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Journal of Clinical Microbiology, December 2004, p. 5582-5587, Vol. 42, No. 12
0095-1137/04/$08.00+0 DOI: 10.1128/JCM.42.12.5582-5587.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Pulsed-Field Gel Electrophoresis Study of Mycobacterium abscessus Isolates Previously Affected by DNA Degradation
Yansheng Zhang,1*
Mitchell A. Yakrus,2
Edward A. Graviss,3
Natalie Williams-Bouyer,3
Christine Turenne,4
Amin Kabani,4 and
Richard J. Wallace Jr.1
Department of Microbiology, University of Texas Health Center, Tyler, Texas,1
National Center for Infectious Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia,2
Department of Pathology, Baylor College of Medicine, Houston, Texas,3
National Reference Center for Mycobacteriology, National Microbiology Laboratory, Health Canada, Winnipeg, Manitoba, Canada4
Received 6 February 2004/
Returned for modification 17 March 2004/
Accepted 25 August 2004

ABSTRACT
DNA degradation (which results in a smear pattern) occurs with
almost 50% of
Mycobacterium abscessus strains during pulsed-field
gel electrophoresis (PFGE). We assessed the potential benefit
of using thiourea-containing buffer with
M. abscessus by studying
69 isolates not previously typeable by PFGE (i.e., those with
a smear pattern). Random (epidemiologically unrelated) isolates
that were typeable (no smear pattern) were included as controls.
Genomic DNA was digested with DraI, XbaI, and AseI. PFGE gels
were run in regular gel buffer with and without 100 µM
thiourea. All 69 isolates that generated smear patterns had
clear band profiles when the thiourea buffer was used. These
isolates were divided into only 30 patterns with DraI, 20 patterns
with XbaI, and 20 patterns with AseI. The molecular profiles
were all closely or possibly related, and the differences between
the isolates ranged from zero to six bands. By multilocus enzyme
electrophoresis (MEE), 45 of 53 smear isolates (85%) belonged
to two closely related electrophoretic types. These isolates
contained at least one enzyme allele seen almost exclusively
in this group. Isolates without smear patterns were unaffected
by thiourea and produced unrelated PFGE profiles, as well as
multiple MEE types. The
hsp65 and 16S rRNA gene sequences of
the isolates with smear patterns were identical to those of
M. abscessus type strain ATCC 19977, which had a nonsmear pattern,
suggesting that this clone is a subgroup within
M. abscessus.
This demonstrates that the inability to type
M. abscessus by
PFGE is associated with a single clone of organisms.

INTRODUCTION
Mycobacterium abscessus is a species of rapidly growing mycobacteria.
It most commonly causes wound infections, abscess formation,
and chronic pulmonary disease (
23). Health care setting-associated
outbreaks or pseudo-outbreaks due to the species have been reported
with increasing frequencies since the mid-1970s (
19,
21,
22,
27). Repetitive insertional sequences have not yet been recognized
in
M. abscessus, and usable probes are still unknown. Plasmids
are also present in only some strains (
5). Pulsed-field gel
electrophoresis (PFGE) analysis of genomic DNA is the standard
molecular technique for epidemiologic studies of the species
(
22). Unfortunately, approximately 50% of
M. abscessus strains
lyse spontaneously during electrophoresis, resulting in a smear
pattern, and these strains cannot be assessed by PFGE (
22).
Random amplified polymorphic DNA PCR (RAPD-PCR) has been used
as an alternative method for these strains (
27). However, this
method often produces highly similar profiles for clinically
unrelated isolates. It is much more method dependent than PFGE
and generally requires the use of multiple primers to differentiate
strains. The potential for the misinterpretation of the results
obtained by RAPD-PCR is also greater than the potential for
misinterpretation of those obtained by PFGE (
27).
In the early 1990s, it was observed that the DNA of two Streptomyces species contained site-specific modifications which undergo Tris-dependent strand scission during gel electrophoresis (3, 8, 28). The use of non-Tris-containing HEPES gel running buffer or the addition of 50 µM thiourea in Tris-containing buffer may prevent DNA degradation (3, 8, 9, 28). Recently, these methods were reported to have been used with a number of bacterial species to prevent DNA degradation (1, 6, 10, 11).
We applied the technique of adding thiourea into regular Tris-borate-EDTA (TBE) gel running buffer to study 69 M. abscessus isolates which had previously given smeared PFGE profiles with regular buffer (referred to as "smear isolates") (22). Some of the isolates (mainly isolates from clinical outbreaks) were previously studied by RAPD-PCR because of their inability to be differentiated by PFGE typing (27).

MATERIALS AND METHODS
Strains.
Sixty-nine clinical
M. abscessus isolates not previously typeable
by PFGE were chosen. This included 49 random (epidemiologically
unrelated) isolates from 18 states within the United States
(1 isolate from Arkansas, 1 from Alabama, 2 from California,
1 from Colorado, 1 from Connecticut, 1 from Georgia, 1 from
Illinois, 3 from Indiana, 3 from Louisiana, 1 from Massachusetts,
3 from Michigan, 2 from Minnesota, 3 from North Carolina, 1
from Ohio, 3 from Pennsylvania, 2 from Rhode Island, 19 from
Texas, and 1 from Utah) and 20 isolates from four separate outbreaks
in Florida, Texas (two outbreaks), and one foreign country (Colombia)
(
19,
21,
27). Random smear isolates were from (i) patients treated
at the University of Texas Health Center at Tyler, (ii) among
those isolates submitted to the Mycobacteria/Nocardia Reference
Laboratory of the University of Texas Health Center at Tyler
for antimicrobial susceptibility testing, or (iii) among those
isolates submitted to the Mycobacteriology Reference Laboratory,
National Center for Infectious Diseases, Centers for Disease
Control and Prevention (Atlanta, Ga.). The outbreak isolates
were provided for epidemiological study by individual investigators.
Three of the four outbreaks have been described previously (
19,
21,
27), while the fourth outbreak in Texas has not been described.
A total of 15 random (epidemiologically unrelated) isolates
that were typeable by PFGE were included as PFGE study controls.
Thirty-one random (epidemiologically unrelated) isolates that
were typeable by PFGE were included as multilocus enzyme electrophoresis
(MEE) controls. The type strain of
M. abscessus (ATCC 19977)
was kindly provided by the American Type Culture Collection,
Manassas, Va.
Species identification of the isolates was performed by published standard growth and biochemical methods and antimicrobial susceptibility patterns (12, 16, 26). Of the 69 isolates, 27 seen after 1995 were also identified by PCR-restriction enzyme analysis (PRA) of the 441-bp Telenti fragment of the hsp65 gene (13, 17, 24, 26). Susceptibility testing was performed by the broth microdilution method by use of the recently published NCCLS standards (4, 25). Isolates had been stored at 70°C in tryptic soy broth with 15% glycerol following isolation.
PFGE.
Analysis of large restriction fragment profiles by PFGE was carried out as described previously (22) with all isolates included in this study. Briefly, organisms cast into low-melting-point agarose plugs were lysed with lysozyme, sodium dodecyl sulfate, and proteinase K. The genomic DNA contained in the plugs was digested with restriction endonucleases DraI, XbaI, and AseI and separated by PFGE with a CHEF Mapper system (Bio-Rad Laboratories, Richmond, Calif.). The gel photos were scanned and analyzed by using Advanced Quantifier 1-D Match software (Bio Image, Ann Arbor, Mich.). PFGE gels were run in regular 0.5x TBE buffer, as well as 0.5x TBE buffer plus 100 µM thiourea. All PFGE procedures, including DNA preparation and restriction endonuclease digestion, were unchanged except for the addition of 50 or 100 µM thiourea to the gel running buffer.
Differences between PFGE profiles were determined, and strain relatedness was determined by use of a modification of the definitions recommended by Tenover et al. (18) (with the recognition that these definitions were designed for studies of outbreaks, not population studies, and were not examined for their validity with mycobacterial species). Isolates were considered indistinguishable, closely related, or possibly related if they exhibited zero, two to three, or four to six band differences with all three enzymes, respectively. In all these circumstances the isolates were considered clonal. Isolates were considered not related if they exhibited seven or more band differences with all three enzymes.
MEE.
Five enzyme systems (6-phosphogluconate dehydrogenase, glutamate oxalacetic transaminase, adenylate kinase, phosphoglucose mutase, and esterase) which exhibited allelic differences for M. abscessus isolates were studied by MEE as described previously (26). Forty-four of the 69 isolates included in this study were evaluated.
hsp65 gene sequencing.
A 360-bp portion of the hsp65 gene was sequenced as described previously (7, 14, 15) by using an Applied Biosystems (Foster City, Calif.) model 377 sequencer. Nine of 16 isolates with smear profiles by PFGE with regular buffer and seven isolates with clear profiles by PFGE were analyzed.
16S rRNA gene sequencing.
The 16S rRNA gene sequences of two selected smear isolates were sequenced as described previously (20), and the results were compared to those obtained with type strain ATCC 19977.

RESULTS
Strains.
All isolates were identified as
M. abscessus, with no differences
between smear and nonsmear isolates in their growth, biochemical
reactions, or drug susceptibility patterns. All smear and nonsmear
isolates had the type I restriction pattern by PRA of the 441-bp
Telenti fragment of the
hsp65 gene sequence (
2).
PFGE.
The use of 50 µM thiourea produced only a partial reduction in DNA smearing. All 69 isolates with smear patterns with regular buffer produced band profiles as clear as those for isolates that were typeable with regular buffer when the smear isolates were run with the buffer containing 100 µM thiourea (Fig. 1). Isolates from three of the four outbreaks each gave indistinguishable PFGE profiles, while isolates from the remaining outbreak had two XbaI profiles with only one band difference between the two.
The results for 1 isolate from each of the three outbreaks which
had indistinguishable PFGE profiles and the 2 isolates from
the other outbreak which had two PFGE profiles, along with the
49 random isolates, were then compared in detail. These 54 isolates
were divided into only 30 profiles by DraI, 20 profiles by XbaI,
and 20 profiles by AseI (Table
1). DraI digestion produced more
easily discernible profiles. The profiles were all indistinguishable,
closely related, or possibly related (Fig.
2). The differences
between the genotypes of a selected reference strain from one
Texas outbreak and the other isolates mostly ranged from zero
to six bands. Only one isolate had a seven-band difference with
DraI, one isolate presented a seven-band difference with XbaI,
and three isolates yielded seven-band differences with AseI.
No single isolate gave seven-band differences with all three
restriction enzymes, and hence, all smear isolates were considered
related. The molecular profiles of the isolates from four outbreaks
in two U.S. states (Florida and Texas) and Colombia were also
closely related. By AseI digestion, isolates from outbreaks
in Texas (Sherman, Texas), Florida, and Colombia were indistinguishable
and differed by only three bands from an isolate from a second
outbreak in Texas.
Fifteen nonsmear isolates, including ATCC 19977, gave unrelated
PFGE profiles which all differed by more than seven bands with
all three enzymes. These profiles are shown in Fig.
3 and are
categorized as unrelated.
The isolates typeable with regular buffer showed the same PFGE
profile with thiourea-containing buffer (Fig.
1).
MEE.
The 44 smear isolates of M. abscessus gave highly similar MEE profiles (Table 2). They were separated into only six electrophoretic types (ETs) (ETs 1 and 13 to 17). Only 2 of 31 (6%) nonsmear control isolates belonged to these ETs. A total of 37 of the 44 (84%) smear isolates belonged to ETs 13 and 14, which were two closely related ETs (Fig. 4). The smear isolates contained a 6-phosphogluconate dehydrogenase enzyme allele (allele 1 in Table 2), which was seen almost exclusively in this group, while the nonsmear isolates had multiple alleles that were seen almost exclusively with the nonsmear isolates. Twenty-nine of the 31 isolates with nonsmear PFGE profiles were separated into 19 other ETs. This also suggested that the smear group of isolates was clonal.
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TABLE 2. M. abscessus enzyme profiles for 44 isolates with PFGE smear patterns and 31 isolates with nonsmear patterns with regular buffer
|
hsp65 gene sequencing.
Nine smear isolates of
M. abscessus and seven nonsmear control
isolates had identical
hsp65 gene sequences which matched (100%)
the sequence of the nonsmear
M. abscessus type strain, ATCC
19977.
16S rRNA gene sequencing.
The 16S rRNA gene sequences of the two smear isolates of M. abscessus showed 100% identity with type strain ATCC 19977 when the nearly complete 16S rRNA gene (1,463 bp) was sequenced.

DISCUSSION
We have been conducting epidemiologic and clinical studies with
M. chelonae,
M. abscessus, and
M. fortuitum isolates for many
years. Strain comparison has relied on PFGE as the standard.
However, the genomic DNA of approximately 50% of
M. abscessus strains and 10% of
M. chelonae strains lyses spontaneously during
gel electrophoresis, resulting in smeared profiles when PFGE
is performed (
6,
22,
27). Different cell treatment procedures
and DNA digestion protocols were applied to prevent DNA degradation,
but with little to no success. Recently, Römling and Tümmler
(
10) reported that more than 50
Pseudomonas aeruginosa strains
which were previously affected by DNA degradation produced macrorestriction
profiles with no background DNA smearing with the addition of
50 µM thiourea to the Tris-containing buffer. Corkill
et al. (
1) found that genomic DNA was degraded in all PCR ribotype
1 isolates of
Clostridium difficile in the absence of thiourea,
while band profiles were observed when thiourea was present.
In our study the addition of thiourea to the TBE gel running
buffer resolved the problem of DNA degradation in
M. abscessus.
Isolates previously affected by DNA degradation gave PFGE profiles
as clear as those not previously affected. The addition of thiourea
had no effect on strains previously typeable with regular buffer.
The use of HEPES buffer has also been recommended for the prevention of DNA degradation (3, 6, 8). We applied and tested this method. With HEPES buffer, however, PFGE gels must run for a longer time to keep the current within the normal range because of the buffer's higher ionic strength, and the bands in the gels were not as clear as those run in gels with thiourea-containing TBE buffer (Y. Zhang, unpublished observations). However, thiourea is a carcinogenic agent, on the basis of the results of animal studies, and special caution should be taken when handling this agent.
The 69 clinical M. abscessus smear isolates included in this study were from 18 states within the United States and one foreign country (Colombia). These isolates were divided into only 30 characterization patterns with DraI, 20 patterns with XbaI, and 20 patterns with AseI. Compared with the profiles for random M. abscessus isolates previously typeable, which produced highly diverse (unrelated) restriction profiles (22), the PFGE profiles of random smear isolates showed that they were all closely or possibly related (Table 1). The large restriction fragment differences between isolates generally ranged from zero to six bands. Thus, M. abscessus isolates requiring thiourea for PFGE represent a single clone by PFGE and MEE characterization, even though they were from a wide range of sources and geographic locations. This is a surprising observation. It suggests that this inability to type and characterize M. abscessus isolates by PFGE with regular gel running buffer is not a random event but is associated with a closely related clone of organisms. No differences between smear and nonsmear isolates were evident by standard growth and biochemical testing, drug susceptibility patterns, PRA of the hsp65 gene, partial sequencing of the hsp65 gene, and sequencing of the 16S rRNA gene, suggesting that this group of smear organisms is a subgroup within M. abscessus and not a different species.
However, we do not know if the isolates with the same characteristics within other species in which some isolates have smear patterns may form a subspecies. Corkill et al. (1) studied a small number of Clostridium difficile isolates and found that genomic DNA was degraded in all PCR ribotype 1 isolates. As shown by Römling and Tümmler (10), P. aeruginosa strains from different sources affected by DNA degradation had various PFGE profiles. They concluded that DNA degradation is not a clonal trait and that it could be lost by individual isolates of a clone (10). The taxonomic significance of this feature for other species may need further studies.
Isolates from four outbreaks involving thiourea-requiring strains had almost indistinguishable PFGE profiles. Three of the four outbreaks were previously studied by RAPD-PCR and are reported elsewhere (19, 21, 27). RAPD-PCR was used as an alternative method for these strains. However, PFGE analysis is still considered the "gold standard" molecular technique for epidemiologic and clinical microbiological studies of the species. While doing epidemiologic studies, one must keep in mind the fact that thiourea-requiring strains have closely or possibly related PFGE profiles, regardless of their sources. Therefore, a definition more strict than that of Tenover et al. (18) should be considered when interpreting strain relatedness by PFGE. In the previous study (27), RAPD-PCR separated outbreak from random smear isolates most of the time. It would be of interest to reassess these smear isolates now that PFGE analysis is possible.

FOOTNOTES
* Corresponding author. Mailing address: Department of Microbiology, University of Texas Health Center, 11937 U.S. Hwy. 271, Tyler, TX 75708. Phone: (903) 877-7683. Fax: (903) 877-7652. E-mail:
yansheng.zhang{at}uthct.edu.


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Journal of Clinical Microbiology, December 2004, p. 5582-5587, Vol. 42, No. 12
0095-1137/04/$08.00+0 DOI: 10.1128/JCM.42.12.5582-5587.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
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