Comparison of isothermal microcalorimetry and BACTEC MGIT960 for the detection of the metabolic activity of Mycobacterium tuberculosis in sputum samples.

INTRODUCTION: This study explores the uses of microcalorimetry to detect Mycobacterium tuberculosis (TB) in sputum. Microcalorimetry measures metabolic heat evolution during cellular proliferation of tuberculosis (TB) and is considered as a possible alternative to conventional diagnostic tools. OBJECTIVES: To compare the time to detection (TTD) from the BACTEC™ MGIT™ 960 and the calScreener™ calorimetric system. METHODS: Sixty-four sputa samples were selected from patients with confirmed pulmonary tuberculosis. Those sample were then decontaminated and analysed using calorimetry and BACTEC MGIT 960 system. RESULTS: The incubation period until detection of M. tuberculosis in the sample was 8·5 ± 3·7 days for the MGIT system and 10·1 ± 4·1 days (mean ± SD) for calorimetry. CONCLUSIONS: The microincubations in the 48-well format calScreener offers potential for rapid and accurate diagnostic of TB in different samples. Although TTD from calorimetry is still longer than with the MGIT, our findings suggest that several improvements are possible. Still, the instrument is ideal for continuous, real-time analysis of net metabolic heat release of limited sample numbers. SIGNIFICANCE AND IMPACT OF THE STUDY: Our result emphasizes that with further optimization, calorimetry can become an alternative detection method for tuberculosis.

Perspectives for personalized therapy for patients with multidrug-resistant tuberculosis.

According to the World Health Organization (WHO), tuberculosis is the leading cause of death attributed to a single microbial pathogen worldwide. In addition to the large number of patients affected by tuberculosis, the emergence of Mycobacterium tuberculosis drug-resistance is complicating tuberculosis control in many high-burden countries. During the past 5 years, the global number of patients identified with multidrug-resistant tuberculosis (MDR-TB), defined as bacillary resistance at least against rifampicin and isoniazid, the two most active drugs in a treatment regimen, has increased by more than 20% annually. Today we experience a historical peak in the number of patients affected by MDR-TB. The management of MDR-TB is characterized by delayed diagnosis, uncertainty of the extent of bacillary drug-resistance, imprecise standardized drug regimens and dosages, very long duration of therapy and high frequency of adverse events which all translate into a poor prognosis for many of the affected patients. Major scientific and technological advances in recent years provide new perspectives through treatment regimens tailor-made to individual needs. Where available, such personalized treatment has major implications on the treatment outcomes of patients with MDR-TB. The challenge now is to bring these adances to those patients that need them most.

Para-aminosalicylic acid plasma concentrations in children in comparison with adults after receiving a granular slow-release preparation.

There are no paediatric data regarding slow-release para-aminosalicylic acid (PAS). We studied PAS plasma concentrations in 10 children receiving a single 150 mg/kg dose daily or 75 mg/kg twice daily and 12 adults receiving 4 g twice daily. Blood specimens pre-dose and 2, 4, 6, 8 and 12 h post-dose from the children and 2, 3, 4, 5, 6, 8 and 12 h post-dose from the adults were analysed by high performance liquid chromatography MS/MS. The mean Cmax in children receiving PAS 75 mg/kg and 150 mg/kg and adults receiving 4 g was 45.40, 56.49 and 51.3 µg/ml, respectively (p = 0.614); the AUC0-12 was 233.3, 277.9 and 368.0 µg/h/ml (p = 0.587). No parameters differed significantly between children and adults nor between the two doses in the same children. A 150 mg/kg PAS dosage given as one or two daily doses leads to plasma concentrations in children similar to those of adults receiving 4 g PAS twice daily.

Randomized pilot trial of eight weeks of bedaquiline (TMC207) treatment for multidrug-resistant tuberculosis: long-term outcome, tolerability, and effect on emergence of drug resistance.

The 2-year follow-up results for a randomized placebo-controlled study of 47 patients with multidrug-resistant pulmonary tuberculosis treated with either the new diarylquinoline TMC207, recently renamed bedaquiline, or placebo, added to the first 8 weeks of a background regimen, are presented. Bedaquiline significantly reduced the time to culture conversion over 24 weeks (hazard ratio, 2.253; 95% confidence interval, 1.08 to 4.71; P = 0.031). With the exception of nausea reported in 26% of patients receiving bedaquiline and none receiving placebo, adverse events occurred at similar frequencies in both groups of patients: bilateral hearing impairment, extremity pain, acne, and noncardiac chest pain occurred in 13 and 21%, 17 and 13%, 9 and 17%, and 4 and 17% of patients, respectively, receiving bedaquiline or placebo. Excluding resistance to ethambutol and ethionamide, only one patient receiving bedaquiline acquired resistance to companion drugs, but five patients receiving placebo (4.8% versus 21.7%; P = 0.18) acquired resistance to companion drugs, and resistance to ofloxacin was acquired in four patients receiving placebo and none receiving bedaquiline (0% versus 22%; 0 = 0.066). In all, 23 patients (49%), including 13 receiving placebo (54%) and 10 receiving bedaquiline (44%), discontinued the study prior to its completion, 12 during the first 24 weeks of treatment. Eight subjects were withdrawn for noncompliance or default, and seven withdrew consent, citing the rigorous program of investigations for safety and pharmacokinetic monitoring. Bedaquiline may contribute to the management of multidrug-resistant tuberculosis by effecting more rapid sputum culture negativity and by preventing acquired resistance to companion drugs.

Factors associated with screening failure and study withdrawal in multidrug-resistant TB.

SETTING: Multidrug-resistant TB (MDR-TB) clinical trial in Lima, Peru and Cape Town, South Africa.OBJECTIVE: To identify baseline factors associated with screening failure and study withdrawal in an MDR-TB clinical trial.DESIGN: We screened patients for a randomized, blinded, Phase II trial which assessed culture conversion over the first 6 months of treatment with varying doses of levofloxacin plus an optimized background regimen (ClinicalTrials.gov: NCT01918397). We identified factors for screening failure and study withdrawal using Poisson regression to calculate prevalence ratios and Cox proportional hazard regression to calculate hazard ratios. We adjusted for factors with P < 0.2.RESULTS: Of the 255 patients screened, 144 (56.5%) failed screening. The most common reason for screening failure was an unsuitable resistance profile on sputum-based molecular susceptibility testing (n = 105, 72.9%). No significant baseline predictors of screening failure were identified in the multivariable model. Of the 111 who were enrolled, 33 (30%) failed to complete treatment, mostly for non-adherence and consent withdrawal. No baseline factors predicted study withdrawal in the multivariable model.CONCLUSION: No baseline factors were independently associated with either screening failure or study withdrawal in this secondary analysis of a MDR-TB clinical trial.

Pretomanid with bedaquiline and linezolid for drug-resistant TB: a comparison of prospective cohorts.

BACKGROUND: There are no data comparing the 6-9 month oral three-drug Nix regimen (bedaquiline, pretomanid and linezolid [BPaL]) to conventional regimens containing bedaquiline (B, BDQ) and linezolid (L, LZD).METHODS: Six-month post end-of-treatment outcomes were compared between Nix-TB (n = 109) and 102 prospectively recruited extensively drug-resistant TB patients who received an ˜18-month BDQ-based regimen (median of 8 drugs). A subset of patients received BDQ and LZD (n = 86), and a subgroup of these (n = 75) served as individually matched controls in a pairwise comparison to determine differences in regimen efficacy.RESULTS: Favourable outcomes (%) were significantly better with BPaL than with the B-L-based combination regimen (98/109, 89.9% vs. 56/86, 65.1%; adjusted relative risk ratio [aRRR] 1.35; P < 0.001) and in the matched pairwise analysis (67/75, 89.3% vs. 48/75, 64.0%; aRRR 1.39; P = 0.001), despite significantly higher baseline bacterial load and prior second-line drug exposure in the BPaL cohort. Time to culture conversion (P < 0.001), time to unfavourable outcome (P < 0.01) and time to death (P < 0.03) were significantly better or lower with BPaL than the B-L-based combinations.CONCLUSION: The BPaL regimen (and hence substitution of multiple other drugs by pretomanid and/or higher starting-dose LZD) may improve outcomes in drug-resistant TB patients with poor prognostic features. However, prospective controlled studies are required to definitively answer this question.

Rapid on-site evaluation of transbronchial aspirates: randomised comparison of two methods.

The value of different staining methods for rapid analysis of transbronchial needle aspirates during bronchoscopy has not been explored. In the present study, we compared a Papanicolaou-based rapid stain, prepared by a technologist and read by a cytopathologist, and a Wright-Giemsa-based rapid stain, prepared and read by a cytopathologist alone. Gold standard was the final laboratory report issued on each aspirate. We harvested 827 aspirates from 218 target sites in 126 consecutive patients. At least one positive aspirate was found in 99 (79%) patients. In those 99 patients, 288 of 574 (50%) aspirates were positive for neoplastic (83%) or non-neoplastic (17%) disease. False-negative aspirates and target sites were more frequent with the rapid Wright-Giemsa than with the rapid Papanicolaou stain (14.2 versus 7.3%, p = 0.008, and 13.7 versus 3.6%, p = 0.021, respectively). The sensitivity of the Wright-Giemsa-based and Papanicolaou-based rapid stains for detecting diagnostic material was 93 and 100% in patients, 83.1 and 95.5% in target sites, and 72.8 and 84.9% in aspirates, respectively. Specificity was 100% for both methods in patients and target sites, and 90.4 and 95% in aspirates. We concluded that a Papanicolaou-based stain has superior yield and accuracy to a Wright-Giemsa-based stain for rapid on-site evaluation of transbronchial needle aspirates.

Time to detection of the growth of Mycobacterium tuberculosis in MGIT 960 for determining the early bactericidal activity of antituberculosis agents.

Evaluation of early bactericidal activity (EBA) by the determination of a fall in viable colony-forming units (CFU) of Mycobacterium tuberculosis in sputum is a first step in the clinical study of new antituberculosis agents. The time to detection (TTD) of growth in liquid media is more sensitive and could substitute for CFU counting on solid media. Overnight sputum samples collected during the evaluation of the novel agent TMC207 in comparison to isoniazid and rifampicin were studied. For the determination of CFU, we incubated 10-fold dilutions of homogenized sputum on selective 7H10 agar. The TTD was measured by incubating decontaminated sputum in the BACTEC MGIT 960 system. The fall in bacillary load over 7 days determined by CFU counting closely matched the prolongation of the TTD in the BACTEC MGIT 960 system. The CFU counts correlated significantly with the TTD. While the ranking of agents and different dosages of TMC207 was similar, the highest dose of TMC207 showed markedly better activity when measured by the TTD than CFU counting when compared to the activity of isoniazid. Automated TTD could augment, or, in future, replace, CFU counting to determine sputum bacillary load in EBA clinical trials pending a more formal evaluation of the correlation of the measurements.

Postmortem lung biopsies from four patients with COVID-19 at a tertiary hospital in Cape Town, South Africa.

BACKGROUND: An outbreak of a novel coronavirus in China in late 2019 has resulted in a global pandemic. The virus (SARS-CoV-2) causes a severe acute respiratory syndrome and had been responsible for >14 000 deaths in South Africa (SA) at the time of writing, 30 August 2020. Autopsies in our setting have not been prioritised owing to the infective risks for staff, resulting in a lack of information on the histopathology of the disease in the SA setting. Postmortem biopsies are relatively quick and easy to perform and reduce the infective risk posed by full autopsies. OBJECTIVES: To determine whether postmortem biopsies of lung tissue could be used to determine cause of death in lieu of full autopsies in patients dying from COVID-19. METHODS: We performed postmortem biopsies of lung tissue on 4 patients with SARS-CoV-2 confirmed by reverse transcriptase polymerase chain reaction who died in the Tygerberg Hospital (Cape Town, SA) intensive care unit (ICU) in June - July 2020, in order to determine their cause of death. The biopsies were performed in the ICU with the necessary personal protective equipment within 2 hours after death. Clinical information was obtained from the hospital records and the histopathology was reviewed by two consultant histopathologists. Microbiology and electron microscopy were also performed on this tissue. RESULTS: All 4 patients were aged >50 years and had multiple comorbidities. Pulmonary pathology was present in only 3 cases, and the findings were surprisingly heterogeneous. One case demonstrated several findings including diffuse alveolar damage, extensive fibrin thrombi in pulmonary arteries with pulmonary infarction, organising pneumonia and bronchopneumonia. Other findings included type 2 pneumocyte hyperplasia, intra-alveolar macrophages and squamous metaplasia. An organising pneumonia was present in 2 other cases, although these findings were not deemed to be severe enough to be the cause of death. Fibrin thrombi were present in pulmonary arteries of 3 cases. One case showed no significant acute pulmonary pathology. The cause of death could only be determined in 1 case. CONCLUSIONS: The pulmonary findings we observed are in keeping with those described in the international literature. However, the pathology was surprisingly heterogeneous between cases, and was only deemed severe enough to be the cause of death in 1 of 4 cases. While lung-targeted, standardised postmortem biopsies may be safe, easy to perform and provide useful insights into the disease, they are not suitable to replace full autopsies in determining cause of death.

Diagnostic yield and safety of ultrasound-assisted biopsies in superior vena cava syndrome.

The yield and safety of ultrasound (US)-assisted transthoracic fine needle aspirations (TTFNA) and cutting needle biopsies (CNB) in the setting of superior vena cava (SVC) syndrome are unknown. The aims of the present prospective study were to asses the diagnostic yield and safety of US-assisted TTFNA and CNB in SVC syndrome with an associated mass lesion abutting the chest wall. Over a 3-yr period, the present authors screened 59 patients with SVC syndrome, and enrolled 25 patients who had an associated mass lesion that extended to the chest wall. US-assisted TTFNA with rapid on-site evaluation (ROSE) was performed in all cases. CNBs were performed where a provisional diagnosis of bronchogenic carcinoma could not be established, and in 57.1% of patients with bronchogenic carcinoma (limited due to safety constraints). ROSE of US-assisted TTFNA confirmed diagnostically useful material in 24 patients, and cytological diagnoses were ultimately made in all of these cases (diagnostic yield 96%). US-assisted CNB had a diagnostic yield of 87.5%. Minor haemorrhage occurred in one out of 25 TTFNA and three out of 16 CNB. Neither procedure resulted in major haemorrhage nor pneumothoraces. US-assisted TTFNA and CNB have a high diagnostic yield and are safe in the setting of SVC syndrome with an associated mass lesion abutting the chest wall.

Tuberculosis research in South Africa over the past 30 years: From bench to bedside.

The South African Medical Research Council Centre for Tuberculosis Research has a rich history of high-impact research that has influenced our understating of this hyper-epidemic which is further exacerbated by the emergence and spread of drug-resistant forms of the disease. This review aims to summarise the past 30 years of research conducted in the Centre which has influenced the way that tuberculosis (TB) is diagnosed and treated. The review includes the development of new technologies for rapid screening of people with probable TB and the repurposing of human diagnostics for wildlife conservation.

Early bactericidal activity and pharmacokinetics of the diarylquinoline TMC207 in treatment of pulmonary tuberculosis.

Tibotec Medicinal Compound 207 (TMC207) is a novel diarylquinoline with a unique mode of action that targets mycobacterial ATP synthase. TMC207 exhibits high in vitro activity against mycobacterial strains either susceptible or resistant to all first-line and many second-line drugs, including fluoroquinolones, and has shown exceptional in vivo activity against several mycobacterial species in different animal models. In this early bactericidal activity study, 75 treatment-naïve patients with smear-positive pulmonary tuberculosis were randomized to once-daily oral TMC207 (25 mg, 100 mg, or 400 mg), 600 mg rifampin (RIF), or 300 mg isoniazid (INH) for 7 days. Sixteen-hour overnight sputum collected at baseline and on each treatment day was plated in serial dilutions on selective agar plates. The bactericidal activity was expressed as the log(10) decrease in CFU/ml sputum/day. Pharmacokinetic sampling was performed on day 7 of TMC207 administration up to 24 h postdose. The decreases in log(10) CFU counts (+/- standard deviation) from baseline to day 7 were 0.04 +/- 0.46 for 25 mg TMC207 (n = 14), 0.26 +/- 0.64 for 100 mg TMC207 (n = 14), 0.77 +/- 0.58 for 400 mg TMC207 (n = 14), 1.88 +/- 0.74 for INH (n = 11), and 1.70 +/- 0.71 for RIF (n = 14). Significant bactericidal activity of 400 mg TMC207 was observed from day 4 onward and was similar in magnitude to those of INH and RIF over the same period. The pharmacokinetics of TMC207 were linear across the dose range. In summary, TMC207 demonstrated bactericidal activity with a delayed onset and was well tolerated, and no study drug-related serious adverse events occurred.

Clinical and radiological grading of superior vena cava obstruction.

BACKGROUND: Superior vena cava obstruction (SVCO) is commonly caused by neoplastic venous compression and presents with typical symptoms and signs. Its clinical severity presumably depends on the degree of obstruction and the adequacy of venous collateral formation. OBJECTIVES: The development of novel clinical and radiological scoring systems based on the postulate that a reproducible relationship exists between the degree of SVCO, the presence of collateral circulation and the extent of clinical symptoms. METHODS: We prospectively evaluated consecutive cases of acute and subacute SVCO with a newly developed clinical scoring system, which is based on easily detectable clinical symptoms and signs of SVCO. In parallel, we recorded and scored the degree of SVCO and the extent of collaterals visible on contrast-enhanced computed tomography (CT). RESULTS: Thirty-four cases of SVCO were evaluated: 8 (23.5%) were clinically mild, 16 (47%) moderate and 10 (29.5%) severe. Lung cancer was the underlying histological diagnosis in 94% of cases. Radiologically, 53% had complete SVCO. A well-developed collateral system was found in 14 (41%). A scoring system subtracting a 'collateral score' from an 'obstruction score' showed a significant correlation with the clinical score (r = 0.75, p < 0.01). CONCLUSIONS: Clinical severity of SVCO depends upon the degree of SVCO and is ameliorated by collateral formation. The novel clinical scoring system can predict the underlying CT features in SVCO and may be valuable in the bedside assessment of SVCO severity.

Droplets, dust and guinea pigs: an historical review of tuberculosis transmission research, 1878-1940.

The transmission of tuberculosis (TB) occurs mainly via inhalation of airborne droplet nuclei; however, the precise details of this process remain uncertain. We reviewed the literature from 1870 to 1940, when Mycobacterium tuberculosis was discovered and the concept of transmission emerged as a hallmark of the infectious disease. By 1940, laboratory experiments, animal studies and clinical observation had demonstrated that cough was central to TB transmission, and that guinea pigs close to patients with cough could be infected, mainly by patients coughing small droplets likely containing only 1-2 bacilli. A minority of pulmonary TB patients, usually during the early stages of the disease, with thin watery sputum, more successfully coughed small infectious droplets than patients with heavily smear-positive tenacious sputum who were often too ill and too weak to cough vigorously. There was ongoing debate regarding the possible importance of desiccated sputum particles found in surface dust. Investigation of TB transmission has a history of more than 130 years.

Risk factors for recurrence of haemoptysis following bronchial artery embolisation for life-threatening haemoptysis.

SETTING: Life-threatening haemoptysis is a frequent and often fatal complication in areas with a high prevalence of tuberculosis (TB). Bronchial artery embolisation remains the standard initial treatment. Subsequent curative measures, such as surgical resection of the focus of haemorrhage, are generally recommended to prevent recurrence, but risk-based selection criteria have not been established. OBJECTIVES: To identify risk factors for the recurrence of haemoptysis following embolisation. DESIGN: Baseline characteristics were obtained from consecutive patients with life-threatening haemoptysis who were successfully embolised and followed up for at least 12 months. RESULTS: Recurrence of haemoptysis was observed in 47% and was associated with increased mortality compared to patients without recurrence (31% vs. 10%, P = 0.021). Patients with recurrence experienced residual mild haemoptysis beyond the first week after embolisation (odds ratio [OR] 7.2), received blood transfusions (OR 5.3) or presented with an aspergilloma (OR 5.1). Conversely, the presence of active TB amenable to treatment (OR 0.3) protected patients from these events. Radiographic or angiographic appearance did not predict recurrence. CONCLUSIONS: Recurrence of haemoptysis following embolisation for life-threatening haemoptysis is common and is associated with high mortality. The results of this study can contribute to the risk assessment of these patients and guide decisions regarding the urgency of definitive therapy.

Direct comparison of liquid-based and smear-based cytology with and without rapid on-site evaluation for fine needle aspirates of thoracic tumors.

BACKGROUND: Liquid-based cytology (LBC) and rapid on-site evaluation (ROSE) are proposed to improve the quality of fine needle aspirates (FNA) and their diagnostic yield compared with conventional smear cytology (CSC). This prospective study directly compared outcomes of sonar-guided FNA of thoracic tumors supported by LBC, CSC, or CSC with ROSE. METHODS: Three aspirates each for both LBC and CSC with separate 22G spinal needles in a randomized, alternating sequence during 64 transthoracic FNA of thoracic tumors were collected. Smears were prepared by cytology staff on site but evaluated with ROSE only when all six samples had been collected. If no diagnostic material was found on the first three CSC additional needle passes guided by ROSE were performed. RESULTS: Final diagnoses were non-small cell lung cancer in 50 (78.1%), small cell lung cancer in 11 (17.2%), mesothelioma in 1 (1.6%), and inflammation in 2 cases (3.1%), respectively. LBC and CSC were diagnostic in 42 (65.6%) and 49 (76.6%) cases, respectively (P = 0.039), with both methods diagnostic in 41 cases (64.1%). Fifteen cases (23.4%) remained undiagnosed following three passes for CSC but 9 (14.1%) of these were diagnosed using FNA and ROSE with a total yield of 58 cases (90.6%; P < 0.001). CONCLUSION: The diagnostic yield of transthoracic FNA submitted for LBC is significantly lower than with CSC when slides are prepared professionally. ROSE significantly increases the yield of transthoracic FNA.

Improved diagnosis of childhood tuberculous meningitis using more than one nucleic acid amplification test.

BACKGROUND: Early treatment is critical to reducing tuberculous meningitis (TBM) related morbidity and mortality. Diagnosis based on cerebrospinal fluid (CSF) culture is impractical due to slow turnaround times, while microscopy has poor sensitivity. Enhanced detection methods are essential to guide early treatment initiation, especially in vulnerable young children. METHODS: We assessed the diagnostic accuracy of the GenoType(®) MTBDRplus and Xpert(®) MTB/RIF assays on CSF collected from paediatric meningitis suspects prospectively enrolled at Tygerberg Hospital, Cape Town, South Africa. Fluorescent auramine-O microscopy, liquid culture for Mycobacterium tuberculosis, GenoType and Xpert assays were performed on all CSF samples. RESULTS: Of 101 meningitis suspects, 55 were diagnosed with TBM and 46 served as non-TBM controls. Using a pre-defined TBM case definition as reference standard, sensitivities and specificities were 4% and 100% for fluorescent microscopy, 22% and 100% for culture, 33% and 98% for GenoType, 26% and 100% for Xpert, 22% and 100% for microscopy and culture combined and 49% and 98% for GenoType and Xpert combined. Culture, GenoType and Xpert combined performed best, with 56% sensitivity and 98% specificity. CONCLUSION: Although commercial nucleic-acid amplification tests performed on CSF revealed incrementally improved diagnostic accuracy, providing rapid microbiological confirmation, they cannot serve as a rule-out test.

Functional evaluation before and after interventional bronchoscopy in patients with malignant central airway obstruction.

Rigid or flexible interventional bronchoscopy is increasingly accepted for treating central tracheal or bronchial obstruction caused by incurable pulmonary malignancy. Immediate and sustained symptomatic relief and improvement of quality of life can be offered to patients suffering from dyspnoea and facing death by suffocation. Selection of patients based on functional aspects is of central importance for success in endoscopic treatment. A thorough workup is conducted with close cooperation between the various specialists involved in thoracic oncology (pulmonologist, oncologist, radio-oncologist, thoracic and upper airway surgeon). Careful history taking, clinical examination, pulmonary function testing (PFT) and radiologic imaging are needed to assess the general condition of the patient and give information of type and stage of the underlying malignancy. In eligible patients, timing and method of the intervention depend on the acuteness of the presentation and the endoscopic assessment of the lesion responsible for the obstruction. Regular postinterventional follow-up should be conducted by the patient's doctor and consist of clinical examination, PFT and chest radiography. Close cooperation with the specialist centre is necessary to refer patients for repeat bronchoscopy and management of suspected recurrences or complications.

Early stationary phase culture supernatant accelerates growth of sputum cultures collected after initiation of anti-tuberculosis treatment.

We investigated the effect of Mycobacterium tuberculosis culture supernatant added to sputum cultures collected during the first 8 weeks of anti-tuberculosis treatment. With ongoing treatment duration, time to culture positivity decreased significantly in supernatant-enriched cultures, possibly due to stimulation of dormant or slowly metabolizing M. tuberculosis cells.