Morphology of Bacteriophages


Bacteriophages are typical viruses of complex structure.

The majority of phages resembles spermatozoids in their shapes but filamentous and some other forms occur. Head of phage contains viral genome – DNA or RNA.

The phage size usually varies from 20 to 200 nm.

Main structural components of phages are nucleic acid (DNA or RNA) and proteins. Bacteriophage RNA is single-stranded. DNA may be single or double-stranded. Nucleic acids of phage are tightly packed into the phage’s head.

Bacteriophages are composed of head, protein tail with hollow core and fibers, attached to tail basal plate.

Phage head is of protein nature with icosahedral symmetry. It can be naked or covered with external envelope (supercapsid). Phage head can carry spikes with receptor activity. Inner head proteins support DNA supercoiling. Phage’s tail of helical symmetry is covered with sheath, composed of contractile proteins, which provide tail contraction. They can be connected with calcium ions, and different enzymes (e.g., endolysins, ATPase or some others).

Small amount of lipids are also present in phages envelope.

Total protein content is about 50% of phage weight, nucleic acid content – 40-50% and lipids are about 1.5-3%.

Well-studied is a group of T- (or type) phages of E. coli, as well as temperate E. coli lambda-phage, some filamentous phages (for instance, M13 phage) and many others.

The differentiation of phages is based upon type of nucleic acid, phage’s morphology, chemical structure, and their interaction with the bacterial cell.

As the result, all phages are divided into DNA- and RNA-containing.

According to their morphology they are separated into several types:

– DNA filamentous phages;

– RNA-containing phages with rudimentary tails;

– DNA phages with short tails;

– DNA phages with long non-contractile tails;

– DNA phages with long contractile tails.

Phages are enough resistant to different physical and chemical factors (radiation, drying, pH fluctuations, or temperature). Nevertheless, they are rapidly inactivated by boiling, chemical disinfectants, UV-light.