Bacteriophages, or simply phages, are viruses that infect bacteria. They possess a protein shell surrounding the phage genome, which with few exceptions is composed of DNA. A bacteriophage attaches to specific receptors on its host bacteria and injects its genome through the cell wall. This forces the host cells to synthesize more bacteriophages. The host cell lyses at the end of this reproductive phase. So-called temperate bacteriophages lysogenize the host cells, whereby their genomes are integrated into the host cell chromosomes as the so-called prophage. The phage genes are inactive in this stage, although the prophage is duplicated synchronously with host cell proliferation. The transition from prophage status to the lytic cycle is termed
spontaneous or artificial induction. Some genomes of temperate phages may carry genes which have the capacity to change the phenotype of the host cell. Integration of such a prophage into the chromosome is known as lysogenic conversion.
Bacteriophages are viruses the host cells of which are bacteria. Bacteriophages are therefore obligate cell parasites. They possess only one type of nucleic acid, either DNA or RNA, have no enzymatic systems for energy supply and are unable to synthesize proteins on their own.
The Importance of the Bacteriophages
Bacteriophages are often used as models in studies of fundamental biological processes: DNAreplication, gene expression, gene regulation, viral morphogenesis, studies of the details, and function of supramolecular
Vectors for gene cloning, adjuvants in sequencing Therapy and prevention An older concept now receiving increased attention. Administration of suitable phage mixtures in therapy and prevention of gastrointestinal infections. In animal husbandry, a number of phages that attack only EHEC (enterohemorrhagic E. coli) are used against EHEC infections.
Bacterial typing. Strains of a bacterial species are classified in phagovars (syn lysotypes) based on their sensitivity to typing bacteriophages. Recognition of the bacterial strain responsible for an epidemic, making it possible to follow up the chain of infection and identify the infection sources. This typing method has been established for
Salmonella typhi, Salmonella paratyphi B, Staphylococcus aureus, Pseudomonas aeruginosa, and other bacteria, although it is now increasingly being replaced by new molecular methods, in particular DNA typing.
Reproduction of Bacteriophages
Ref: Kayser, Medical Microbiology © 2005 Thieme