A genetically modified strain of M. tuberculosis or Mycobacterium bovis BCG is provided, wherein the genetically modified strain comprises at least one modified sequence comprising SEQ ID NO: 1, SEQ ID NO: 2, or both, having at least one mutation at T2, Q4, F8, A14, L28, L29, W43, G45, Y51, Q55, Q56, N66, M83, V90, M93, or F94 in SEQ ID NO:1; or at least one mutation at Q3, F7 A13, L27, W42, G44, Y50, Q54, N65, N67, M82, V89, M92, or F93 in SEQ ID NO:2, or a deletion at the terminal end of less than 20 amino acids. In a preferred embodiment, the mutation is at least one of T2H, Q4L, F8I, AI4R, L28A, L29S, W43R, G45T, Q55I, Q56A, N66I, N67A, M83I, V90R, M93T, or F94Q in SEQ ID NO:1, and Q3L, F7I, A13R, L27A, L28S, W42R, Q44T, Q54I, N65I, M82I, V89R, M92T, and F93Q in SEQ ID NO:2. Similarly, the genetically modified strain may also secrete ESAT-6 with a hexa-histidine tag, tetra-cysteine tag, or FLAG-tag, a GFP-fusion, or a short truncation at the C-terminal end of less than 20 amino acids.