Interaction of Phages with Bacterial Cells

There are four basic stages in phage-bacterial interaction that generally reflect the common steps of any viral replication cycle: adsorption, cell penetration, phage reproduction, and release of newly formed phage particles.

Adsorption is provided by the attachment of receptor fiber proteins of the phage tail to the specific receptors on the bacterial cell wall.

Reversible and irreversible phases of adsorption are indicated. The adsorption is accelerated by divalent cations (Ca2+ and Mg2+).

Penetration is simulated by the action of phage enzymes. For instance, phages T4 of E. coli possess lysozyme-like enzyme endolysin, which degrades the minimal site of the bacterial cell wall. The tip of the tail opens allowing viral DNA to move through the channel of the phage tail. Phage ATPase generates energy for tail contraction, and genomic nucleic acid is injected by into bacterial cell, passing through the cell wall and cytoplasmic membrane. The phage capsid remains outside the cell.

Reproduction stage covers the period between the phage penetration and release of newly created phage particles. At the beginning of reproduction there is short-term eclipse phase, when phage biopolymers (genomic DNA and proteins of the coat) are synthesized.

Phage DNA serves both as the template for replication of new phage DNA molecules and for transcription of matrix RNA, which encodes phage proteins.

After the DNA penetration a number of “early” enzymes and other proteins are formed within the cell. They are generally termed as phage-induced proteins. Their synthesis occurs due to the partial transcription of phage DNA by cellular polymerases. Some of these proteins (for instance, phage-induced nuclease) block bacterial cell DNA replication, and the phage switches bacterial intracellular machinery to its own purposes.

As the result, the enzymes of bacterial cell actively supply the process of phage reproduction with energy and monomers for protein and nucleic acid synthesis. Bacterial ribosomal apparatus produces phage proteins.

Then the assembly of structural components of phage (its head, tail and tail spikes, fibers) is activated.

Late-induced enzymes provide the assembly of phage particles and phage release out of bacterial cell.

Phage assembly is a complex process. After phage head formation DNA is packed into it, then the tail is attached with subsequent addition of tail spikes.

Overall, for above mentioned T4-phage eclipse phase is finished within about 30 minutes; and new viable phage particles can be detected inside the bacterial cell.

Phage release is initiated by late-induced enzymes with cell wall hydrolyzing activity (endolysins or murein hydrolases). They break down bacterial peptidoglycan layer with subsequent phage dissemination.

Finally, hundreds of newly formed phage particles are liberated, and these virions are capable of infecting the neighbouring bacterial cells.