Gene transfer in Mycobacterium tuberculosis: Shuttle phasmids to enlightenment

Infectious diseases have plagued humankind throughout history and have posed serious public health problems. Yet vaccines have eradicated smallpox and antibiotics have drastically decreased the mortality rate of many infectious agents (1). Although the precise viral agents had not yet been characterized, the smallpox vaccine work of Edward Jenner was critical in demonstrating that inoculation with pus from cowpox lesions could protect from a subsequent challenge with smallpox. These pioneering transfer experiments laid the groundwork for the eventual eradication of smallpox, as announced by the World Health Organization in 1979. The discovery of DNA as genetic material and the understanding of how this information translates into specific phenotypes have changed the paradigm for developing new vaccines, drugs, and diagnostic tests. Knowledge of the mechanisms of immunity and mechanisms of action of drugs has led to new vaccines and new antimicrobial agents. For example, the discovery of the Australia antigen (HBsAg) led to the subsequent engineering of the first recombinant vaccine, whose remarkable efficacy offers hope that eradication of hepatitis B in humans is not an unreasonable expectation (1, 2). Similarly, HIV infections, which not so long ago were uniformly fatal, are now controlled with drugs that were developed by understanding the HIV genome and gene products required for the HIV life cycle. The key to the acquisition of the knowledge of these mechanisms has been identifying the elemental causes (i.e., genes and their products) that mediate immunity and drug resistance. The identification of these genes is made possible by being able to transfer the genes or mutated forms of the genes into causative agents or surrogate hosts. Such an approach was limited in Mycobacterium tuberculosis by the difficulty of transferring genes or alleles into M. tuberculosis or a suitable surrogate mycobacterial host. The construction of shuttle phasmids, chimeric molecules that replicate in Escherichia coli as plasmids and in mycobacteria as mycobacteriophages, was instrumental in developing gene transfer systems for M. tuberculosis. This review will discuss M. tuberculosis genetic systems and their impact on tuberculosis (TB) research.