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Journal of Clinical Microbiology, February 2003, p. 892-895, Vol. 41, No. 2
0095-1137/03/$08.00+0 DOI: 10.1128/JCM.41.2.892-895.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
Molecular Typing of Clostridium perfringens from a Food-Borne Disease Outbreak in a Nursing Home: Ribotyping versus Pulsed-Field Gel Electrophoresis
Barbara Schalch,1* Lutz Bader,2 Hans-Peter Schau,3 Rolf Bergmann,3 Andrea Rometsch,1 Gertraud Maydl,2 and Silvia Keßler3
Institute for Hygiene and Technology of Food of Animal Origin, Veterinary Faculty, Ludwig-Maximilians University Munich, 80539 Munich,1
Max von Pettenkofer Institute for Hygiene and Medical Microbiology, Medical Faculty, Ludwig-Maximilians University Munich, 80336 Munich,2
Department of Medical Microbiology, Thuringian Federal Authorities for Food Safety and Consumers' Protection, TLLV, 99089 Erfurt, Germany3
Received 8 May 2002/
Returned for modification 16 July 2002/
Accepted 11 November 2002

ABSTRACT
In 1998, 21 inhabitants of a German nursing home fell ill with
acute gastroenteritis after consumption of minced beef heart
(P. Graf and L. Bader, Epidemiol. Bull. 41:327-329, 2000). Two
residents died during hospital treatment. Seventeen
Clostridium perfringens strains were collected from two different dishes
and from patients' stool samples and autopsy materials. A majority
of these isolates was not typeable by restriction fragment length
polymorphism-pulsed-field gel electrophoresis (PFGE). Subsequent
ribotyping of
C. perfringens distinguished four different groups.
The same ribopattern was detected in a minced beef heart dish,
in autopsy material from the two deceased patients, and additionally
in stool samples from six further residents who had fallen ill
with diarrhea. Three further ribopatterns from food and autopsy
materials could be differentiated. As chromosomal macrorestriction
with subsequent PFGE is generally regarded more useful than
ribotyping for molecular strain analysis, four selected isolates
were lysed in parallel with a standard protocol and two nucleases
inhibiting modifications. Neither of these methods could differentiate
all of the isolates. These results suggest that PFGE with the
current standard protocols is not able to characterize all
C. perfringens isolates from food-borne disease investigations
and that ribotyping is still a helpful method for molecular
identification of clonal relationships.

TEXT
Clostridium perfringens is a gram-positive, spore-forming, anaerobic
rod. This bacterium can spoil food, and some strains produce
an enterotoxin (
C. perfringens enterotoxin [CPE]) that is released
upon lysis of the vegetative cell during sporulation in the
small intestine. CPE causes food-borne disease in humans and
some animals (
3,
14,
17). As
C. perfringens is also part of
the intestinal flora, molecular typing is important for investigating
clonal relationships in outbreaks and for studying the molecular
epidemiology of this microorganism. Several molecular methods
have been used successfully for
C. perfringens strain differentiation;
among them are serotyping (
12), plasmid isolation (
4,
11), sodium
dodecyl sulfate-polyacrylamide gel electrophoresis (
9a), pulsed-field
gel electrophoresis (PFGE) (
16,
19), and ribotyping (
1). We
describe the examination of 17 food-borne disease-related
C. perfringens isolates by ribotyping (
7) and PFGE with three different
cell lysis methods. PFGE is presumed to offer greater discriminatory
power, typeability, and reproducibility than many other DNA-based
typing methods, including ribotyping, and has been applied successfully
for genotyping of various
C. perfringens isolates (
16,
19,
20).
Nevertheless, DNA degradation problems with certain strains
due to endogenous bacterial nucleases, which are rather common
among clostridial isolates, have been reported (
5,
7,
19,
20).
In our study, preliminary PFGE of isolates involved in a food-borne
disease outbreak (
6) failed. Therefore, ribotyping was performed
subsequently and PFGE was repeated, including two protocols
with nuclease-inhibiting supplements.
Strains
Table 1 shows the origins of the 17 C. perfringens isolates investigated. Isolates were cultured, purified, identified, and kept on a Microbank (Mast Diagnostica, Reinfeld, Germany) at -18°C. Before PFGE and ribotyping were performed, the purity of the isolates was ensured by culturing them twice on Columbia sheep blood agar (Unipath, Ltd., Basingstoke, Hampshire, United Kingdom).
Ribotyping was carried out as described by Grimont and Grimont
(
7). Briefly, clostridia were grown anaerobically overnight
in brain heart infusion broth (Unipath, Ltd.). DNA was isolated
by the guanidium thiocyanate method of Pitcher et al. (
15),
including the modifications described by Björkroth and
Korkeala (
2). Five micrograms of DNA was cleaved with
EcoRI
(Qbiogene, Heidelberg, Germany) in accordance with the manufacturer's
instructions. DNA concentrations were determined with a UV spectrophotometer
(UV/VIS spectrometer Lambda 2; Perkin-Elmer, Norwalk, Conn.).
DNA fragments were separated in 0.8% agarose gels (24 h, 25
V). Digoxigenin-labeled phage lambda DNA (Roche Diagnostics,
Mannheim, Germany) was used as a molecular size marker. DNA
fragments were Southern blotted (
18) to a nylon membrane (Roche
Diagnostics). DNA was fixed at 180°C for 0.5 h, prehybridized
for 2 to 4 h in a 58°C water bath, and hybridized overnight
at 58°C; the rest of the procedure was done in accordance
with the instructions supplied with the Roche Diagnostics digoxigenin
DNA labeling and detection kit. The DNA probe was prepared from
Escherichia coli 16S and 23S rRNA (Roche Diagnostics) in accordance
with the instructions supplied with the kit. The pattern was
read visually.
PFGE was first carried out as described recently (8, 13, 19) and repeated after ribotyping with the standard buffer plus two modifications. After being cultured overnight in brain heart infusion broth, cells were harvested by centrifugation (1,000 x g, 4°C, 15 min) and resuspended in parallel in 600 µl of pure PIV buffer (10 mM Tris, 1 M NaCl, pH 7.6), PIV buffer with formaldehyde (10 mM Tris, 1 M NaCl, 3.7% formaldehyde) as previously reported (10), and PIV buffer with urea (10 mM Tris, 1 M NaCl, 7 M urea). Restriction of chromosomal DNA with SmaI (New England Biolabs, Beverly, Mass.) followed. Electrophoresis was performed in a 1% pulsed-field agarose gel (Bio-Rad, Hercules, Calif.; dissolved in 0.5x Tris-borate-EDTA) in the Gene NavigatorÔ System (Pharmacia Biotech, Uppsala, Sweden) at 200 V. The electrophoresis time was 21 h with initial and final switching times of 1 and 30 s. The gel was stained in ethidium bromide solution (0.5 µg ml-1 in 0.5x Tris-borate-EDTA) and analyzed visually with the Gel Doc 1000 imaging system (Bio-Rad).
The CPE-producing capability of the isolates was tested with a commercially available CPE reverse passive latex agglutination test kit (PET-RPLA; Oxoid, Basingstoke, United Kingdom) in accordance with the manufacturer's recommendations.
PFGE was carried out with all 17 strains in two different laboratories with the standard PIV buffer (19) without interpretable results. After ribotyping of all 17 strains, PFGE was repeated in the third laboratory with four isolates (653, 730, 11227a, and 11227b) representing three different ribotypes by using in parallel the standard PIV buffer procedure (19) and two further nuclease-inhibiting modifications with 3.7% formaldehyde and 7 M urea, respectively (Table 1).
The results of ribotyping and PFGE done with the three different DNA preparation methods are listed in Table 1 together with the results of CPE detection. As shown in Fig. 1, four distinct ribotype patterns were detected among the 17 C. perfringens isolates. The results of one membrane with the ribopatterns of 11 C. perfringens isolates possibly involved in the outbreak are shown in Fig. 2.
Table
1 shows that six
C. perfringens isolates (no. 728, 729,
730, 732, 733, and 734) from patients' stool samples, three
isolates from the two deceased patients' autopsy material (no.
866, 867, and 869), and one food isolate from minced beef heart
(no. 11227a) belonged to ribopattern 1. Ribopattern 2 was detected
twice, in minced beef heart (no. 11227 b) and in patient B's
autopsy material (no. 650). Ribopattern 3 was detected only
once, in a meat dish called Wilderertöpfchen (no. 18006).
Four
C. perfringens isolates derived from autopsy materials
from patients B and S (no. 648, 652, 653, and 654) belonged
to ribopattern 4.
PFGE of all 17 strains in two laboratories produced no interpretable results. After ribotyping of all 17 strains, PFGE was repeated in the third laboratory with four isolates (no. 653, 730, 11227a, and 11227b) representing three different ribotypes and using in parallel the standard PIV buffer procedure (19) and two further nuclease-inhibiting modifications. Only one C. perfringens isolate (no. 653), representing ribotype 4, gave a weakly visible pattern with all three protocols. The other three isolates showed degraded DNA on all lanes far beyond the 15-kb band of the marker.
The ribotyping results obtained suggest that all 17 of the C. perfringens isolates tested could, theoretically, have been involved in the outbreak. Three C. perfringens isolates with three different ribopatterns were detected in foods. Ribopattern 1 (Table 1 and Fig. 1) was detected in isolates that most probably represent the outbreak, as it was found in minced beef heart, six stool samples from different patients, and various autopsy materials of the two deceased patients. These findings, together with the positive CPE results, give a strong hint that the food isolate with pattern 1 was responsible for the outbreak. Ribopattern 2 was also detected in the minced beef heart isolate but only in one autopsy isolate from one patient. Furthermore, these two isolates were CPE negative. Therefore, these C. perfringens isolates can be classified as not involved in the outbreak.
Ribopattern 3 was exclusively detected in the meat dish Wilderertöpfchen. It is unlikely to be involved in the outbreak, because it was not found in any isolate originating from the patients. Ribopattern 4 was detected in four isolates of autopsy material from the two deceased patients. It is remarkable that these two individuals carried isolates with this same ribopattern. Nevertheless, no obvious connection with the outbreak can be stated and the CPE result was also negative.
As PFGE gave no interpretable results, the use of another DNA-based typing technique would be helpful in evaluating the present results of these isolates.
When ribotyping and PFGE are compared in this context, it is remarkable that 1 single C. perfringens isolate (no. 653) of 17 yielded results after repeated PFGE in three different laboratories. This shows that the use of PFGE, which is regarded as superior to ribotyping, can imply difficulties in the investigation of the genetic relationships of certain isolates involved in epidemic situations. It can be concluded that the current protocols for restriction fragment length polymorphism-PFGE seem to be not necessarily applicable to all strains of C. perfringens.
Besides the standard PFGE protocol, two special nuclease-inhibiting procedures were used in our study. Neither of them could differentiate all of the isolates beyond the results obtained with the standard protocol. This shows, in accordance with many researchers who have reported problems with Clostridium sp. DNA degradation (1, 8, 9, 10, 16, 19), that modified PFGE protocols with improved protection against endonucleases activity need to be developed. However, few results concerning the PFGE typeability of C. perfringens strains involved in food-borne diseases have been published (16; Klein et al., Proc. 37th Arbeitstagung Arbeitsgebietes Lebensmittelhyg.). The results of this study show that PFGE with the current standard protocols is insufficient for reliable differentiation of C. perfringens isolates from epidemiological investigations, e.g., in food-borne disease outbreaks, and that ribotyping is still a helpful tool for the characterization of certain isolates.

ACKNOWLEDGMENTS
We thank P. Graf, Department of Health and Environment, Municipal
Authority, Munich, Germany, and H. Beck, Department of Health
Service, South Bavaria, Oberschleissheim, Germany, for providing
data concerning the outbreak and isolates.

FOOTNOTES
* Corresponding author. Mailing address: Institute for Hygiene and Technology of Food of Animal Origin, Veterinary Faculty, Ludwig-Maximilians University Munich, Veterinärstr. 13, 80539 Munich, Germany. Phone: 49-89-2180-2525. Fax: 49-89-2180-3872. E-mail:
Barbara.Schalch{at}lmhyg.vetmed.uni-muenchen.de.


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Journal of Clinical Microbiology, February 2003, p. 892-895, Vol. 41, No. 2
0095-1137/03/$08.00+0 DOI: 10.1128/JCM.41.2.892-895.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
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