Transformation and transposition of the genome of Mycobacterium marinum

To develop and evaluate protocols for genetic manipulations (transformation and transposition) of the fish pathogen, Mycobacterium marinum.

Isolates of M. marinum obtained from fish and humans.

Electrocompetent cells were prepared from isolates of M. marinum grown to various growth phases at several temperatures and with or without the addition of ethionamide or cycloheximide. Mycobacterial cells were transformed by electroporation with a replicative Escherichia coli-mycobacteria shuttle vector (pYUB18) as well as suicide vectors (pYUB285 and pUS252) that carried transposable elements (IS1096 and IS6110, respectively). Mutants from both isolates of M. marinum were recovered on 7H10 agar plates supplemented with kanamycin. Transformation and transposition efficiencies for various protocols were compared. Southern hybridization analysis was performed on mycobacterial mutants to confirm transposition events.

Competent cells prepared at room temperature (23-25 C) from organisms in late-exponential growth phase yielded higher transposition efficiency, compared with cells prepared at 4 C or from organisms in early- or mid-exponential growth phase. Naturally developing kanamycin-resistant colonies of M. marinum were not detected. Only the IS1096-derived transposition was able to efficiently mutate M. marinum. Southern hybridization of M. marinum mutants revealed random integration of IS 1096 into the M. marinum genome.

Transposition and transformation efficiencies were comparable, suggesting that the limiting factor in transposition is the transformation step. Most of the experiments resulted in transposition of IS1096; however, better approaches are needed to improve transposition efficiency.