Site-Directed Mutagenesis in Mycobacterium tuberculosis

Precise manipulation of the mycobacterial genome is required to study the effect of specific mutations without confounding influences from the diverse genetic backgrounds in clinical isolates. This is particularly important for interrogating gene function and drug resistance mechanisms, where background mutations may enhance or suppress the effects of a mutation of interest. Here, two targeted site-directed mutagenesis approaches, two-step allelic exchange using a suicide vector and oligonucleotide-mediated recombineering, are described and compared for introducing defined mutations into an isogenic Mycobacterium tuberculosis background. Two-step allelic exchange uses a suicide vector lacking a mycobacterial origin of replication and carrying a mutated DNA fragment, selectable markers such as sacB for sucrose sensitivity, lacZ for blue-white screening, and antibiotic resistance genes (kan or hyg). The process involves generating the mutated fragment via a multi-step polymerase chain reaction (PCR) or synthesis, cloning it into the vector, electroporating it into M. tuberculosis, and selecting single-crossover (SCO) integration events using antibiotic resistance and blue-white colony screening. This is followed by counter-selection with sucrose and X-gal to eliminate bacteria carrying the wild-type sequence. This method ensures stable, unmarked mutations but is time-consuming and labor-intensive. Recombineering uses a recombinase-expressing plasmid, such as pJV75amber expressing Che9c gp61 recombinase under an inducible acetamidase promoter, together with a single-stranded DNA (ssDNA) oligonucleotide to introduce precise mutations. Transformants are selected on appropriate antibiotics and confirmed by PCR and Sanger sequencing. This approach is simpler, faster, and more cost-effective, but may be limited by low transformation efficiency, the need for fresh competent cells, and variable efficiency depending on the genomic region and oligonucleotide design. Together, two-step allelic exchange and recombineering offer complementary strategies for introducing defined mutations into M. tuberculosis. Both methods enable experimental validation of mutations, such as variants associated with gene function and drug resistance.