Differentially culturable Mycobacterium tuberculosis in cough-generated aerosols of patients with pulmonary tuberculosis DCTB in cough-generated aerosols

Abstract

Introduction. While Mycobacterium tuberculosis cells in sputum in sputum have been studied extensively, little is known of their properties in exhaled aerosols.Hypothesis. As differentially culturable tubercle bacteria (DCTB) are readily found in sputum, we hypothesized that DCTB might also be present in aerosols and potentially contribute to transmission.Aim. To test cough aerosols from recently diagnosed pulmonary tuberculosis (TB) patients for DCTB.Methodology. Cough-generated aerosols and sputum samples were collected from active pulmonary TB patients (n=27). A cough aerosol sampling system was modified to include both an Andersen Cascade Impactor using solid agar and a BioSampler liquid impactor. We performed the most probable number of assays to detect DCTB, using media supplemented with Mycobacterium tuberculosis culture filtrate (CF).Results. Briefly, 63% of patients (n=17) had advanced TB, and 55.6% (n=15) had a 3+ sputum smear for acid-fast bacilli. Evidence for DCTB was found in 8 patients' aerosols (29.5%) and more than half of the 19 sputum samples tested (n=10; 52.6%). Two patients had DCTB in only one of the collected samples (cough aerosols or sputum). Among cough aerosol specimens, two patients (7%) only had CF-dependent DCTB.Conclusion. We detected DCTB in sputum and evidence for their presence in cough samples from pulmonary TB patients. These data suggest that bacilli undetected by traditional mycobacterial cultures may be aerosolized from pulmonary TB patients.

Keywords: Mycobacterium tuberculosis; cough-generated aerosols; culturability; culture filtrate; differentially culturable.

Fig. 1.. Study design to detect dormant MTB bacilli, activated by CF, from cough aerosol cultures of pulmonary TB patients. Initially, patients were screened through a questionnaire, AFB smear and the molecular test ‘GeneXpert’ result. Then, considering inclusion and exclusion criteria, eligible patients had their sputum collected for (i) quantitative culture, (ii) positivity on the Mycobacteria Growth Indicator Tube (MGIT) system and (iii) for the LDM to estimate dormant and active bacilli. Also, the CASS assay was performed, adapted with a bioaerosol sampler for dormant bacilli detection and with the Andersen Cascade Impactor for active bacilli quantification.

Fig. 2.. Proportion of MTB bacilli recovered by different assays applied to CASS samples. Twenty-one patients produced at least one positive culture in their aerosol samples. Six patients produced no positive aerosol cultures. *Biosampler MPN+CF counts for one patient were excluded for technical reasons. Percentages are shown against the 27 participants, with numbers of individuals in parentheses.