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Journal of Clinical Microbiology, August 2000, p. 3131-3132, Vol. 38, No. 8
0095-1137/00/$04.00+0
LETTERS TO THE EDITOR
Evaluation of the BACTEC MGIT 960 and MB BAC/T Systems for
Routine Detection of Mycobacterium tuberculosis
 |
LETTER |
Dr. Alcaide and colleagues have recently reported an evaluation of
the BACTEC MGIT 960 (Becton Dickinson) and MB/BacT (Organon Teknika)
systems for recovery of mycobacteria from clinical specimens (1). We have recently completed a similar evaluation at the University College Hospital Galway.
Our evaluation was conducted on 493 specimens submitted between
November 1999 and February 2000. There were 365 respiratory tract
specimens and 128 nonrespiratory tract specimens (including pleural
fluid, urine, and cerebrospinal fluid). Sputum samples were predigested
with an equal volume of Sputasol (Oxoid) following centrifugation at
3,000 × g for 15 min. A slide was prepared from the
sediment for microscopy, and the remaining sediment was decontaminated with 4% aqueous NaOH for 20 min followed by neutralization with 14%
KH2PO4. Following centrifugation, the sediment
was resuspended in approximately 1 ml of neutralization buffer and 0.5 ml of material was inoculated into each of the two systems.
Bronchoalveolar lavage specimens were treated in a similar manner.
Urine specimens were concentrated by centrifugation, and the deposit
was decontaminated with 2.5 ml of 5% sulfuric acid for 30 min. Other
contaminated specimens were decontaminated with 4% NaOH followed by
neutralization. Slides for microscopy were stained with Auramine O. Antimicrobials were added to the culture vials prior to inoculation
(PANTA for BACTEC MGIT 960 and MAS [MB/BACT antibiotic supplement]
for MB/BacT). Cultures were incubated for 6 weeks. From positive
cultures, a smear was stained by the Ziehl-Neelsen method.
Chocolate agar media were inoculated to check for contamination with
bacteria other than mycobacteria.
Isolates of mycobacteria were identified by conventional methods
(2, 3). In total, 18 isolates (18 in MB/BacT; 16 in BACTEC
MGIT 960) of M. tuberculosis were obtained. The 16 specimens positive in both systems were from the respiratory tract and were positive on microscopy. The two isolates detected only in the MB/BacT
system after 28 days of incubation were pleural biopsy and pleural
fluid specimens (same patient) and were negative on microscopy. These
results are consistent with those of Alcaide et al., indicating no
significant difference in the isolation rate of M. tuberculosis between the BACTEC MGIT 960 and the MB/BacT systems
(1).
Four MOTT (mycobacteria other than M. tuberculosis) isolates
(one M. kansasii and three Mycobacterium avium-M.
intracellulare) were detected. The M. kansasii isolate
was detected only in the BACTEC MGIT 960 system; the M. avium-M.
intracellulare isolates were detected in both systems. The numbers
of MOTT isolates are small, and we note that Alcaide et al. found that
the MB/BacT was significantly better than the BACTEC MGIT 960 at
isolating M. kansasii.
The mean times to detection (TTD) of a positive culture of M. tuberculosis were 8.5 days (range, 4 to 23 days) in the BACTEC MGIT 960 system and 13.4 days (range, 2 to 39 days) in the MB/BacT system. This is consistent with the finding of Alcaide et al. that the
mean TTD is significantly shorter in the BACTEC MGIT 960 than in the
MB/BacT culture system. In this study, the contamination rate was 8.5%
in the BACTEC MGIT 960 system, similar to the results described in
previous reports (5). The contamination rate of 25% in the
MB/BacT culture system is high relative to the results of Alcaide et
al. and others (1, 4) and resulted in contamination with
staphylococci or streptococci in 5 of 18 (28%) cultures positive for
M. tuberculosis. In our experience, both systems are
effective in isolating mycobacteria; however, the BACTEC MGIT 960 system is preferable to the MB/BacT system in particular because of
lower contamination rates and also because of a superior mean TTD for M. tuberculosis. Since much of the contamination was due to
staphylococci and streptococci, it is possible that the routine
addition of vancomycin to the MB/BacT system may reduce the
contamination rate.
| |
FOOTNOTES |
*
Phone: 353 91 544146
Fax: 353 91 524216
E-mail: Martin.Cormican{at}bsi.ie
| |
REFERENCES |
| 1.
|
Alcaide, R.,
M. A. Benitez,
J. M. Escriba, and R. Martin.
2000.
Evaluation of the BACTEC MGIT 960 and MB/BacT systems for recovery of mycobacteria from clinical specimenc and for species identification by DNA AccuProbe.
J. Clin. Microbiol.
38:398-401[Abstract/Free Full Text].
|
| 2.
|
Collins, C. H.,
J. M. Grange, and M. D. Yates.
1985.
Organization and practice in tuberculosis bacteriology.
Butterworth Ltd., London, United Kingdom.
|
| 3.
|
Collins, T., and P. N. Levett.
1989.
Radiometric studies on the use of selective inhibitors in the identification of Mycobacterium spp.
Med. Microbiol.
30:175-181.
|
| 4.
|
Roggenkamp, A.,
M. W. Hornef,
A. Masch,
B. Aigner,
I. B. Autenrieth, and J. Heesemann.
1999.
Comparison of MB/BacT and BACTEC 460 TB systems for recovery of mycobacteria in a routine diagnostic laboratory.
J. Clin. Microbiol.
37:3711-3712[Abstract/Free Full Text].
|
| 5.
|
Tortoli, E.,
P. Cichero,
C. Piersimoni,
M. Tullia Simonetti,
G. Gesu, and D. Nista.
1999.
Use of the BACTEC MGIT 960 for recovery of mycobacteria from clinical specimens: multicenter study.
J. Clin. Microbiol.
37:3578-3582[Abstract/Free Full Text].
|
| | | | |
Tom Whyte
Belinda Hanahoe
Tom Collins
Department of Medical Microbiology
University College Hospital Galway
Galway, Ireland
|
| | | | |
Geraldine Corbett-Feeney
Martin Cormican*
Department of Bacteriology
National University of Ireland
Galway, Ireland
|
| |
AUTHORS' REPLY |
The findings of Dr. Whyte and colleagues are similar to the results we
reported previously for the detection of Mycobacterium tuberculosis by the BACTEC MGIT 960 and MB/BacT systems
(1). Although no significant differences in the isolation
rate of M. tuberculosis were found by Whyte et al., the
MB/BacT system was better at isolating this species (100%) than the
BACTEC MGIT 960 system (88.9%). Surprisingly, only two M. tuberculosis isolates were obtained from smear-negative specimens,
and they were only recovered in the MB/BacT system. This fact might
explain the short mean time to detection observed in this study,
especially with the MGIT 960.
The contamination rate was the greatest difference between the study of
Whyte et al. and other comparison studies (1, 2, 5). A
bacterial overgrowth rate of 
9% in the MB/BacT system was initially
reported when the original antibiotic supplement was used
(4). Since 1998, a revised supplement with vancomycin has
been introduced by the manufacturer, and it has practically resolved
this problem. However, the contamination rates obtained in the study of
Whyte et al. are among the highest reported for the MB/BacT system
(25%). This wide variation with the results of other studies may
reflect the different antibiotic supplement and
digestion-decontamination procedure used by the authors. Interestingly, despite the high rate of contamination found by Whyte et al., the
MB/BacT system showed a better recovery rate for M. tuberculosis than the MGIT 960 system did. In our experience with
3,823 clinical specimens collected between July 1999 and April 2000, the contamination rate was 4.2% for the MB/BacT system and the
percentage of positive cultures was >7.7%. We have followed the
conventional N-acetyl-L-cysteine-NaOH digestion-decontamination procedure (3), and the MB/BacT
antibiotic supplement was added only to the bottles for culture of
nonsterile specimens, as recommended by the manufacturer. Therefore, we
think the bacterial overgrowth in the MB/BacT system is not, at
present, a significant problem.
In conclusion, both the MGIT 960 and the MB/BacT are efficient systems
for the isolation of mycobacteria in a clinical laboratory, which could
replace the radiometric method.
| |
FOOTNOTES |
*
Phone: 34-93-2607930
Fax: 34-93-2607547
E-mail: falcaide{at}csub.scs.es.
| |
FOOTNOTES |
| |
REFERENCES |
| 1.
|
Alcaide, F.,
M. A. Benítez,
J. M. Escribà, and R. Martín.
2000.
Evaluation of the BACTEC MGIT 960 and the MB/BacT systems for recovery of mycobacteria from clinical specimens and for species identification by DNA AccuProbe.
J. Clin. Microbiol.
38:398-401.
|
| 2.
|
Hanna, B. A.,
A. Ebrahimzadeh,
L. B. Elliott,
M. A. Morgan,
S. M. Novak,
S. Rusch-Gerdes,
M. Acio,
D. F. Dunbar,
T. M. Holmes,
C. H. Rexer,
C. Savthyakumar, and A. M. Vannier.
1999.
Multicenter evaluation of the BACTEC MGIT 960 system for recovery of mycobacteria.
J. Clin. Microbiol.
37:748-752[Abstract/Free Full Text].
|
| 3.
|
Metchock, B. G.,
F. S. Nolte, and R. J. Wallace, Jr.
1999.
Mycobacterium, p. 399-437.
In
P. R. Murray, E. J. Baron, M. A. Pfaller, F. C. Tenover, and R. H. Yolken (ed.), Manual of clinical microbiology, 7th ed. ASM Press, Washington, D.C.
|
| 4.
|
Rohner, P.,
B. Ninet,
C. Metral,
S. Emler, and R. Auckenthaler.
1997.
Evaluation of the MB/BacT system and comparison to the BACTEC 460 system and solid media for isolation of mycobacteria from clinical specimens.
J. Clin. Microbiol.
35:3127-3131[Abstract].
|
| 5.
|
Tortoli, E.,
P. Cichero,
C. Piersimoni,
M. T. Simonetti,
G. Gesu, and D. Nista.
1999.
Use of BACTEC MGIT 960 for recovery of mycobacteria from clinical specimens: multicenter study.
J. Clin. Microbiol.
37:3578-3582.
|
| | | | |
Fernando Alcaide*
Miguel Angel Benítez
Rogelio Martín
Servei de Microbiologia
Ciutat Sanitària i Universitària de Bellvitge
C/Feixa Llarga s/n
08907 L'Hospitalet de Llobregat
Barcelona, Spain
|
| | | | |
Josep M. Escribà
Cap de Drassanes
Institut Catalá de la Salut
Barcelona, Spain
|
Journal of Clinical Microbiology, August 2000, p. 3131-3132, Vol. 38, No. 8
0095-1137/00/$04.00+0
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