Antibiotic susceptibility profile of Mycobacterium avium subspecies hominissuis is altered in low-iron conditions

Abstract

Antibiotic susceptibility testing of the Mycobacterium avium complex is often characterized by a lack of correlation between in vitro results and clinical response. The reason for this discrepancy might lie in the difference between in vitro susceptibility testing conditions and the actual environment experienced by mycobacteria in the host. The availability of iron is one such difference, which is limited in host macrophages upon infection, but abundant in susceptibility testing media. Accordingly, the aim of our study was to determine whether iron limitation affects the antibiotic susceptibility profile of M. avium subspecies hominissuis. Susceptibilities to multiple antibiotics targeting various cellular processes were determined in media with normal- and low-iron concentrations using the resazurin microplate assay. Differences in susceptibilities were evaluated by monitoring changes in the MIC and growth inhibition at subinhibitory antibiotic concentrations (sub-MICs). Cultures grown in low-iron conditions were less susceptible to the DNA synthesis inhibitors 6-mercaptopurine and levofloxacin at sub-MICs. Decreased susceptibility to the protein synthesis inhibitors azithromycin (>2-fold) and streptomycin (at sub-MICs) was observed only during adaptation to low-iron conditions. On the contrary, increased susceptibility to antibiotics that interfere with cell wall synthesis [isoniazid (4-fold), d-cycloserine (2-fold) and ethambutol (at sub-MICs)], mycobactin synthesis [4-aminosalicylate (at sub-MICs)] and mRNA synthesis [rifampicin (4-fold)] was observed in low-iron conditions. The susceptibility profile in low-iron conditions significantly differs from that observed in normal-iron conditions. Mimicking the host environment in terms of iron availability should be considered for in vitro susceptibility testing of mycobacteria, especially for antibiotics interfering with iron metabolism, such as 4-aminosalicylate.