Differences in the susceptibility of mycobacterium tuberculosis to the 1st and 2nd line antituberculosis drugs under aerobic and anaerobic conditions.

Abstract

Although Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), is now considered a facultative anaerobe, and bacilli isolated from sputum specimen possess morphologies identified from bacilli growing aerobically and under oxygen deprived conditions, most of the targets for the antituberculosis drugs are readily found on bacilli that are thriving aerobically. This raises questions on the efficiency of antituberculosis drugs on eradicating the pathogen from the host during treatment. In this study to determine whether the antituberculosis drugs that are used currently for the treatment of TB have similar effect of these different populations of this mycobacterium, we grew this organism under aerobic and oxygen deprived environments and then subjected them to the antimicrobial agents. The minimum inhibitory concentration (MICs) of these isolates against nine antituberculosis drugs were determined under aerobic conditions for the 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) assay and under both aerobic and anaerobic conditions using the Microscopic Observation Drug Susceptibility (MODS) assays. In addition the bactericidal activities of isoniazid, rifampicin, kanamycin and ofloxacin were tested and compared amongst MDR isolates that were growing aerobically and anaerobically. There were some differences in the MICs determined by the MTT assay and the MODS assay for some isolates. For the susceptible isolates the MICs from the MTT assay were higher than the MICs from the MODS assay. The reverse was true for the drug resistant isolates. The reference strain H37Rv was resistant to some of the antimicrobial agents that were tested in this study. This was under both methods. However, MICs measured under anaerobic conditions with anaerobic bacilli did not yield viable results due to absence of growth as the bacilli are known to replicate at a negligible rate under anaerobic conditions. The bacilli in the inoculum were viable as following 40 days of anaerobic incubation but upon aerobic incubation of these cultures, growth was observed. And again with the bactericidal assays that were conducted on the multidrug resistant (MDR) isolates proved this. Rifampicin was the most potent antimicrobial agent against the anaerobic M. tuberculosis as susceptibility to this antimicrobial agent increased under anaerobic conditions.