Mycobacterium heckeshornense lung infection diagnosed by matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS)

A Case of Pulmonary Mycobacterium heckeshornense Infection in which the Causative Microorganism was Difficult to Identify

A 44-year-old woman underwent a follow-up examination for Crohn's disease 9 years ago. Chest computed tomography (CT) showed an infiltration shadow with a cavity in the right upper lobe. After a CT-guided lung biopsy, epitheloid granuloma was noted, and an acid-fast bacilli examination was smear-positive, but a culture examination was negative. Because the abnormal chest shadow with cavity gradually increased and right shoulder pain appeared, we performed bronchoscopy again six months later. Mycobacterium heckeshornense was isolated from the bronchoalveolar lavage fluid specimen, so we diagnosed her with pulmonary M. heckeshornense disease. Isoniazid, rifampicin, and ethambutol were administered, and the abnormal chest shadow improved.

[Cavernous Nontuberculous Mycobacterial Lung Infection in an HIV-positive Patient]

Since we first described Mycobacterium heckeshornense, a rare species of mycobacteria in 2000, only 21 cases of infection with this mycobacterium have been described in humans. We relate the diagnosis and therapy of another case of this uncommon nontuberculous mycobacterium in an immune-suppressed patient.

Performance of lipid fingerprint-based MALDI-ToF for the diagnosis of mycobacterial infections

OBJECTIVES

Bacterial diagnosis of mycobacteria is often challenging because of the variability of the sensitivity and specificity of the assay used, and it can be expensive to perform accurately. Although matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) has become the workhorse of clinical laboratories, the current MALDI methodology (which is based on cytosolic protein profiling) for mycobacteria is still challenging due to the number of steps involved (up to seven) and potential biosafety concerns. Knowing that mycobacteria produce surface-exposed species-specific lipids, we here hypothesized that the detection of those molecules could offer a rapid, reproducible and robust method for mycobacterial identification.

METHODS

We evaluated the performance of an alternative methodology based on characterized species-specific lipid profiling of intact bacteria, without any sample preparation, by MALDI MS; it uses MALDI-time-of-flight (ToF) MS combined with a specific matrix (super-2,5-dihydroxybenzoic acid solubilized in an apolar solvent system) to analyse lipids of intact heat-inactivated mycobacteria. Cultured mycobacteria are heat-inactivated and loaded directly onto the MALDI target followed by addition of the matrix. Acquisition of the data is done in both positive and negative ion modes. Blinded studies were performed using 273 mycobacterial strains comprising both the Mycobacterium tuberculosis (Mtb) complex and non-tuberculous mycobacteria (NTMs) subcultured in Middlebrook 7H9 media supplemented with 10% OADC (oleic acid/dextrose/catalase) growth supplement and incubated for up to 2 weeks at 37°C.

RESULTS

The method we have developed is fast (<10 mins) and highly sensitive (<1000 bacteria required); 96.7% of the Mtb complex strains (204/211) were correctly assigned as MTB complex and 91.7% (22/24) NTM species were correctly assigned based only on intact bacteria species-specific lipid profiling by MALDI-ToF MS.

CONCLUSIONS

Intact bacterial lipid profiling provides a biosafe and unique route for rapid and accurate mycobacterial identification.

A case of pulmonary Mycobacterium heckeshornense infection in a healthy Japanese man

A 72-year-old man, healthy, smoker, with long-standing cough, was referred to our hospital and his chest X-ray (CXR) revealed a cavity lesion in the right upper lobe. Direct sputum smears, but not culture in solid medium, were positive for acid-fast bacilli (AFB) without tuberculosis DNA. The preliminary diagnosis was of a non-tuberculosis infection that progressed slowly, and the CXR showed the condition to worsen daily. Four years later, a commercialized mycobacteria growth indicator tube system was used to culture the colonies of AFB successfully in liquid medium, and the species Mycobacterium heckeshornense was identified by matrix-assisted laser desorption ionization-time of flight mass spectrometry. The patient responded well to triple therapy with rifampicin, ethambutol, and clarithromycin, the sputum cultures remained negative and the roentgenogram showed minor improvement over the following 6 months.