bacteriophage

Bacteriophages, or phages, are viruses that infect and replicate within bacteria. They are among the most abundant biological entities on Earth and play important roles in microbial ecology, evolution, and biotechnology.

Structure and genome: Most phages consist of a protein capsid enclosing a nucleic acid genome, and many possess a tail that mediates attachment to bacterial receptors. Phage morphologies include myoviruses with contractile tails, siphoviruses with long non-contractile tails, and podoviruses with short tails. The genomes are predominantly double-stranded DNA, though RNA phages also exist. Genome sizes and gene content vary widely among families.

Life cycles: Phages can follow lytic or lysogenic life cycles. Lytic (virulent) phages hijack the host’s machinery to produce progeny, culminating in cell lysis and release of new virions. Temperate phages can integrate their genome into the bacterial chromosome as a prophage, replicating with the host until induction triggers the lytic cycle. Transduction, the transfer of bacterial genes by phages, is an important mechanism of horizontal gene transfer.

Host range and infection process: Phage infection is highly specific to particular bacterial species or strains, determined by receptor recognition. The infection sequence typically includes adsorption, genome injection, replication, assembly of new particles, and host cell lysis or extrusion.

Applications and significance: Phages are used in phage therapy as targeted antibacterial agents, especially amid rising antibiotic resistance. They also serve as tools in biotechnology (phage display, phage vectors), bacterial detection, and studies of microbial ecology and evolution. Phage research continues to explore coevolution with bacteria, CRISPR–based bacterial immunity, and the roles of phages in environment and disease.