In-vitro Effect of Iron on the Growth Density of Mycobacterium tuberculosis and Drug Susceptibility Testing

Abstract

This work was carried out to study the effect of iron on the growth density of Mycobacterium tuberculosis as well as drug susceptibility testing. Fifty two smear-positive acid fast bacilli out of 100 sputum specimens were obtained from patients who were referred to National Health Laboratory, Federal Ministry of Health, Sudan. The smear positive specimens were cultured onto control LJ medium and other three sets of LJ medium containing ferrous sulfate (iron) with different concentration; 100 mg/l, 200 mg/l and 400 mg/l. The growth was graded as negative (0 = no growth), +1 (1-19 colonies), +2 (20-100 colonies) +3 (100-200 colonies), +4 (200-500 colonies) and +5 (more than 500 colonies). At the same time, proportion method was applied to test susceptibility of mycobacterial isolates (20 isolates) to anti-tuberculosis drugs; isoniazid (INH), rifampicin (RIF), ethambutol (EMB) and streptomycin (STM) in the presence and absence of ferrous sulfate. Chi squire test was used to analyze categorical variables and the significance level was set at P = .05. Collectively, the results showed that the growth density of M. tuberculosis increased significantly when ferrous sulfate was added to LJ medium. Resistance to the four first line anti-tuberculosis drugs (INH, RIF, EMB, STM), INH and EMB were also respectively increased from 10% to 15℅, 20% to 40% and 10% to 15% in the presence of ferrous sulfate. Contrary to other findings, it was observed that addition of ferrous sulfate to LJ medium enhanced the activity of STM while no effect was seen on RIF and multi-drug resistance (INH and RIF). The study concluded that the growth density of M. tuberculosis can be enhanced by supplementing LJ medium with 100-200 mg/l iron. This would be useful in recovery of the organisms from specimens with a low bacterial load particularly in laboratories in low income setting. Supplementation of LJ medium with iron must be avoided if drug susceptibility testing is required.