Antimycobacterial activity of cyclic dipeptides isolated from Bacillus sp. N strain associated with entomopathogenic nematode

Abstract Context: Tuberculosis (TB) is one of the leading causes of morbidity and mortality with a global mortality rate of two million deaths per year; one-third of the world's population is infected with Mycobacterium tuberculosis. Objective: The aim of this study was to determine the antimycobacterial activity of six diketopiperazines (DKPs) purified from a Bacillus sp. N strain associated with entomopathogenic nematode Rhabditis (Oscheius) sp. Materials and methods: The minimum inhibitory concentration (MIC) and minimum bactericidal concentration of DKPs were determined using the broth dilution method on Middlebrook 7H11 against M. tuberculosis H37Rv. Time-kill assay was used to determine the rate of killing of M. tuberculosis H37Rv by DKPs. The cytotoxicity of the DKPs was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay against the VERO cell line. Results: Out of six DKP-tested cyclo-(d-Pro-l-Leu), cyclo-(l-Pro-l-Met) and cyclo-(d-Pro-l-Phe) recorded antimycobacterial activity, the cyclo-(l-Pro-l-Met) showed the highest activity and MIC values of 4 μg/ml for M. tuberculosis H37Rv. The MIC value for rifampicin was 0.06 μg/ml. Growth curve study by the MIC concentration of cyclic dipeptides recorded significant inhibition when compared with control. Time-kill curve showed maximum reduction of colony count was between 3 and 5 weeks. The DKPs are nontoxic to the VERO cell line up to 200 µg/ml. The antimycobacterial activity of cyclo-(d-Pro-l-Leu), cyclo-(l-Pro-l-Met) and cyclo-(d-Pro-l-Phe) is reported in this study for the first time. Discussion and conclusion: In conclusion, the potency, low cytotoxicity and selectivity of these compounds make them valid lead compounds for treatment against TB.