Phages

Phages, or bacteriophages, are viruses that infect bacteria. They are among the most abundant biological entities and play pivotal roles in microbial ecology. A typical phage particle consists of a nucleic acid genome enclosed in a protein capsid; many also possess a tail that attaches to bacterial surfaces and delivers the genome. Phages show extensive diversity in size, morphology, and genome organization. Most known phages have double-stranded DNA genomes, but both RNA and single-stranded forms exist.

Life cycles: Lytic phages replicate quickly, hijack the host machinery, produce many progeny, and cause cell lysis. Lysogenic or temperate phages integrate their genome into the bacterial chromosome as a prophage, remaining dormant until induction triggers the lytic cycle. Environmental factors such as stress or UV light can induce lytic growth. Phages are often host-specific and may recognize particular surface receptors.

Taxonomy and morphology: Many bacteriophages are tailed phages, belonging to the order Caudovirales, with families Myoviridae, Siphoviridae, and Podoviridae; there are also tailless and filamentous forms. Genome content ranges from a few thousand to hundreds of thousands of base pairs, encoding proteins for replication, assembly, and lysis. Plaques on bacterial lawns are a traditional sign of lytic activity.

Significance and applications: Phages shape bacterial ecology and have been explored as alternatives or complements to antibiotics in phage therapy. They are used in food safety to reduce bacterial contamination and as biocontrol agents in agriculture and aquaculture. In biotechnology, phage display libraries exploit phage proteins to study interactions. Phages contribute to horizontal gene transfer and influence bacterial evolution, and their study has informed tools such as CRISPR-based defense systems.