Interaction of phages with outer membrane vesicles: Immunological aspect

Bacteriophages (phages) are viruses that infect bacteria. Although it is clear that phages may help the host immune system to kill bacterial pathogens, many details of bacteria-phage-host immune interactions remain poorly understood. Bacterial outer membrane vesicles (OMVs) are one of the virulence factors of Gram-negative organisms. OMVs are spherical membrane-enclosed microparticles produced during bacterial growth. During bacterial infections, OMVs are recognized by the host immune system and participate in the elicitation of immune responses. OMVs have been found to interact with bacteriophages in a host-specific manner. The effects of such interactions on the biological functions of OMVs remain largely unknown. In this project, research into the immunological consequences of bacteriophages interacting with OMVs was assessed by investigating whether OMV-phage interaction affects the inflammatory response elicited by OMVs alone.

An important methodological development from this project was to optimise a protocol to isolate and purify OMVs from E. coli. OMVs were isolated in a highly pure form. They were interacted with purified E. coli specific phages. OMVs, phages and OMV-phage mixtures were then interacted with macrophage cells, and it was found that the OMV-phage interaction led to a reduction of the pro-inflammatory response elicited by OMVs alone. This suggests that phages may impact bacterial-host immune system interactions not only by killing the pathogens, but also by altering host responses elicited by the conserved bacterial virulence factor, OMVs.

Furthermore, preliminary results revealed that interactions of a bacterial lysogen with macrophages or whole blood samples resulted in the induction of prophages from lysogenized bacteria. This may suggest that during the interaction of bacteria with the host immune system, some bacteria may be killed not by the immune assault per se, but by the induced prophages.

Three bacteriophages infecting K. pneumoniae were isolated from sewage samples by the traditional method of phage isolation. Genomic and biological characterisations were performed on two of these phages. These phages were used in this current research and may be used in the future to investigate their impact on the immunological responses elicited by Klebsiella OMVs, once a methodology to isolate sufficient quantities of OMVs from this pathogen is developed.