Susceptibilities of Mycobacterium malmoense determined at the growth optimum pH (pH 6.0)

Setting: Pulmonary disease caused by Mycobacterium malmoense is increasing. Conventional in vitro antimicrobial susceptibilities correlate poorly with response to treatment for this organism. Radiometrically determined minimum inhibitory concentrations (MICs) allow quantitative susceptibility testing for non-tuberculous mycobacteria. The M. avium complex (MAC) has been investigated extensively with this approach, and clear interpretative criteria have been established at pH 6.8. However, there has been little work with the acidophilic M. malmoense, which grows poorly at pH 6.8.Objective: To determine whether MICs at pH 6.0 provide results compatible with the interpretative criteria established for the MAC.Design: MICs were performed in Middlebrook PZA medium (pH 6.0) and 7H12 medium (pH 6.8) for ten strains of M. malmoense.Results: MICs can be determined at pH 6.0 for M. malmoense using the criteria adopted for the M. avium complex.Conclusion: The low optimal pH of M. malmoense suits this organism for growth in acid conditions. As with MAC, M. malmoense multiplies within macrophages in vivo, and MICs determined at pH 6.0 may reflect in vivo activity. The combination of radiometric MIC testing at optimal growth pH and interpretation based on pharmacokinetic parameters may be helpful in designing therapeutic regimens.