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Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry Identification of Nontuberculous Mycobacterial Species in Lung Transplant Recipient

Background: Nontuberculous mycobacteria (NTM) are very commonly isolated from the airways in the patients with bronchiectasis, especially those with cystic fibrosis (CF). These patients are particularly prone to NTM infection after lung transplantation. However, the overall rates of invasive disease are low and the increased risk for post-transplant NTM disease is restricted largely to those with pretransplant Mycobacterium abscessus islolates [1]. According to the International Guidelines for the Selection of Lung Transplant Candidates developed by the International Society of Heart and Lung Transplantation colonization with NTM in lung transplant candidates is considered relative contraindication for lung transplantation [2]. However, eradication attempts should be performed prior to lung transplantation. Also, medical treatment effectively controls NTM disease after lung transplantation and therefore fast identification of mycobacterial species isolated from lung transplant recipient’s clinical samples is of crucial importance [3]. Previously used biochemical tests are surpassed by still technically demanding molecular methods including line probe assays and gene sequencing. A new tool for species identification of NTM is matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) [4], [5]. We describe identification of NTM isolated in the patient with mycobacteriosis after lung transplantation using this new technology. In 57-year-old woman lung transplantation was indicated for chronic inflammatory bronchiectasis. She was chronically infected with Pseudomonas aeruginosa and was hospitalized repeatedly and treated with different antimicrobial regimens. Mycobacterim gordonae and Nocardia sp. were also isolated in single clinical samples four years before transplantation. After transplantation she developed mycobacteriosis accompanied by respiratory insufficiency and was mechanically ventilated. Methods: Tracheal aspirate was taken from the patient and conventional auramin stain, culture on solid Lowenstein-Jensen (L-J) medium and in Mycobacterium Growth Indicator Tube (MGIT) culture bottles incubated in BACTEC® MGIT™ 960 System (Becton, Dickinson and Company, Sparks, USA) were done based on standard procedures. To exclude Mycobacterium tuberculosis complex (MTBC), BD MGIT TBc Identification Test (Becton, Dickinson and Company, Sparks, USA) and niacin test were done on positive MGIT culture bottles and on acid-fast bacilli grown on L-J medium, respectively. When MTBC was excluded, AFB grown in MGIT bottle were identified with MALDI-TOF MS. For MALDI-TOF MS identification an extraction method with zirconia/silica beads was used. The extract was analyzed with MALDI Biotyper 3.1 (Bruker Daltonik GmbH, Bremen, Germany) and the spectra obtained were compared to Mycobacteria Library 1.0. Results: Tracheal aspirate was smear-positive with high positivity (AFB score 3+). After 7 days of incubation MGIT culture bottles turned positive for AFB and BD MGIT TBc Identification Test was negative indicating the growth of nontuberculous mycobacteria. MALDI-TOF MS resulted in identification of M. abscessus with score value >2.0 corresponding to an identification at species level. Isolation and identification of grown mycobacteria was achieved in 8 days. Conclusion: Nontuberculous mycobacteria often colonize airways of lung transplant candidates and can cause infections after transplantation. Eradication is recommended prior to lung transplantation and medical treatment effectively controls NTM disease if develops after transplantation. MALDI-TOF MS for identification of NTM could have a major clinical impact in these patients by very fast species identification. It can guide decision about the necessity of antimicrobial treatment and can assist in selecting appropriate treatment because of the predictable susceptibility profiles in specific NTM species.

MALDI-TOF; identification; non-tuberculous mycobacteria

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