) changed from pattern I/B to a new SacII pattern type
(Fig. 2, lane 20). Change was also visible in the location of one
fragment of the SmaI and SalI patterns (results
not shown). Ribotyping of HaeIII digests of the selected FB
6371 and BTI isolates showed that when PFGE pattern II/A was changed to
pattern I/B, the respective change was seen in the ribopatterns as well
(Fig. 3, lanes 1 to 4). Similarly, when
PFGE pattern I/B of strain BTI was changed, the ribopattern was also changed (Fig. 3, lanes 5 and 6). Passaged isolates with the changed genotype of strain FB 6371 changed from serotype 57 to serotype 27. The
serotype of the isolate with the unchanged genotype remained 57. The
serotype, 27, of strain BTI remained the same in isolates with
unchanged and changed genotypes.
0099-2240/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Stability of Related Human and Chicken
Campylobacter jejuni Genotypes after Passage through Chick
Intestine Studied by Pulsed-Field Gel Electrophoresis
ABSTRACT
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70°C in sterile skim milk with 15% glycerol. The
strains were grown on brucella (Oxoid Ltd., Basingstoke, Hampshire,
United Kingdom) blood agar plates for 2 days and in brucella broth for
1 day in a microaerobic atmosphere at 37°C.
5 to 108
dilutions were spread on modified charcoal cefoperazone deoxycholate agar (Oxoid) medium. All modified charcoal cefoperazone deoxycholate agar plates were incubated microaerobically at 42°C. Four colonies were subcultured from a 106 or 108 dilution
from each chick and confirmed by colony morphology, gram staining, and
positive catalase and hippurate tests to be C. jejuni. Two
colonies from each chick were used for PFGE pattern analysis.
TABLE 1.
Serotypes, PFGE patterns, and ribotypes of C. jejuni strains used for studies on genomic stability
View larger version (39K):
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FIG. 1.
SmaI (a), SalI (b), and
SacII (c) patterns of C. jejuni strains with
highly similar SmaI pattern I or II. Lanes: 1, strain FB
6032 (genotype I/B), 2, FB 6371 (II/A); 3, FB 7052 (I/B); 4, 50A (I/E);
5, 48A (I/E); 6, FB 5194 (I/J); 7, FB 5592 (I/K); 8, FB 5622 (I/B); 9, FB 6021 (I/L). Molecular size marker, 48.5-kb lambda concatemer (lanes
m.w.).
View larger version (121K):
[in a new window]
FIG. 2.
Patterns of C. jejuni FB 6371 and BTI after
passage through chicks' intestines. Lanes: 2, 7, and 12, SmaI, SalI, and SacII patterns of
strain FB 6371, respectively, before the colonization study; 3 to 6 (SmaI), 8 to 11 (SalI), and 13 to 16 (SacII), patterns of four colonies of strain FB 6371 isolated from chicken fecal samples after the colonization experiment.
Changed patterns of three colonies A/a, A/d, and B/c, are seen in lanes
8, 9, and 11 and 13, 14, and 16. Similarly, the control
SacII pattern of strain BTI is shown in lane 18, the
unchanged patterns are in lanes 19 and 20, and the changed pattern is
in lane 20. Molecular size markers (48.5-kb lambda concatemer) are
shown in lanes 1 and 17.
View larger version (69K):
[in a new window]
FIG. 3.
Ribotypes (HaeIII digests) of selected
isolates of strains FB 6371 and BTI before and after colonization in
chicks' intestines. Lanes: 1, strain FB 6371 before inoculation,
ribotype 2; 2 and 3, changed ribotype 1 of two colonies (chicks A/a and
A/d) of strain FB 6371 after colonization; 4, colony (chick B/a) with
unchanged ribotype 2; 5, strain BTI before inoculation, ribotype 1; 6, isolate (BTI, chick A/c) after colonization, ribotype 8; mw, molecular
size marker (Marker II; Boehringer Mannheim, Mannheim, Germany).
During the last 10 years, molecular subtyping systems have been developed and extensively used to support studies on the epidemiology of microbial infections (1, 7, 16, 22, 25). Several methods are based on the analysis of RFLP of a single locus or on entire-genome macrorestriction analysis (PFGE). Information on the stability of the patterns over a short (during an epidemic) or long (years, decades) time span or during in vitro culture and storage is required for evaluation of the performance characteristics of the methods (22). Our results suggest that intestinal colonization may favor genetic recombination (rearrangement, insertions, deletions, point mutations) in C. jejuni which will become visible in the PFGE patterns and in ribopatterns, because in 2 of our 12 strains such variability was evident. Recently, Wassenaar et al. (26) studied PFGE genotype distribution in poultry meat batches and found variable, related C. jejuni PFGE genotypes; they suggested that this variation might have resulted from genetic recombination occurring under stressful conditions during meat storage. However, when a C. jejuni genotype was passaged in two day-old chicks, the genotype remained stable. On the contrary, in our study, we were able to find changed patterns in 2 of 12 strains, suggesting that either the method used was not sensitive enough to reveal genetic variations in all of the strains at different rates or that the genotypes differ in genomic stability. PFGE is a rather laborious and expensive method, and the number of colonies which can be analyzed is limited. In our study, both the number of strains and that of the chicks used for in vivo passage were higher than in the studies of Wassenaar et al. (26), thus increasing the possibility that we would detect genomic instability. We were able to show simultaneous change in the genotype and in the serotype. Recent studies have revealed that intergenomic and intragenomic recombinations between flaA and flaB genes is evident (9), suggesting that this genomic region is unstable in C. jejuni.
The strains in our study were selected from a large collection of human and chicken strains with a wide variety of PFGE patterns (7). SmaI pattern I strains formed the predominant group among human domestic campylobacter strains in 1996 or were isolated from chickens during the same time period. Pattern II/A is uncommon among Finnish strains, and it has been identified in four strains in 1996 to 1998 (unpublished results). SmaI pattern XI strains were either human isolates from two small outbreaks (patterns XI/S and XI/R) or chicken isolates (patterns XI/Q, XI/U, and XI/T). C. jejuni is highly adapted for growth in the intestines of chickens, and if a flock is colonized with C. jejuni, most of the animals acquire the organism and excrete it in high numbers, i.e., 106 to 108 CFU/g of fecal material (2, 10). Several studies have shown that usually only one or a few serotypes or genotypes colonize a flock (1, 2, 10, 21). Thus, no direct evidence of natural genetic instability within commercially reared flocks is available. If several genotypes have been identified in a flock, this has been explained by several contamination sources (21). Ayling et al. (1) performed a longitudinal study of 96 broiler houses by use of PCR-RFLP analysis of flaA and flaB genes and found that, in most cases, one genotype was predominant, suggesting genotype stability during the rearing of a flock. Detection of genomic instability, however, depends on the number of samples taken and the methods used. PFGE has been shown to be a more distinguishing typing method than PCR-RFLP for flagellar genes (4). Although we did find genomic instability in genotypes I/B and II/A, we could not identify this phenomenon in the other genotypes or in the second strain of genotype I/B. Ribotyping of isolates with changed genotypes indicated that during intestinal colonization, new restriction sites were also formed in ribosomal regions. In serotype 57 strain 6371, serotype conversion was seen in addition to genetic change.
Genotype I/B was isolated for the first time 10 years ago from fecal samples of cattle (8), suggesting long-term stability of this genotype. We have also identified one of the genotypes, I/E, in chickens over at least a 3-year period, also indicating the stability of this genotype (unpublished results). Steinbrueckner et al. (20) found that the PFGE patterns of the strains of 48 campylobacter patients remained stable in stool samples studied at a mean interval of 3 weeks. However, in one patient, the first and second isolates exhibited a difference of four bands in their PFGE patterns. It was not clear if this patient had two infecting strains or if the original pattern had changed during the infection (20).
All of the common typing methods suggest that C. jejuni is a heterogeneous organism (6, 7, 16, 17, 19). This heterogeneity may also result from genomic instability. Genomic plasticity may thus be an important characteristic for survival in various hosts known to be colonized by C. jejuni (24). More data on the genetic stability of PFGE patterns and other genotypes is needed.
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ACKNOWLEDGMENTS |
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This work was supported by funds from the Finnish Veterinary Medical Association.
We thank Urzula Hirvi and Tiina Okkonen for excellent technical assistance.
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FOOTNOTES |
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* Corresponding author. Mailing address: Faculty of Veterinary Medicine, Department of Food and Environmental Hygiene, P.O. Box 57, FIN-00014 University of Helsinki, Finland. Phone: 358-9-70849704. Fax: 358-9-70849718. E-mail: marja-liisa.hanninen{at}helsinki.fi.
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