Recovery of nontuberculous mycobacteria from non-cystic fibrosis respiratory specimens using a decontamination-free selective agar

Digestion and decontamination during acid-fast bacilli (AFB) culture processing are performed to suppress growth of normal microbiota in respiratory specimens; however, these steps may render AFB nonviable. This study aimed to evaluate the performance of NTM Elite agar, a decontamination-free selective medium, for the recovery of nontuberculous mycobacteria (NTM) compared to standard of care (SOC) processing and solid and liquid media inoculation for AFB culture in five hundred lower respiratory samples from non-cystic fibrosis patients. Concurrently, the same specimens were directly inoculated onto NTM Elite agar (NTM Elite-Direct Inoculation) or centrifuged and washed with saline prior to inoculation onto NTM Elite agar (NTM Elite-Concentrated) and incubated up to 28 days. The overall median time to positivity was 7.0 days (IQR = 7–18) for NTM Elite-Direct Inoculation and 14 days (IQR = 7–14) for NTM Elite-Concentrated, which were similar to SOC broth but shorter than SOC solid agar at 35 days (IQR = 2–41). Breakthrough non-AFB growth rate was 1.4% (7/500) NTM Elite-Direct Inoculation, which was less than NTM Elite-Concentrated at 9.0% (45/500) and SOC media at 10.0% (50/500). Forty-five unique isolates were included in the analysis for sensitivity of NTM detection. Sensitivity was 43.5% (95% CI = 30.2–57.8) for SOC media, 20.0% (95% CI = 10.9–33.8) for NTM Elite-Direct Inoculation, and 82.6% (95% CI = 69.3–90.9) for NTM Elite-Concentrated. Combined with SOC broth culture, sensitivity was 93.3% (95% CI = 82.1–97.7) for NTM Elite-Concentrated. The high sensitivity of the latter procedure indicates potential for NTM Elite agar to replace SOC solid agar for detection of NTM in areas of high NTM incidence but low incidence of tuberculosis.