RGM medium versus MGIT™ for isolation of nontuberculous mycobacteria from sputum of patients with cystic fibrosis.

Background: Isolation of non-tuberculous mycobacteria (NTM) from sputum of patients with cystic fibrosis (CF) is challenging as, due to contaminant overgrowth, many cultures need to be abandoned. RGM medium is a novel agar-based culture medium which contains growth factors and selective agents to allow isolation of NTM. Material and methods: In this prospective study, all patients of the clinic on regular follow-up and without lung transplant (n = 188, 18y + :57%) were eligible if able to expectorate at least 2 ml either spontaneously or at the end of a full session of physiotherapy during a routine visit. The samples were inoculated onto both RGM medium and MGIT, the latter having been preceded by a decontamination step using NALC (0.5%)-NaOH (4%). Both media were incubated for 42 days. Identification of NTM was confirmed by MALDI TOF MS and rpoB sequencing. Results: Out of 123 eligible patients, data are currently available for 106 (M: 57; PS: 17; mean age: 27.1 y ± 12.6, range: 6.8–69; mean FEV1: 79% pr ± 23; chronic colonization by P. aeruginosa: 31%). Potentially pathogenic NTM were recovered from 9 patients. Concordant positive results were obtained in only 3 patients, all with M abscessus (MABSC). RGM and MGIT enabled the isolation of 7 and 5 NTM strains respectively, with 2 undetected NTM for RGM (MABSC:2) and 4 for MGIT (M chelonae:2, M avium:1, M kansasii:1). Whereas 40 MGIT (34.5%) were contaminated and had to be discontinued, 11 RGM (9.5%) were contaminated (p < 0.001) and remained interpretable due to weak contaminant overgrowth. In addition, contaminants isolated with RGM all were relevant pathogens in CF (A. xylosoxidans:7, B. gladioli:1, B. multivorans:1, yeast:2). Conclusion: Whether RGM cultures are as sensitive as MGIT to isolate relevant NTM remains to be investigated. However, RGM cultures are far less contaminated, easier to perform and cheaper than liquid media. These characteristics make them an attractive tool allowing for more regular NTM screening