RGM medium offers a simple and effective culture method for the isolation of rapidly-growing mycobacteria from sputum samples from patients with CF without decontamination of samples. RGM medium allows for the systematic screening of all sputum samples routinely referred for culture from patients with CF.

A novel selective agar (RGM medium) has been advocated for the isolation of rapidly growing mycobacteria from the sputa of cystic fibrosis (CF) patients. The aim of this study was to compare RGM medium to selective agar (BCSA) and a standard acid-fast bacillus (AFB) culture method for the isolation of nontuberculous mycobacteria (NTM) from patients with CF. The applicability of matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) for the identification of NTM isolated on RGM medium was also assessed. Respiratory samples ( = 869) were collected from 487 CF patients and inoculated directly onto RGM medium and BCSA. Cultures were incubated at 30°C and examined for up to 28 days. A subset of 212 samples (from 172 patients) was also cultured by using a mycobacterial growth indicator tube (MGIT) and on Lowenstein-Jensen medium following dual decontamination. By using a combination of all methods, 98 mycobacteria were isolated from 869 samples (11.3%). The sensitivity of RGM medium (96.9%) was significantly higher than that of BCSA (35.7%) for the isolation of mycobacteria ( < 0.0001). The sensitivity of RGM medium was also superior to that of standard AFB culture for the isolation of mycobacteria (92.2% versus 47.1%; < 0.0001). MALDI-TOF MS was effective for the identification of mycobacteria in RGM medium. RGM medium offers a simple and highly effective tool for the isolation of NTM from patients with CF. Extended incubation of RGM medium for 28 days facilitates the isolation of slow-growing species, including members of the complex (MAVC).

cIsolation of nontuberculous mycobacteria (NTM) from the sputum of patients with cystic fibrosis (CF) is challenging due to overgrowth by rapidly growing species that colonize the lungs of patients with CF. Extended incubation on Burkholderia cepacia selective agar (BCSA) has been recommended as an expedient culture method for the isolation of rapidly growing NTM in this setting. The aim of this study was to assess five selective media designed for the isolation of Burkholderia cepacia complex, along with two media designed for the isolation of mycobacteria (rapidly growing mycobacteria [RGM] medium and Middlebrook 7H11 agar), for their abilities to isolate NTM. All seven media were challenged with 147 isolates of rapidly growing mycobacteria and 185 isolates belonging to other species. RGM medium was then compared with the most selective brand of BCSA for the isolation of NTM from 224 sputum samples from patients with CF. Different agars designed for the isolation of B. cepacia complex varied considerably in their inhibition of other bacteria and fungi. RGM medium supported the growth of all isolates of mycobacteria and was more selective than any other medium. NTM were recovered from 17 of 224 sputum samples using RGM medium, compared with only 7 samples using the most selective brand of BCSA (P ‫؍‬ 0.023). RGM medium offers a superior option, compared to other selective agars, for the isolation of rapidly growing mycobacteria from the sputum of patients with CF. Furthermore, the convenience of using RGM medium enables routine screening for rapidly growing NTM in all submitted sputum samples from patients with CF. Burkholderia cepacia complex (BCC) and nontuberculous mycobacteria (NTM) are both recognized as potentially important pathogens when isolated from the lungs of patients with cystic fibrosis (CF). The use of a selective culture medium is recommended for the isolation of BCC (1), and several brands of such media are commercially available. Rapidly growing mycobacteria (RGM) represent a subset of NTM that generate colonies on solid culture media within 7 days of incubation (2). The predominant species of NTM within the CF population in Europe is Mycobacterium abscessus complex (MABSC) (3-5), and there is convincing evidence that the prevalence of infection by MABSC in the CF population is increasing (3,(5)(6)(7). This rapidly growing species comprises three subspecies, i.e., Mycobacterium abscessus subsp. abscessus, Mycobacterium abscessus subsp. bolletii, and Mycobacterium abscessus subsp. massiliense.Culture of mycobacteria from sputum samples has traditionally relied on methods that were designed to accommodate slowly growing mycobacteria, particularly Mycobacterium tuberculosis, which is comparatively rare among cystic fibrosis patients. Such methods are laborious and expensive, as they involve chemical decontamination of samples and subsequent culture on both liquid and solid media (8, 9). Furthermore, contamination of cultures by faster growing microorganisms may mean that cultures have to be aban...