Background: Pulmonary non-tuberculous mycobacteria (NTM) represent an increasing threat. Consideration of NTM pathogenicity has increased but diagnosis remains difficult due to slow growth of mycobacteria and to pulmonary flora that can be abundant, especially in the case of cystic fibrosis (CF) patients. RGM medium has been designed at the Freeman Hospital (Newcastle Upon Tyne, UK). This medium shows increased specificity and sensitivity compared to reference methods. NTM Elite agar (bioMérieux) is the industrial version derived from RGM medium dedicated to the isolation of NTM from respiratory specimens without prior decontamination. RGM and NTM Elite agar have been compared with pure strains and sputum specimens.
Materials/methods: Pure strains (51 NTM; 46 non-mycobacteria) and 83 sputa (77 patients including 70 with CF) were inoculated on both RGM and NTM Elite agar. Media were incubated at 30°C for 28 days with regular readings, minimum once a week. The number of colonies isolated on both media were then compared. Clinical isolates were identified by MALDI-TOF MS.
Results: Among the 51 NTM strains inoculated, 46 grew on RGM (90%) and 47 on NTM Elite agar (92%) after 28 days incubation. Among the 46 non-mycobacteria strains, 34 were inhibited on RGM (74%) and 36 on NTM Elite agar (78%). From 83 sputa, respectively, 5 and 6 isolates belonging to Mycobacterium avium complex were isolated on NTM Elite agar and RGM media. Seven isolates belonging to Mycobacteroides abscessus were isolated on both media. A higher number of Mycobacteroides chelonae isolates were recovered on NTM Elite agar, but this may be artefactual since an important proportion grew only after 21 days of incubation. Among the 83 sputa tested, no overwhelming growth of pulmonary flora was observed at final reading. Fifteen non-mycobacteria were isolated on both media (17 recovered in total) and 4 NTM were isolated in the presence of contaminants.
Conclusions: NTM Elite agar, industrial version of RGM, appears as efficient as RGM for the culture of NTM. These media enable a simple, sensitive and highly selective screening of NTM in sputa without prior decontamination. Therefore, they have a real contribution to the diagnosis of NTM pulmonary infection.