Background: Non-tuberculous mycobacteria (NTM) are important respiratory pathogens in patients with underlying lung diseases and particularly in patients with cystic fibrosis (CF). As a cornerstone of diagnosis, international guidelines recommend culture of sputum that has been decontaminated via chemical treatment e.g. using 2% N-acetyl L-cysteine with 2% sodium hydroxide (NALC-NaOH). RGM medium is a highly selective agar-based culture medium allowing isolation of NTM without decontamination of sputum samples. RGM medium therefore provides a convenient tool to quantitatively assess the impact of various decontamination strategies on the viability of NTM.
Materials/methods: Sputum samples from 41 distinct patients known to be colonized with NTM were subdivided and the aliquots were subjected to six different decontamination strategies followed by quantitative culture for NTM using serial dilutions inoculated onto RGM medium. 38 samples were from patients with CF. Thirty samples contained Mycobacterium abscessus complex (MABSC) and 11 contained Mycobacterium avium complex (MAC). Decontamination strategies included treatment with NALC-NaOH, 4% NaOH, 1% chlorhexidine, N/2 sulfuric acid, 5% oxalic acid and double decontamination with NALC-NaOH followed by 5% oxalic acid. As a control, a separate aliquot was treated with sterile saline (0.85%).
Results: The standard NALC-NaOH treatment resulted in an average reduction in colony count of 85% for MABSC when compared with saline-treated controls. 4% NaOH, which is commonly used in the UK, caused a 98% reduction in colony count. Standard treatments using sulfuric or oxalic acids were less deleterious but still resulted in an average reduction in colony count of at least 30%. The viability of MAC was much less affected by most decontamination treatments with the biggest impact caused by 1% chlorhexidine, which caused a 66% reduction in viability.
Conclusions: The viability of MABSC was severely compromised by the standard decontamination regimen recommended by international guidelines. This supports an abundance of recently acquired evidence to show that optimal recovery of MABSC is achieved by culture on selective agar media without decontamination of sputum samples.