A holistic insight of mycobacteriophage induced changes in mycobacterial cells

Mycobacteriophages are phages that interact with mycobacteria resulting in their killing. Although lysis is the major mechanism by which mycobacteriophages cause cell death, other mechanisms may also be involved. The present study was initiated with the objective of investigating the changes that take place at the cellular level following the infection of mycobacterial cells by phage D29. To investigate this issue, we took recourse to performing immunofluorescence and electron microscopic studies. Transmission electron microscopic examination revealed the adsorption of phages on to the surface of mycobacteria, following which penetration of the tail through the thick mycoloic acid layer was seen. At later time points discrete populations of cells at different stages of lysis were observed, which comprised of completely lysed cells, in which the cells were fragmented and those at the early onset stage exhibited formation of membrane pores through which the phages and intracellular contents were released. SEM results also indicated that phages may come out through the entire surface of the cell, or alternatively through gaps in the surface. In some of the images we observed structures that apparently resembled membrane blebs which are normally encountered when cells undergo programmed cell death (PCD). In addition, we observed significant increase in DNA fragmentation as well as membrane depolarization, which are also indicative of occurrence of PCD. As several bacterial PCD pathways are mediated by the toxin-antitoxin (TA) modules, the expression profile of all the TA systems was examined before and after phage infection. Apart from specifically addressing the issue of PCD in mycobacteriophage infected cells, this investigation has led to the development of facile tools necessary for investigating mycobacteriophage-mycobacteria interactions by means of microscopic methods.