Viruses are organisms which parasitize the host cell's protein synthesis machinery. Bacteriophages (phages or bacterial viruses) are viruses that infect and replicate in bacterial cells. Phages may have dsDNA, ssDNA, dsRNA or ssRNA as chromosome and the chromosome can be circular or linear. Phages have either a lytic or a lysogenic cell cycle.
Lytic phages injects their chromosome into the bacterial host. To be able to do this, the phage has to adhere to specific receptors on the surface of the host bacterium. Then, the chromosome is replicated and phage proteins are synthesized by means of the host bacterial ribosome etc. When new phage progeny has formed, the bacterial host cell will lyse and the phage particles will find new bacterial host cells.
Lysogenic (= temperate) phages does not lyse the host bacterium immediately, but its chromosome can instead be integrated into the bacterial genome and there it exist as a so-called prophage (endogenous phage). The prophage will then replicate during the bacterial cell division and is passed on to successive generations. The phage will exist as a prophage until the environment for the bacteria deteriorate. Then the prophage become active, form new phage particles and finaly lyse the host cell.
Some bacteria (e.g. Corynebacterium diphtheria, Clostridium botulinum, Shigella dysenteriae, Escherichia coli type VTEC and Streptococcus pyogenes) are pathogenic only if they carry a prophage. They are then said to have undergone lysogenic conversion and it is thus the genes of the prophage that encode important virulence factors.
Phages as diagnostic tools
Phages are not only species-specific, but in many cases also strain-specific and therefore, they can be used for subtyping of bacteria. Phagetyping is used for epidemiological studies of Salmonella enterica subsp. enterica. Phagetyping is perfomed by testing as a set of phage types and examine which of these can lyse the current salmonella strain. To perform the testing, a drop of bacteria is spread onto an agar plate, which is then allowed to dry on the surface. Then, small drops of phages with defined specificities are added. After incubation, the agar plate is inspected for plaques, which are formed where phages for which the bacterium is sensitive, have been applied. Exemples of other bacterial genera for which phage typing is used: Bacillus, Campylobacter, Clostridium and Staphylococcus.
Since the problem of multi-drug resistant strains of bacteria increases worldwide, the interest in treating bacterial infectious diseases by phage therapy has increased. For this purpose, lytic phages are best suited. The method is still in the experimental stage, but many researchers believe that it has great potential in both human and veterinary medicine.