Introduces bacteriophages as viruses that infect bacteria, then covers their structure, with a table of capsid, tail and tail-fibre roles and a note on phage T4. It classifies phages by nucleic acid type in a table with examples, contrasts the lytic and lysogenic life cycles, and defines prophage, lysogeny and induction. The note closes with clinical uses such as phage therapy, typing and genetic tools, phage-carried bacterial toxin genes, and a table of advantages and limitations.
Topic 101 of 113 in the MU Sofia syllabus for Microbiology (Dental Medicine, Year 2). The free AceMed note covers it in 9 sections, about 277 words (3 min of reading), with 14 flashcards.
In this note
Bacteriophages
I. Definition
II. Structure of Bacteriophages
III. Classification of Phages
IV. Life Cycles of Bacteriophages
V. Specialized Terms
VI. Clinical Applications
VII. Virulence and Toxin Genes via Phage (Lysogenic Conversion)
VIII. Advantages and Limitations of Phage Use
I. Definition
Bacteriophages (or phages) are viruses that infect bacteria.
The word comes from Greek "phagein" meaning "to eat" – i.e., "bacteria-eaters."
They are host-specific and do not infect eukaryotic cells.
II. Structure of Bacteriophages
Composed of nucleic acid core (DNA or RNA) enclosed in a protein capsid.
May have tail fibers that help in adsorption to bacterial surface receptors.
Phage T4 (E. coli phage) is the classic example of a complex bacteriophage.
III. Classification of Phages
Based on nucleic acid type:
IV. Life Cycles of Bacteriophages
A. Lytic Cycle (Virulent Phages)
Phage attaches to bacterial surface → injects DNA
Viral DNA hijacks host machinery → replicates → phage assembly
Phage-encoded lysozyme degrades bacterial wall → lysis → release of new phages
Host cell dies
Example: T4 phage of E. coli
B. Lysogenic Cycle (Temperate Phages)
Phage DNA integrates into bacterial chromosome as a prophage
Bacteria reproduces with prophage → vertical gene transfer
Under stress, prophage may exit chromosome → switch to lytic cycle
Example: Lambda phage (λ-phage)
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