Molecular mechanisms of transport and metabolism of vitamin B12 in mycobacteria

Mycobacterium tuberculosis (MTB) encodes three enzymes that are dependent on vitamin B12–derived cofactors for activity, including a B12-dependent methionine synthase (MetH). Previously, work in the Molecular Mycobacteriology Research Unit (MMRU) demonstrated vitamin B12 auxotrophy in a mutant strain disrupted in the alternative, B12-independent methionine synthase, MetE. This observation established the ability of MTB to transport corrinoids despite the absence of an identifiable B12-specific transporter. In addition, it suggested that MTB does not synthesize vitamin B12 in vitro. Notably, bioinformatic analyses identified PPE2 as the only B12-related transport candidate in MTB, though as a putative B12-regulated cobalt transporter. PPE2 is unusual in possessing directly upstream of its predicted start codon one of only two B12-dependent riboswitches in the MTB genome, and it lies in a putative operon with B12 biosynthetic genes, cobU and cobQ1. In this study, the possibility that PPE2 functions in the transport of vitamin B12 or cobalt was investigated. Transcriptional and phenotypic data suggested that PPE2 was not involved in B12 transport. Instead, it was shown that cobalt can supplement the growth of an MTB metE mutant in liquid medium, strongly supporting the ability of MTB to synthesize B12 de novo. Moreover, the ability to utilise exogenous cobalt was dependent on functional PPE2, thereby establishing a role for a PPE-family member in cobalt assimilation in MTB. Vitamin B12 comprises a central corrin ring co-ordinated to 5,6-dimethylbenzimidazole (DMB) as α-axial ligand. Substituting DMB with adenine yields the alternate form, pseudo-B12. The ability of mycobacteria to utilize pseudo-B12 precursors (cobinamide and adenine) to support full function of B12-dependent metabolic pathways was evaluated. Although the pseudo-B12 precursors appeared to complement chemically the mycobacterial B12 auxotrophs, growth of the mutants on cobinamide alone complicated this interpretation. To address this limitation, DMB synthesis was targeted by disrupting the MTB bluB homologue, Rv0306. Neither site-directed mutagenesis of key Rv0306 residues, nor full-gene deletion was sufficient to eliminate growth on cobinamide. Instead, this observation highlights the need to establish biochemically the nature of the active B12 form synthesized and utilized by MTB under different conditions. In combination, the results presented here support the inferred flexibility of vitamin B12 biosynthesis in MTB, and reinforce the potential role of B12-dependent metabolism in mycobacterial pathogenesis

The Myxobacterial Antibiotic Myxovalargin: Biosynthesis, Structural Revision, Total Synthesis, and Molecular Characterization of Ribosomal Inhibition

Resistance of bacterial pathogens against antibiotics is declared by WHO as a major global health threat. As novel antibacterial agents are urgently needed, we re-assessed the broad-spectrum myxobacterial antibiotic myxovalargin and found it to be extremely potent against Mycobacterium tuberculosis. To ensure compound supply for further development, we studied myxovalargin biosynthesis in detail enabling production via fermentation of a native producer. Feeding experiments as well as functional genomics analysis suggested a structural revision, which was eventually corroborated by the development of a concise total synthesis. The ribosome was identified as the molecular target based on resistant mutant sequencing, and a cryo-EM structure revealed that myxovalargin binds within and completely occludes the exit tunnel, consistent with a mode of action to arrest translation during a late stage of translation initiation. These studies open avenues for structure-based scaffold improvement toward development as an antibacterial agent

Paper Data: Detection of Mycobacterium tuberculosis bacilli in bio-aerosols from untreated TB patients (October 2017)

Data used in paper submitted to _Gates Open Research

Detection of Mycobacterium tuberculosis bacilli in bio-aerosols from untreated TB patients [version 2; referees: 3 approved]

Background: Tuberculosis (TB) is predominantly an airborne disease. However, quantitative and qualitative analysis of bio-aerosols containing the aetiological agent, Mycobacterium tuberculosis (Mtb), has proven very challenging. Our objective is to sample bio-aerosols from newly diagnosed TB patients for detection and enumeration of Mtb bacilli. Methods: We monitored each of 35 newly diagnosed, GeneXpert sputum-positive, TB patients during 1 hour confinement in a custom-built Respiratory Aerosol Sampling Chamber (RASC). The RASC (a small clean-room of 1.4m 3) incorporates aerodynamic particle size detection, viable and non-viable sampling devices, real-time CO 2 monitoring, and cough sound-recording. Microbiological culture and droplet digital polymerase chain reaction (ddPCR) were used to detect Mtb in each of the bio-aerosol collection devices. Results:  Mtb was detected in 27/35 (77.1%) of aerosol samples; 15/35 (42.8%) samples were positive by mycobacterial culture and 25/27 (92.96%) were positive by ddPCR. Culturability of collected bacilli was not predicted by radiographic evidence of pulmonary cavitation, sputum smear positivity. A correlation was found between cough rate and culturable bioaerosol. Mtb was detected on all viable cascade impactor stages with a peak at aerosol sizes 2.0-3.5μm. This suggests a median of 0.09 CFU/litre of exhaled air (IQR: 0.07 to 0.3 CFU/l) for the aerosol culture positives and an estimated median concentration of 4.5x10 7 CFU/ml (IQR: 2.9x10 7-5.6x10 7) of exhaled particulate bio-aerosol. Conclusions:  Mtb was identified in bio-aerosols exhaled by the majority of untreated TB patients using the RASC. Molecular detection was more sensitive than mycobacterial culture on solid media, suggesting that further studies are required to determine whether this reflects a significant proportion of differentially detectable bacilli in these samples

Detection of Mycobacterium tuberculosis bacilli in bio-aerosols from untreated TB patients [version 1; referees: 2 approved, 1 approved with reservations]

Background: Tuberculosis (TB) is predominantly an airborne disease. However, quantitative and qualitative analysis of bio-aerosols containing the aetiological agent, Mycobacterium tuberculosis (Mtb), has proven very challenging. Our objective is to sample bio-aerosols from newly diagnosed TB patients for detection and enumeration of Mtb bacilli. Methods: We monitored each of 35 newly diagnosed, GeneXpert sputum-positive, TB patients during 1 hour confinement in a custom-built Respiratory Aerosol Sampling Chamber (RASC). The RASC (a small clean-room of 1.4m3) incorporates aerodynamic particle size detection, viable and non-viable sampling devices, real-time CO2 monitoring, and cough sound-recording. Microbiological culture and droplet digital polymerase chain reaction (ddPCR) were used to detect Mtb in each of the bio-aerosol collection devices. Results: Mtb was detected in 27/35 (77.1%) of aerosol samples; 15/35 (42.8%) samples were positive by mycobacterial culture and 25/27 (92.96%) were positive by ddPCR. Culturability of collected bacilli was not predicted by radiographic evidence of pulmonary cavitation, sputum smear positivity, or cough rate. Mtb was detected on all viable cascade impactor stages with a peak at aerosol sizes 2.0-3.5μm. This suggests a median of 0.09 CFU/litre of exhaled air (IQR: 0.07 to 0.3 CFU/l) for the aerosol culture positives and an estimated median concentration of 4.5x107 CFU/ml (IQR: 2.9x107-5.6x107) of exhaled particulate bio-aerosol. Conclusions: Mtb was identified in bio-aerosols exhaled by the majority of untreated TB patients using the RASC. Molecular detection was more sensitive than mycobacterial culture on solid media, suggesting that further studies are required to determine whether this reflects a significant proportion of differentially detectable bacilli in these samples

Real-time investigation of tuberculosis transmission: developing the Respiratory Aerosol Sampling Chamber (RASC)

Knowledge of the airborne nature of respiratory disease transmission owes much to the pioneering experiments of Wells and Riley over half a century ago. However, the mechanical, physiological, and immunopathological processes which drive the production of infectious aerosols by a diseased host remain poorly understood. Similarly, very little is known about the specific physiological, metabolic and morphological adaptations which enable pathogens such as Mycobacterium tuberculosis ( Mtb ) to exit the infected host, survive exposure to the external environment during airborne carriage, and adopt a form that is able to enter the respiratory tract of a new host, avoiding innate immune and physical defenses to establish a nascent infection. As a first step towards addressing these fundamental knowledge gaps which are central to any efforts to interrupt disease transmission, we developed and characterized a small personal clean room comprising an array of sampling devices which enable isolation and representative sampling of airborne particles and organic matter from tuberculosis (TB) patients. The complete unit, termed the Respiratory Aerosol Sampling Chamber (RASC), is instrumented to provide real-time information about the particulate output of a single patient, and to capture samples via a suite of particulate impingers, impactors and filters. Applying the RASC in a clinical setting, we demonstrate that a combination of molecular and microbiological assays, as well as imaging by fluorescence and scanning electron microscopy, can be applied to investigate the identity, viability, and morphology of isolated aerosolized particles. Importantly, from a preliminary panel of active TB patients, we observed the real-time production of large numbers of airborne particles including Mtb , as confirmed by microbiological culture and polymerase chain reaction (PCR) genotyping. Moreover, direct imaging of captured samples revealed the presence of multiple rod-like Mtb organisms whose physical dimensions suggested the capacity for travel deep into the alveolar spaces of the human lung

Particle production as a function of respiration in a clinical TB patient.

<p>CO₂ concentration (solid line and left ordinate) and particle counts (dots and right ordinate) in the 1–2.5 μm size range for a TB patient.</p

The Respiratory Aerosol Sampling Chamber (RASC).

<p>(A) Photograph of the RASC (with the door open) on site in a community TB clinic (1) aerodynamic particle sizer (2) Filter samplers (3) Andersen impactor (4) Mixing fan (5) CO2, temperature and RH (6) PM10 impactor (7) Chair for participant. (B) Block diagram depicting the fluidic and electronic configuration of the RASC. Thick connecting lines indicate airflow and aerosol paths; thin lines indicate electronic connections. All air leaving the RASC is HEPA filtered.</p

Isolation of <i>Mtb</i> from a TB patient.

<p>SEM image of patient sample impacted on the lower plate of the PM10 impactor. The dimensions and morphology of the rod-shaped structure (denoted by *) are consistent with the presence of <i>Mtb</i> bacilli in the untreated TB patient. There is also evidence of multiple “splats” of unknown identity (one example is denoted by **) which might comprise organic matter derived from patient lung or respiratory tract. Note the “halo” structures (dark shadows) surrounding each particle.</p