Laboratory diagnosis of nontuberculous mycobacteria.

In conclusion, it is important to realize that there is no "stand alone" assay for the identification of NTM. Many new species may not be recognized in all assays. Newer molecular tests are more accurate for identification than phenotypic tests and have significantly improved turnaround time. Clinical significance of an isolate should be determined, however, before committing resources for the identification of a mycobacterial isolate to the species level. In addition, there are significant differences in the range and quality of services provided by different laboratories. Today, techniques and equipment are increasingly complex and costly, making it more difficult to upgrade every local laboratory to perform these assays. But because specimen delivery and communication of results can be rapidly and easily achieved, utilization of reference laboratories for rarely performed sophisticated tests is a more practical approach.

Use of molecular methods to identify the Mycobacterium tuberculosis complex (MTBC) and other mycobacterial species and to detect rifampin resistance in MTBC isolates following growth detection with the BACTEC MGIT 960 system.

A prospective study was organized by using a total of 1,585 consecutive clinical specimens to determine whether biomass obtained from positive growth in the MGIT 960 system could be used directly in AccuProbe DNA hybridization tests, the PCR-based Inno-LiPA Rif.TB (LiPA) assay, and a PCR-based DNA sequencing of the rpoB gene for the rapid identification of the Mycobacterium tuberculosis complex (MTBC) and other mycobacterial species and for the determination of rifampin (RIF) resistance in MTBC strains. The results were compared to routine culture, identification, and susceptibility testing techniques performed on the same samples. The study results revealed that the DNA AccuProbe assay (on the day of growth positivity) readily identified 95.7%, the LiPA assay readily identified 98.6%, and rpoB sequencing readily identified 97.1% of the 70 MTBC isolates from mycobacterial growth indicator tubes (MGIT). In addition, application of the LiPA for the identification and RIF susceptibility testing of the MTBC in growth-positive MGIT resulted in a turnaround time of less than 2 weeks after specimen receipt. Although DNA sequencing of rpoB required a slightly longer (16 days) turnaround time, this method was capable of identifying several species of nontuberculous mycobacteria in addition to identifying MTBC and determining RIF susceptibility or resistance. The molecular methods were also found to rapidly identify RIF-susceptible and -resistant MTBC in two of the three mixed mycobacterial cultures weeks earlier than conventional methods. In conclusion, the biomass obtained in MGIT at the time of growth positivity in the 960 system is sufficient for use in all three molecular tests, and this approach can reduce the turnaround time for reporting results.

Rapid and simple approach for identification of Mycobacterium tuberculosis complex isolates by PCR-based genomic deletion analysis.

Although the virulences and host ranges differ among members of the Mycobacterium tuberculosis complex (TBC; M. tuberculosis, M. africanum, M. canettii, M. microti, M. bovis, and M. bovis BCG), commercially available molecular assays cannot differentiate these organisms because of the genetic identities of their 16S rRNA gene sequences. Comparative genomic analyses with the complete DNA sequence of M. tuberculosis H37Rv has provided information on regions of difference (RD 1 to RD 16) deleted in members of the TBC other than M. tuberculosis. To determine whether deletion analysis could accurately differentiate members of TBC, we used PCR to assess the presence or absence of specific regions of the genome in 88 well-characterized isolates of M. tuberculosis, M. africanum, M. microti, M. bovis, and M. bovis BCG. The identifications obtained by use of the specific deletion profiles correlated 100% with the original identifications for all TBC members except M. africanum, but further characterization resulted in profiles specific for all members. Although six RD regions were used in the analyses with the original 88 isolates, it was found that the use of RD 1, RD 9, and RD 10 was sufficient for initial screenings, followed by the use of RD 3, RD 5, and RD 11 if the results for any of the first three regions were negative. When 605 sequential clinical isolates were screened, 578 (96%) were identified as M. tuberculosis, 6 (1%) were identified as M. africanum, 8 (1%) were identified as M. bovis, and 13 (2%) were identified as M. bovis BCG. Since PCR-based assays can be implemented in most clinical mycobacteriology laboratories, this approach provides a rapid and simple means for the differentiation of members of TBC, especially M. bovis and M. tuberculosis, when it is important to distinguish between zoonotic sources (i.e., cattle and unpasteurized dairy products) and human sources of tuberculosis disease.

Method for inactivating and fixing unstained smear preparations of mycobacterium tuberculosis for improved laboratory safety.

The inactivation of smears that contain Mycobacterium tuberculosis for microscopy before removal of the material from a biosafety cabinet is an important safety factor in preventing the potential transmission of tuberculosis to laboratory workers. The fixing and inactivating properties of heat flaming, 70% ethanol, and 1, 3, and 5% phenol in ethanol for smears containing M. tuberculosis were investigated. Heat flaming failed to inactivate the smear material, whereas 5% phenol in ethanol successfully fixed and inactivated all smears containing M. tuberculosis both from concentrated sputum samples and from culture material.