Characterization of the antimycobacterial effect of a pseudomonas-derived activity

Abstract

The emergence of multidrug-resistant strains of Mycobacterium tuberculosis (MTB), the causative agent of tuberculosis, reinforces the need for novel antimycobacterial compounds. Secondary metabolites from various microorganisms have provided most antibacterials introduced clinically since 1935. Previously, a putative Pseudomonas isolate was identified that inhibited growth of the non-pathogenic M. smegmatis (MSM). Here, we demonstrate the stable isolation of the inhibitory compound(s) in crude extract, and present microbiological data characterizing the antimycobacterial effect. A parallel extraction was performed on an unrelated Pseudomonas isolate which failed to inhibit growth of MSM, thereby confirming that the inhibitory effect is limited to our strain, designated Pseudomonas MB (anti-mycobacterial). Moreover, the crude extract inhibited growth of all Gram-positive organisms assayed, including other actinobacteria, but not the Gram-negative E. coli, suggesting the possibility of a Gram-restricted target range. As the cell wall constitutes the dominant target of natural-product antibacterials, we hypothesised that the active compound(s) might inhibit cell wall metabolism. However, preliminary data are inconclusive and the target of the extract remains to be elucidated, perhaps reflecting the presence of more than one active compound. Notably, the crude extract was shown by broth microdilution assay to inhibit growth of MTB at a concentration of 14-16 μg/ml, a value ten-fold higher than key frontline anti-TB agents tested. Therefore, although the identity of the constituent compound(s) and its mode of action are unknown, the apparent anti-MTB activity suggested by our preliminary experiments identifies the Pseudomonas-derived active agent(s) as a compelling candidate for further investigation as a potential lead compound(s) against a major human pathogen increasingly associated with drug resistance.