Detection and identification of Mycobacterium directly from BACTEC bottles by using a DNA-rRNA probe

Ocular infections associated with atypical mycobacteria: A review

Atypical mycobacteria or non-tuberculous mycobacteria (NTM) are a group of acid-fast bacteria that are pathogenic to different parts of the eye. The organisms can cause a spectrum of ocular infections including keratitis, scleritis, uveitis, endophthalmitis and orbital cellulitis. Trauma, whether surgical or nonsurgical, has the highest correlation with development of this infection. Common surgeries after which these infections have been reported include laser in situ keratomileusis (LASIK) and scleral buckle surgery. The organism is noted to form biofilms with sequestration of the microbe at different inaccessible locations leading to high virulence. Collection of infective ocular material (corneal scraping/necrotic scleral tissue/abscess material/vitreous aspirate, etc.) and laboratory identification of the organism through microbiologic testing are vital for confirming presence of the infection and initiating treatment. In cluster infections, tracing the source of infection in the hospital setting via testing of different in-house samples is equally important to prevent further occurrences. Although the incidence of these infections is low, their presence can cause prolonged disease that may often be resistant to medical therapy alone. In this review, we describe the various types of NTM-ocular infections, their clinical presentation, laboratory diagnosis, management, and outcomes.

Diagnosis of nontuberculous mycobacterial infections

This section discusses the methods of laboratory diagnosis of nontuberculous mycobacteria (NTM) using conventional biochemical and nutritional requirements, acid-fast smear microscopy, high performance liquid chromatography (HPLC), antibiotic susceptibility testing, and newer genetic methods such as molecular probes, polymerase chain reaction restriction fragment length polymorphism analysis (PRA), and 16S rDNA sequence analysis. This article discusses how laboratory results are applied by clinicians, and some of the difficulties and controversies regarding the diagnosis of NTM disease after the laboratory work is complete.

Use of nucleic acid probes for identification of Mycobacterium tuberculosis directly from MB/BacT bottles

The feasibility of using nucleic acid probes directly from positive MB/BacT broth to identify mycobacteria was determined in this study. A total number of 2,727 specimens were cultured into the MB/BacT (Organon Teknika) automated system and on conventional Loweinstein-Jensen (LJ) slants. The Gen-Probe AccuProbe culture identification tests (DNA probes) were used on samples from bottles which were identified as positive for mycobacteria by MB/BacT. Samples of positive MB/BacT broth (0.1 ml) were used directly in the broth culture method for the DNA probes as published by Gen-Probe. Centrifugation of the contents of the bottle was not done prior to probe testing. The number of mycobacteria detected by MB/BacT and LJ was 253 (221 isolates of M. tuberculosis and 32 isolates of mycobacteria other than M. tuberculosis [MOTT]). A total of 96.4% (213 of 221) of the bottles growing M. tuberculosis produced a positive direct DNA probe result for M. tuberculosis complex. One hundred percent (16 of 16) of the bottles growing M. gordonae produced a positive direct DNA probe result for M. gordonae. A total of 3.6% (8 of 221) of the bottles growing M. tuberculosis did not yield a positive direct DNA probe result for M. tuberculosis complex. The testing of subcultures made onto solid media from the positive bottles by AccuProbe identified six of these eight M. tuberculosis isolates. Two (0.9%) M. tuberculosis isolates gave a negative result for the M. tuberculosis probe test applied on the MB/BacT broth and its subculture. The rest of the positive MB/BacT bottles growing MOTT (16 of 32) were negative for M. gordonae, M. avium, M. intracellulare, and M. kansasii probes. The sensitivity and specificity of AccuProbe for the identification of M. tuberculosis and M. gordonae directly from MB/BacT broth were 96.4 and 100% for M. tuberculosis and 100 and 100% for M. gordonae, respectively. The direct testing of positive MB/BacT broth by AccuProbe, without prior centrifugation, allows for the accurate and rapid identification of M. tuberculosis and M. gordonae.

Rapid detection of genetic mutations using the chemiluminescent hybridization protection assay (HPA): overview and comparison with other methods

The detection of genetic mutations is of paramount importance for the study, diagnosis, and treatment of human genetic disease. Methods of detection generally fall into one of two categories: those to scan for unknown mutations and those to detect known mutations. This review focuses on methods for the detection of known mutations. The hybridization protection assay (HPA) is described in detail. The HPA method utilizes short oligonucleotide probes covalently labeled with a highly chemiluminescent acridinium ester (AE). The assay format is completely homogeneous, requiring no physical separation steps, and can rapidly and sensitively detect all single-base mismatches as well as multiple mismatches, insertions, deletions, and genetic translocations. When very low copy number targets are assayed, HPA is coupled with transcription-mediated amplification (TMA), an isothermal method that amplifies DNA or RNA targets. Other methods that are described for the detection of known mutations include hybridization with sequence-specific oligonucleotides, hybridization to oligonucleotide arrays, allele-specific amplification, ligase-mediated detection, primer extension, and restriction fragment analysis. The advantages and limitations of each of these methods are discussed. Methods to scan for unknown mutations are briefly described.

Cord formation in BACTEC medium is a reliable, rapid method for presumptive identification of Mycobacterium tuberculosis complex

Serpentine cord formation in BACTEC 12B medium was evaluated as a rapid method for the presumptive identification of M. tuberculosis complex. Kinyoun acid-fast stained smears were prepared from 666 positive BACTEC 12B bottles and examined for the presence or absence of serpentine cording. Cord formation had a sensitivity, specificity, positive predictive value, and negative predictive value of 89.2, 99.2, 98.5, and 94.2%, respectively. The evaluation of the presence of cord formation in BACTEC 12B medium is reliable and permits the rapid presumptive reporting of M. tuberculosis.

Early detection of Mycobacterium tuberculosis in BACTEC cultures by ligase chain reaction

The LCx Mycobacterium tuberculosis ligase chain reaction system (Abbott Diagnostic Division, Abbott Park, Ill.) was used to detect M. tuberculosis in 150 consecutive BACTEC vials on the day on which a positive growth index (GI) was recorded. By LCx, M. tuberculosis DNA was detected in BACTEC vials on average 2.6 days before the presence of acid-fast bacilli could be confirmed by microscopic examination. A total of 106 of 108 M. tuberculosis isolates were detected without centrifugation from bottles presenting very low GIs (average, 70; median, 33). No false-positive result was obtained from nontuberculous mycobacteria or from isolates with contaminants.

Evaluation of the MycoAKT latex agglutination test for rapid diagnosis of Mycobacterium avium complex infections

Rapid diagnosis of Mycobacterium avium complex (MAC) bacteremia is important for management of patients with the acquired immunodeficiency syndrome who have disseminated MAC. The purpose of this study was to determine the reliability of the MycoAKT latex agglutination test for direct detection of MAC in positive mycobacterial blood cultures. First, colonies of isolates of previously identified mycobacteria, including 35 MAC, were tested. Of the 55 isolates evaluated, 33 were identified as MAC by the latex test, including 31 of the known MAC and 2 M. chelonae (sensitivity, 88.6%; specificity, 90.0%). Second, broth from 20 ESP II and 20 BACTEC 12B bottles seeded with isolates of MAC were tested. Aliquots from 19 (95%) ESP II cultures and 16 (80%) 12B cultures were positive by the latex test. In phase 3, broth from 115 signal-positive ESP II blood cultures were tested by latex agglutination. Forty-three subcultures from these bottles grew mycobacteria (41 MAC and 2 Mycobacterium tuberculosis complex); the remainder grew no organisms. Broth from 40 of the blood cultures (39 that grew MAC and 1 from which no organisms were recovered) were latex positive; thus, the sensitivity, specificity, and positive and negative predictive values of the latex test for direct identification of MAC in ESP II blood cultures were 95.1, 98.6, 97.5, and 97.3%, respectively. The mean time to detection of MAC was 14.6 days (range, 6-34 days) with the direct latex test, compared with 18.3 days (range, 9-36 days) with subculture and probe (p < 0.05).

Mycobacterium growth indicator tube testing in conjunction with the AccuProbe or the AMPLICOR-PCR assay for detecting and identifying mycobacteria from sputum samples

We have compared the ability of the Mycobacterium Growth Indicator Tube (MGIT) system, a new culture method with an oxygen-sensitive fluorescent sensor, to recover mycobacteria from sputum samples with the abilities of egg-based medium and the Septi-Chek AFB system. We have also assessed the clinical utility of the AccuProbe or the AMPLICOR-PCR assay to directly identify Mycobacterium tuberculosis complex and M. avium-M. intracellulare complex (MAC) from positive MGITs. From 382 sputum samples, 99 isolates of M. tuberculosis complex and 20 isolates of MAC were recovered. The MGIT system had the highest recovery rates for M. tuberculosis complex (97.0%) and MAC (100%), compared to recovery rates of 51.5 and 65.0%, respectively, with the egg-based medium and 81.8 and 85.0%, respectively, with the Septi-Chek AFB system. The shortest recovery times were also achieved with the MGIT system: 16.6 days for M. tuberculosis complex and 12.0 days for MAC, compared to 27.1 and 20.1 days, respectively, with the egg-based medium and 21.4 and 13.2 days, respectively, with the Septi-Chek AFB system. The AccuProbe identified 74 (77.1%) of the 96 M. tuberculosis complex-positive MGITs and 17 (85.0%) of the 20 MAC-positive vials. The AMPLICOR system correctly identified 94 (97.9%) of the 96 M. tuberculosis complex-positive MGITs and all 20 MAC-positive vials. Therefore, the MGIT system used in conjunction with the AMPLICOR system is a rapid and sensitive method for detecting and identifying M. tuberculosis complex and MAC isolates from sputum samples.

Detection of Mycobacterium tuberculosis in BACTEC 12B broth cultures by the Roche Amplicor PCR assay

To evaluate the ability of the Amplicor MTB Assay to detect Mycobacterium tuberculosis complex (MTBC) organisms in BACTEC 12B broth cultures, 249 cultures with a growth index (GI) of > or = 20 from 160 patients were tested retrospectively. Specimens were processed by standard methods, and then BACTEC 12B vials and Middlebrook 7H11/7H115 plates were inoculated, incubated, and interpreted in accordance with the manufacturer's instructions and laboratory protocol. From 12B vials with a GI of > or = 20, and aliquot of broth was removed and frozen at -20 degrees C until assayed by PCR. PCR results were compared to those obtained by the usual laboratory protocol, whereby MTBC organisms were identified by a DNA probe assay performed on broth from 12B vials with a GI or > or = 300 or on colonies from solid medium. Of the 249 broth cultures evaluated, 142 contained mycobacteria, including 44 that contained MTBC organisms. Of these 44 cultures, 41 were PCR positive; the 3 that were PCR negative were blood specimens collected in an Isolator tube. All 98 cultures with nontuberculous mycobacteria and the 107 that did not contain mycobacteria were PCR negative. Thus, the sensitivity and specificity of PCR were 93 and 100%, respectively. For those culture sin which MTBC organisms were identified by both the DNA probe and PCR assays, the mean time from specimen inoculation to detection and identification of MTBC organisms was 16 (range, 4 to 26) days for the PCR and 28 (range, 13 to 43) days for the DNA probe assay (P < 0.0001). In summary, PCR is a rapid, reliable method for detection of MTBC organisms in BACTEC 12B broth cultures with a GI of > or = 20.

Selective utilization of DNA probes for identification of Mycobacterium species on the basis of cord formation in primary BACTEC 12B cultures

Primary BACTEC 12B cultures with serpentine cords observed in Kinyoun-stained smears were tested with a probe for Mycobacterium tuberculosis complex, while cultures without cords were tested with a probe for M. avium complex. The sensitivity, specificity, and positive and negative predictive values of cording for the presumptive identification of M. tuberculosis were 95, 95, 90, and 98%, respectively. With experience, the selection of a probe for testing of primary BACTEC 12B cultures on the basis of cord formation and history of tuberculosis can provide a rapid and reliable approach to the laboratory diagnosis of tuberculosis.

Use of Gen-Probe AccuProbes to identify Mycobacterium avium complex, Mycobacterium tuberculosis complex, Mycobacterium kansasii, and Mycobacterium gordonae directly from BACTEC TB broth cultures

To evaluate the utility of Gen-Probe AccuProbes for the identification of mycobacteria directly from BACTEC TB 12B vials containing acid-fast bacilli, culture results for 11,375 clinical specimens other than blood received from 1 January 1992 to 30 September 1993 were reviewed retrospectively. During this period, a total of 359 of 11,375 BACTEC vials were positive for acid-fast bacilli and were evaluated for mycobacteria with one or more probes: 224 were probed for Mycobacterium tuberculosis complex, 253 were probed for Mycobacterium avium complex, 64 were probed for Mycobacterium kansasii, and 77 were probed for Mycobacterium gordonae. After initial testing with the probes, 75 vials were positive for M. tuberculosis complex, 99 were positive for M. avium complex, 11 were positive for M. kansasii, and 55 were positive for M. gordonae. Repeat testing of vials that were initially probe negative or testing of colonies from subcultures of these vials identified an additional 11 M. tuberculosis, 27 M. avium complex, 1 M. kansasii, and 9 M. gordonae that were not detected on initial screening. On the basis of these data, the percentage of organisms identified directly from the BACTEC TB 12B vials upon initial screening with each of the four AccuProbes was 87.2% for M. tuberculosis complex, 78.6% for M. avium complex, 91.7% for M. kansasii, and 85.9% for M. gordonae.