Background Isocitrate lyase (ICL) was previously demonstrated to play a pivotal role in the intracellular metabolism of Mycobacterium tuberculosis (MTB).Presently severallines of evidence suggest that ICL from MTB (MTB-ICL) may play some roles in the interaction between MTB and host macrophage.However,there has been no research on the interaction between MTB-ICL and host macrophage.Methods MTB-icl and M. smegmatis (MS)-icl genes were amplified by polymerase chain reaction (PCR) and cloned into the E. coli-mycobacterium shuttle plasmid pUV15 to obtain recombinant shuttle plasmids PMTB-icl and pMS-icl.Following transformation into MS by electroporation,the expression of pMTB-icl and pMS-icl was verified by reverse transcriptase (RT)-PCR.The expression of recombinant plasmids derived from pUVl 5 when rMS was phagocytized by macrophage was also verified via fluorescence microscope.Ms 1-2c,rMS-pUV15,rMS-pMS-icl and rMS-pMTB-icl were used to infect RAW264.7 cells and the survivaI of intracellular MS was monitored by bacterial culture at 0.24 and 48 hours after infection.The culture supatants from macrophage;nfected by Ms 1-2c,rMS-pUV15,rMS-pMS-icl and rMS-pMTB-icl were collected and the interferon (IFN)-V and nitric oxide (NO) concentrations were measured by ELISA or by Griess assay,respectively.The apoptosis of macrophage was assayed by the in situ TUNEL technique.Results RT-PCR showed that both pMTB-icl and pMS-icl could be expressed in MS.Fluorescence microscopic observation showed that recombinant plasmids derived from pUV15 (pUV15-IG) could also be expressed in MS when MS were phagocytized by macrophage.Bacterial culture data demonstrated that rMS-pMTB-icl exhibited significantly increased intracellular survival in the murine macrophage cellline RAW264.7 compared with Ms 1-2c.rMS-pUV15 and rMS-pMS-icl.This increased intracellular survival was not accompanied by the upregulation of IFN-Y and NO in host macrophage.But a lower apoptosis rate of macrophages infected with rMS-pMTB-icl was observed when compared with macrophages infected with other strains of MS.Conclusions MTB-ICL could promote the intracellular survival of MS.Suppressing the apoptosis of host macrophage may be one of the impoRant mechanisms involved in this increased intracellular survival.