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Journal of Clinical Microbiology, June 1998, p. 1653-1659, Vol. 36, No. 6
0095-1137/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Assessment of Resolution and Intercenter Reproducibility of
Results of Genotyping Staphylococcus aureus by Pulsed-Field
Gel Electrophoresis of SmaI Macrorestriction
Fragments: a Multicenter Study
Alex
van
Belkum,1,*
Willem
van
Leeuwen,1
Mary Elizabeth
Kaufmann,2
Barry
Cookson,2
Françoise
Forey,3
Jerome
Etienne,3
Richard
Goering,4
Fred
Tenover,5
Christine
Steward,5
Frances
O'Brien,6
Warren
Grubb,6
Panayotis
Tassios,7
Nicholas
Legakis,7
Anne
Morvan,8
Névine
El
Solh,8
Raf
de
Ryck,9
Marc
Struelens,9
Saara
Salmenlinna,10
Jaana
Vuopio-Varkila,10
Mirjam
Kooistra,11
Adriaan
Talens,11
Wolfgang
Witte,12 and
Henri
Verbrugh1
Erasmus Medical Center Rotterdam,
Department of Medical Microbiology & Infectious Diseases, 3015 GD
Rotterdam, The Netherlands1;
Central Public Health Laboratory, London NW9 5HT, United
Kingdom2;
Laboratoire de Microbiologie,
Hôpital Edouard Herriot, 69437 Lyon Cedex 3, France3;
Creighton University School of
Medicine, Department of Medical Microbiology, Omaha, Nebraska
681784;
Nosocomial Pathogens Laboratory
Branch, Centers for Disease Control and Prevention, Atlanta,
Georgia 303335;
School of Biomedical
Sciences, Curtin University of Technology, Perth 6845, Australia6;
Department of Microbiology,
Medical School, National University of Athens, 115 27 Athens,
Greece7;
Institut Pasteur, Centre
National de Référence des Staphylocoques, 75724 Paris Cedex 15, France8;
Department
of Microbiology, Hôpital Erasme, 1070 Brussels,
Belgium9;
National Public Health
Institute, 00300 Helsinki, Finland10;
Regional Laboratory for Public Health, 9700 RM Groningen,
The Netherlands11; and
Robert Koch
Institute BGA, D-38855 Wernigerode, Germany12
Received 4 December 1997/Returned for modification 20 January
1998/Accepted 17 March 1998
 |
ABSTRACT |
Twenty well-characterized isolates of methicillin-resistant
Staphylococcus aureus were used to study the optimal
resolution and interlaboratory reproducibility of pulsed-field gel
electrophoresis (PFGE) of DNA macrorestriction fragments. Five
identical isolates (one PFGE type), 5 isolates that produced related
PFGE subtypes, and 10 isolates with unique PFGE patterns were analyzed
blindly in 12 different laboratories by in-house protocols. In
several laboratories a standardized PFGE protocol with a commercial kit was applied successfully as well. Eight of the centers correctly identified the genetic homogeneity of the identical isolates by both
the in-house and standard protocols. Four of 12 laboratories failed to
produce interpretable data by the standardized protocol, due to
technical problems (primarily plug preparation). With the five
related isolates, five of eight participants identified the same subtype interrelationships with both in-house and standard protocols. However, two participants identified multiple strain types in this group or classified some of the isolates
as unrelated isolates rather than as subtypes. The remaining laboratory
failed to distinguish differences between some of the related
isolates by utilizing both the in-house and standardized protocols.
There were large differences in the relative genome lengths of the
isolates as calculated on the basis of the gel pictures. By visual
inspection, the numbers of restriction fragments and overall banding
pattern similarity in the three groups of isolates showed
interlaboratory concordance, but centralized computer analysis of data
from four laboratories yielded percent similarity values of only 85%
for the group of identical isolates. The differences between the data sets obtained with in-house and standardized protocols could be the
experimental parameters which differed with respect to the brand of
equipment used, imaging software, running time (20 to 48 h), and
pulsing conditions. In conclusion, it appears that the standardization
of PFGE depends on controlling a variety of experimental intricacies,
as is the case with other bacterial typing procedures.
 |
INTRODUCTION |
The use of electric field
pulsing techniques in conjunction with agarose gel
electrophoresis for discrimination of large DNA molecules was
introduced by Schwarz and Cantor in 1984 (9). During the
past decade the methodology has been adapted and improved by various
research groups to the point that pulsed-field gel electrophoresis (PFGE) for bacterial strain typing is now
utilized with relative ease in a variety of laboratories
(1). The combination of contour-clamped homogeneous
field electrophoresis and PFGE for the molecular analysis of
Staphylococcus aureus has been reported since the late 1980s
(7, 19). At present, PFGE is considered to have
both the reproducibility and resolving power of a standard technique
for the epidemiological typing of bacterial isolates (10,
15).
Molecular typing systems can identify different strains within a
species, generating data useful for taxonomic or epidemiologic purposes
(10, 14). A frequently observed shortcoming of typing systems in general is their lack of reproducibility: most typing systems do not provide a definitive strain identification, which is
usually due to the variability of the technique and the lack of large databases containing fragment patterns from a wide variety of
organisms to which unknowns can be compared. These problems were
recently described in detail for two molecular typing systems. A
multicenter study on random amplification of polymorphic DNA for
discrimination of S. aureus strains revealed a lack of
interlaboratory reproducibility among the banding patterns generated by
the participating centers, although the epidemiological interpretation
of the data was similar for all the centers involved (16).
For PFGE, a similar lack of interlaboratory reproducibility of patterns
was observed, although the interpretation of the experimental data also
differed per participating center (2). The latter study
analyzed 12 different methicillin-resistant S. aureus
(MRSA) strains with different techniques optimized in each center and
different sources and types of equipment. Since interlaboratory
discrepancies with respect to classification of the strains were
observed, the study concluded that there is a clear need for
standardization of the technique, including the construction of a panel
of reference strains to assist the individual researcher in the
optimization of the PFGE protocol.
The aim of the present study was to compare the fragment patterns of a
well-defined collection of MRSA isolates in 12 laboratories using
in-house and a standard set of PFGE parameters to determine whether
standardization of experimental parameters (DNA preparation and
switching protocols) would improve intercenter reproducibility of PFGE
analysis.
 |
MATERIALS AND METHODS |
Study design.
Twenty isolates of MRSA were selected for
analysis by Wolfgang Witte (Wernigerode, Germany). The collection was
composed of 10 genetically unrelated isolates, 5 isolates exhibiting
similar but not identical PFGE fingerprints, and 5 isolates with
indistinguishable PFGE patterns. The original S. aureus
NCTC 8325 was provided by Richard Goering (Omaha, Neb.). This strain
served as a source for molecular size standards together with
concatameric lambda DNA molecules. Isolates were stored in the
coordinating center (EMCR, MM&ID, Rotterdam, The Netherlands), and
cultures were coded and distributed in agar stabs to the participating
centers. Prior to PFGE analysis, isolates were cultured on blood agar
plates at least once, and single colonies were used for further
testing.
PFGE with the restriction enzyme SmaI was performed in
duplicate on DNA from all isolates in all centers with the equipment available in the individual laboratories. The lambda concatamers were
to be run in every sixth lane (i.e., five isolates included between the
two sets of markers), although not all laboratories complied with this
aspect of the protocol. DNA preparation was performed according to the
in-house protocols of each laboratory. In addition, all participants
analyzed the isolates by using a recently developed, commercially
available PFGE kit (Genepath; Bio-Rad, Veenendaal, The Netherlands)
(GP), which contained all ingredients for both plug and gel
preparation. Each laboratory performed PFGE by using their in-house
switching protocol and PFGE equipment as summarized in Table
1. PFGE of DNA prepared by the commercial
kit was performed as follows: initial switching time, 5.3 s; final
switching time, 34.9 s; run time, 20 h; 6 V/cm; 120° angle;
14°C. All gels were stained with ethidium bromide and photographed
with a Polaroid or charge-coupled device camera. The digital images
were set to a resolution of >500 pixels from well to bottom of the
gel. For the purpose of additional comparison, bacteriophage-typing and
antimicrobial susceptibility testing were performed for all isolates of
the test panel. The isolates were typed by arbitrary primed PCR
(AP-PCR), binary typing, and target 916-Shine-Dalgarno PCR (tar
916-shida PCR) as described previously (3, 16, 18).
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TABLE 1.
Evaluation of the different experimental parameters as
applied in the different participating laboratories
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|
Local data analysis.
All Polaroid pictures or digital images
were interpreted by the individual researchers. Interpretation was
performed on the basis of guidelines for interpreting banding pattern
differences (10, 15). Specific types were identified by
capital letters, with subtypes identified by additional numbers. Each
center analyzed both gels by using its own software package to
calculate Dice coefficients and to generate a dendrogram by UPGMA
(unweighted pair group method using arithmetic averages) clustering.
The in-house data and the data generated by using the GP kit were
interpreted separately, and the agreement between the two methods was
assessed in each center (number of fragments per strain and
classification into types and subtypes based on the overall number of
band differences). For all isolates, the genome size was calculated
based on the cumulative sizes of the restriction fragments observed.
Centralized data analysis.
All pictures and accompanying
interpretations were sent to a second center for interlaboratory
comparison. The tiff images were imported into Bio Image Advanced
Quantifier 1-D Match (AQ) version 2.5 and normalized by using the
lambda concatameric standards on each gel. To evaluate center-to-center
reproducibility, lanes from each normalized image were compared by
using Dice coefficients and a UPGMA-derived dendrogram. For intergel
Advanced Quantifier analysis, the same lambda standards were used. One
of the gels from the five laboratories that sent data for centralized
analysis did not include lambda standards and could not be matched to
the other gels. Another laboratory sent only the in-house image for analysis. Therefore, only four in-house gels and three gels prepared with the commercial kits were analyzed with computer-aided technology.
 |
RESULTS |
General remarks.
Twelve laboratories in nine countries
participated in this study. Although the goal was to compare a
standardized PFGE protocol to in-house PFGE protocols, complete data
sets on the 20 test isolates were not achieved by all laboratories.
Most of the centers found it necessary to modify the commercially
standardized GP protocol. In one instance, the restriction enzyme was
inactive and had to be replaced by that of another manufacturer.
However, most of the problems centered around the deterioration of the agarose plugs during overnight proteinase K treatment at 56°C. The
in-house protocols, which proved to be more effective for typing the
isolates in most centers, are presented in Table 1. Finally, there was
a set order of the strains which did help the reproducibility of data
interpretation.
Local analysis.
Table 2 shows
the visual interpretation of isolate interrelationships based on the
PFGE gels generated by both the standardized and in-house protocols.
Four centers (4, 5, 6, and 12) were not able to generate typing data by
using the standardized protocol. Of the eight laboratories that did
report standardized data, five (1, 2, 8, 9, and 11) correctly
categorized all 20 isolates into the indistinguishable, related, and
unrelated groupings. Laboratories 3, 7, and 10 correctly classified the
indistinguishable and unrelated isolates but misclassified some of the
related subtypes.
Despite widely differing in-house PFGE protocols and variable quality
of the gel pictures (Fig.
1), all of the
laboratories
were capable of correctly discriminating the five
identical isolates
(Fig.
1, lanes 1 to 5) from the unrelated isolates
(Fig.
1, lanes
11 to 20) (Table
2). Two of 12 laboratories
(centers 4 and 7)
were unable to discriminate two of the related
isolates (MRSA
9 and 10), which were identified as
indistinguishable in these
laboratories but were considered related
subtypes in the other
10 laboratories. This may have been the result of
using a pulsing
time of up to 80 s or, more likely, tolerance
settings applied
during the analysis of the data. One laboratory
(center 10) identified
three types and two subtypes among the related
isolates (lanes
5 to 10) by using both the in-house and the
standard protocol.

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FIG. 1.
Comparative analysis of gel pictures obtained after PFGE
of DNA macrorestriction fragments derived from the panel of 20 MRSA
isolates. From top to bottom the experimental outputs of participating
centers 8, 1, and 7 are shown. White arrowheads in the two top panels
highlight potential fragment doublets. Note that these two pictures
clearly overlap with respect to resolution and number of DNA fragments.
The arrowhead in the lower panel identifies a floating plug. Although
the lower panel shows a lesser degree of band resolution, it has to be
emphasized that pattern identification obtained from this gel picture
was as expected except for patterns belonging to isolates 9 and 10. However, the quality of the PFGE profiles shown in the lower panel is
markedly inferior to those in the upper two panels. Numbering above the
lanes corresponds with strain numbers, L identifies the lambda
concatamers and N indicates the macrorestriction pattern generated for
the S. aureus NCTC 8325 reference strain.
|
|
Table
3 shows the molecular size values
as deduced from the experimental data with concatameric (48.5 kbp)
n lambda
DNA and/or
S. aureus NCTC 8325 DNA macrorestriction fragments
as size standards.
In general, the spread in the molecular sizes
was quite large among
centers, and in some instances, the values
for the identical
isolates (1 to 5) differed even within centers
for both the
in-house and standard protocols. The average values
as calculated and
shown in Table
3 demonstrate that the GP procedure
indicates smaller
genome sizes than those calculated on the basis
of results obtained
with the in-house procedures. Note that the
genome size of these
isolates of MRSA seems to vary between 2,153
and 2,768 (in house) or
2,035 and 2,816 (GP) kbp.
Table
4 shows the similarity of the
isolates in the three different clusters as determined in each center
by using Dice coefficients
obtained by using commercial or in-house
software. The number
of bands that the participants detected per group
of isolates
is also indicated. Overall, the clustering of the identical
and
related isolates is well documented in all centers as is the
unrelated
nature of isolates 11 to 20. The in-house procedures produce
similar
fragment numbers for all of the identical isolates with a
single
exception (Table
4, center 7), while the standard protocol was
associated with a wider range of values (12 to 16). Average clustering
values for the indistinguishable isolates were higher for the
in-house
than the standard protocol (99.8 versus 97.3, respectively).
Also, the
average clustering values for the related isolates were
higher with the
in-house than with the GP protocol (87.2 versus
85.9, respectively).
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TABLE 4.
Evaluation of the experimental data obtained by PFGE of
MRSA: pattern similarity (mean values) and range of the total
number of bands per strain by epidemiological group
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|
Centralized analysis.
Gel images from seven data sets, four
in-house gels and three standardized gels, were available for analysis.
The other gel images either did not contain the appropriate lambda
standards or were of insufficient quality for analysis. The gels could
not be analyzed by GelCompar as a single file because the lambda
standards were not sufficient to normalize the gels. Gelcompar was used to assess improvements in data homology upon copying of PFGE
conditions. When the primary data obtained in centers 4 and 8 were
compared, relatively low homology values were calculated. When
center 8 adopted the experimental parameters proposed by center 4, however, the homology between primary data obtained in center 8 and the novel data generated in center 4 increased significantly (results not
shown). This emphasizes the importance of exact experimental standardization. The gels could be analyzed in more detail using the
Bio Image software. Among the gels analyzed, the number of lambda
fragments, which were used for normalization of the gels, varied from 9 to 15 per lane, which hampered the analysis. We focused the analysis on
the first five lanes of the gel images containing the five
indistinguishable isolates. Using the Bio Image software and a 3.5%
band tolerance, which was previously determined at Centers for Disease
Control and Prevention to be optimal for analysis of S. aureus isolates (data not shown), the five indistinguishable
isolates from seven data sets matched at a level of 84% similarity by
using UPGMA clustering (Fig. 2). Both
in-house and GP kit data from center 1 and the in-house data from
center 2 clustered at 100% similarity, while the GP kit and in-house
data from center 3 and the in-house data from center 2 were 97%
similar. These two clusters were linked at 84% similarity. Data from
center 3 for the kit was not available for analysis.

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FIG. 2.
Dendrogram of results of seven data sets, four in-house
(IH) and 3 standardized GP kits (KIT) from four centers (Ctr). The
numbers after the colons are lane numbers. The percent similarity scale
is based on UPGMA clustering of Dice coefficients generated by Bio
Image software.
|
|
Strain characteristics.
The data were obtained by conventional
and genetic analysis of the isolates used for the present study
(results not shown). The binary type (18), the tar
916-shida fingerprints (3), and the AP-PCR characteristics
(16) for isolates 1 through 5 were indistinguishable.
The PFGE subtypes for isolates 6 to 10 appear homogeneous by
AP-PCR and tar 916-shida PCR, confirming their relatedness. Binary
typing shows some heterogeneity among these isolates,
but this is limited to differences detected by a single
probe. AP-PCR discriminates each of the isolates in the final group (11 to 20), although tar 916-shida PCR fails to discriminate isolates 11 and 18 from the cluster of indistinguishable isolates (1 to 5, which
were PFGE identical). For this panel of 20 isolates, both bacteriophage
typing and the antibiogram data are in good agreement with the
genotypic data. Since the study isolates were selected on the basis of
PFGE differences detected in a single laboratory, the additional
typing data were necessary to confirm the interstrain relationships.
 |
DISCUSSION |
Standardization of molecular typing methods is an issue of debate
in the field of medical microbiology. For several microorganisms, such
as Pseudomonas aeruginosa, an array of typing methods has been compared in multiple laboratory studies, leading to suggestions for the appropriate use of molecular data (4, 8, 12). To
date, however, only typing based on IS6110 sequences of
Mycobacterium tuberculosis has been standardized to any
great extent (5, 17). This has resulted in a method and
database capable of importing and analyzing new information generated
by different laboratories around the world. However, this required
several years and the concerted efforts of many individuals and
institutions to achieve. This situation is a rare exception in
microbiology, and efforts in establishing similar systems for other
bacterial species are urgently required. Emerging multiresistant
microorganisms are an important group of species in this context; in
this study, we have attempted to achieve similar results with MRSA.
Several comparative typing studies have been performed for
S. aureus in the recent past (e.g., 2, 6, 11,
13, 16, 18). Some of these studies focused on the detailed
analysis of single techniques, performed in single centers (6, 11, 13, 18), whereas others assessed interlaboratory
reproducibility (2, 16). The study by Cookson et al.
(2) was the first attempt to compare PFGE data from
different laboratories and merge the data into a single file for
analysis. The present study continues this initiative, by utilizing a
set of reference MRSA isolates to investigate interlaboratory
reproducibility of PFGE typing, including the use of standardized DNA
preparation and electrophoretic switching protocols.
In the present study, experimental problems were encountered during
generation of DNA plugs when using the standard protocol. Apparently,
the use of agarose of sufficient heat tolerance is critical for
resolving band differences. The separation of bands could be improved
by altering the pulsing protocol, a parameter which was not optimal in
the current version of the procedure, in which large portions of the
gel were not used. Some of the participating centers modified the
commercial protocol in order to achieve better band separation (see
legend to Table 2). The GP protocol needs to be reexamined critically
and its major deficiencies need to be corrected before it could be
recommended for widespread used.
The in-house procedures performed quite well in all of the
participating laboratories. Despite differences in DNA
preparation methodology, electrophoretic equipment, and electric
current switching times, none of the centers had difficulty in
recognizing the identical and unrelated groups of isolates (1 to
5 and 11 to 20, respectively). Differences between centers were noted,
however, with interpretation of data concerning the related isolates
(numbers 6 to 10). In some instances this was apparently due to
electric current switching protocols which did not clearly
differentiate closely sized (but nonidentical) restriction fragments in
different isolates. However, other differences were not associated with
questions of in-house versus standardized PFGE methodologies but,
instead, were specifically related to the algorithm that individual
investigators employed for pattern interpretation. Investigators were
instructed to employ specific guidelines for assessing isolate
interrelationships (11, 15). A key aspect of this approach,
for the purpose of hospital epidemiology, involves the choice of a
predominant (epidemic) type (occurring more than once in the group) to
which all other patterns are compared (15). Isolates that
differ by three or fewer restriction fragment positions (i.e., up to
six band differences when comparing lanes) are considered subtypes of
this common strain pattern, while organisms differing in four or more
positions are identified as different strain types. With groups of
isolates that are identical or clearly different (i.e., a multitude of differently positioned fragments), application of the algorithm is
straightforward, leading to reproducible interlaboratory interpretation as demonstrated here. However, an interesting aspect of the study design was the inclusion of only one isolate for each of the related isolates, 6 to 10. It was thus left to the investigator to determine which isolate from within this group would represent the standard type
to which the other four would be compared. In addition, two of the
isolates (numbers 6 and 7) exhibited an increased staining intensity of
specific but different restriction fragments which could be interpreted
as a difference between the isolates in comigrating fragment doublets
(Fig. 1). These two features were the reason for the different
relationships as defined in differing centers, even if the PFGE
patterns generated in different laboratories were
indistinguishable. As can be deduced from the results in Fig. 1
(reading left to right), most centers chose isolate 6 as the
standard and each arrived at the same interpretation for the isolates.
Alternative interpretations noted in some instances (Table 2) may
reflect differences in the choice of a standard as well as perceived
differences in restriction fragment positions. These results underscore
the importance of the choice of a predominant PFGE pattern as an
issue separate from that of DNA preparation or the
reproducibility of electrophoretic separation for purposes of
epidemiological interpretation.
An unexpected finding was the fact that data sets generated in
different centers did not lend themselves to numerical analysis by
GelCompar when combined into a single digitized image. This situation
improved when two different in-house protocols were replicated at the
coordinating center, but a 100% homology score was never reached
despite the fact that the visual data appeared highly similar to
that generated by the respective participating center. Different
modes of data processing did not result in the improvement of the
correlation among data sets. This observation may relate to issues
regarding the use of computerized analysis in the comparative
normalization of restriction fragment positions between different gels.
However, data analysis was somewhat more successful with Bio
Image software. When the data sets representing images from four
in-house gels and three standard kit gels were merged, the result was
two clusters of highly related isolates (>97% similarity) linked to
each other at the 84% similarity level. Given the disparity in the
positions of standards and the number of identifiable bands in each
lambda standard lane, this level of similarity suggests that
interlaboratory comparisons are clearly possible, although every
effort should be made to standardize the positioning of standards on
the gels to facilitate comparisons.
At present, it appears that the computerized analysis of
PFGE patterns may be more useful in identifying closely related
or identical strains for further testing or analysis than for reliably establishing differentiating nonrelated strains (Table 4). The differences in the number of DNA fragments that were successfully identified in the different laboratories were most probably related to
differences in the quality of the gel images, rather than the artifactual absence or presence of specific DNA fragments, since a number of laboratories produced apparently identical gel images with
identical numbers of DNA fragments. An important point to make here is
that visual inspection is still an essential complementary procedure to
so-called automated analysis, which subjects objective computerized
analysis to subjective review.
The basis of a scientific method is reproducibility. Different
laboratories performing the same procedure in the same way are
expected to generate the same results. Based on this premise, one
can argue that the present study indicates that standardization of PFGE
typing for MRSA has not yet been achieved. In terms of numerical output
of computerized PFGE analysis this is clearly the case. Any
method involving manipulations performed by hand and data
inspections with an element of human visualization possesses an
inherent potential for at least some degree of variability and bias.
For this reason, absolute numerical standardization of PFGE may never
be achieved. But epidemiological questions commonly involve
answers which are not an absolute "yes" or "no" but,
instead, often involve an assessment of qualitative degrees of
interrelationship. In this context the overall qualitative
similarity of PFGE results observed here, despite a variety of
different in-house DNA preparation procedures, PFGE equipment,
and switching protocols, indicates that continuing efforts to minimize
the variability of these parameters will lead to acceptable numerical
as well as methodological standardization of PFGE procedure and
analysis. As a step in this direction, the difficulties with the
standardized GP protocol noted in this study should be corrected and
reevaluated in a future effort.
 |
ACKNOWLEDGMENTS |
The GP kits used for the standardized part of the present study
were provided free of charge by the Dutch Bio-Rad agency, Veenendaal,
The Netherlands, for which John Kuijpers is gratefully acknowledged. We
thank Loretta Carson at CDC (Atlanta, Ga.) for technical assistance.
The coordinating center was based in the Department of Medical
Microbiology & Infectious Diseases, Erasmus Medical Center Rotterdam,
Rotterdam, The Netherlands. This study was an initiative of the
European Study Group on Epidemiological Markers (ESGEM), which is an
official working party of the European Society of Clinical Microbiology
and Infectious Diseases.
 |
FOOTNOTES |
*
Corresponding author. Mailing address: Erasmus Medical
Center Rotterdam, Dept. Medical Microbiology & Infectious Diseases, Dr.
Molewaterplein 40, 3015 GD Rotterdam, The Netherlands. Phone: 31-10-4635813. Fax: 31-10-4633875. E-mail:
vanbelkum{at}bacl.azr.nl.
 |
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Journal of Clinical Microbiology, June 1998, p. 1653-1659, Vol. 36, No. 6
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Copyright © 1998, American Society for Microbiology. All rights reserved.
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