Bacterial Cell Entry

Bacterial cell walls are strong and relatively thick, to protect them from osmotic lysis and predation, and to give them shape. GRAM-POSITIVE cells have a single internal lipid bilayer, and a thick PEPTIDOGLYCAN cell wall.

GRAM-NEGATIVE cells have an internal membrane, a thin peptidoglycan layer, another membrane, and often a polysaccharide-based CAPSULE.

Bacterial viruses (BACTERIOPHAGES) have therefore to have some means of breaching a quite formidable barrier if they are to enter the cell. They also generally have SPECIFIC RECEPTOR SITES on the bacteria, to which SPECIFIC ATTACHMENT PROTEINS bind: these receptor sites may be lipopolysaccharides, cell wall proteins, teichoic acid, or flagellar or pilus proteins.

Phage T4 - Enterobacteria phage T4, genus "T4-like Viruses", family Myoviridae, or viruses with 34-170 kbp dsDNA genomes, isometric heads and contractile tails - infects the gram-negative bacterium E coli. It has one of the more complex entry mechanisms, involving an active injection process. This is shared by others of the so-called T-even phages of the family Myoviridae. The process is shown in the still image above left (courtesy Russell Kightley)

The animated graphic showing T4 phage attaching and injecting its DNA was derived from negative-stained electron micrograph images taken by Linda Stannard, (then Dept Medical Microbiology, UCT).

See here for a high-resolution video, and here for a recent YouTube video

Click here for an non-animated view of the infection process if your browser does NOT support GIF animation.

The phage tail fibres are the attachment sites; these individually bind the bacterial cell surface - specifically to certain lipopolysaccharides and to the surface outer membrane protein OmpC. This is REVERSIBLE binding, and is probably due to electrostatic interactions as it is Mg2+ and Ca2+ dependent. After TAIL FIBRE binding has consolidated, the BASEPLATE then settles down onto the surface and binds firmly to it as a result of conformational changes in the SHORT TAIL FIBRES. After this occurs, a conformational change takes place in the TAIL SHEATH, which then CONTRACTS, pushing the TAIL CORE through the cell wall, possibly in an ATP-driven process: this is aided by a lysozyme activity associated with the tip of the tail tube. This is an IRREVERSIBLE process. DNA is then extruded from the phage head. This is then used for initial transcription and virus expression.

Phage lambda - Enterobacteria phage λ, genus "λ-like Viruses", family Siphoviridae, a tailed phage with an isometric head and a 49 kbp dsDNA genome - attaches to the maltose receptor on the surface of the E coli cell via the J protein in the tail tip. The receptor is a porin, responsible for transport of maltose across the outer membrane. Although the tail is non-contractile, a DNA injection mechanism similar to that of T-even phages allows entry of DNA into the cell, via a sugar transport protein (ptsG) in the inner membrane, leaving the capsid behind.

MS2 phage - Enterobacteria phage MS2, genus Levivirus, family Leviviridae - an isometric single-stranded RNA-containing virus infecting E coli - attaches to the PILIN (the building block of PILI) of the F(ertility) pili via its single attachment or A PROTEIN. The A protein is covalently linked to the 5'-end of the genomic RNA; binding pilin causes cleavage of the A protein and releases it from the capsid. Tthus, when the pilus is retracted into the cell, the A protein and RNA are pulled with it, leaving the empty capsid outside.

Here is an animated diagram of a DNA-containing enveloped isometric phage entering a gram-negative bacterium.

Interestingly, it is possible that where phages attach may have an effect of the outcome - specifically whether or not the infection is lytic or lysogenic. Click on the links below to see a recent MicrobiologyBytes post.

The injection of DNA from bacterial viruses into cells is usually enabled by a "molecular motor" housed at the junction of the tail and head of the virus particle: this is also responsible for filling pre-assembled phage heads with DNA in the infected cell as part of the virion assembly and maturation process. Thus the motors are nanomachines which can run in both directions.

This is generally an ATP-driven process, and in this is similar to other cellular molecular motors such as myosin, kinesin and dynein in eukaryotes.

Figure 1

Structure of epsilon15 bacteriophage.

a, 200-kV CCD image of epsilon15 particles embedded in vitreous ice.

b, Surface rendering of the 20A ° resolution three-dimensional map of entire epsilon15 bacteriophage reconstructed without symmetry imposition.

The capsid (dark green) exhibits good icosahedral symmetry as indicated by the icosahedral lattice (grey).

c, d, The structural components of epsilon15 bacteriophage are annotated in the central section density (c) and the cut-away surface view (d) of the three-dimensional density map.

Figure 3

Structure of the portal complex and internal core.

a, Side view of the portal complex (pink), internal core (light green) and the putative straight dsDNA terminus (dark blue). The portal complex and internal core are coloured semi-transparently to show that their central channels are filled with the putative dsDNA terminus.

b, Bottom view of the portal complex with the nearly 12-fold structural features labelled around the filled central channel.

c, The apparent 12-fold arrangement of densities for the portal complex in a section image of the three-dimensional map. The location of the section is indicated by the dashed line in a.

d, Top view of the internal core (light green) showing the central channel filled with the putative dsDNA terminus (dark blue).


Eubacteria Gram-positive cell walls	Eubacteria Gram-negative cell walls

Archaea Gram-positive and -negative cell walls	A generic bacterial cell
copyright Russell Kightley


	M13-type (fd) phage	Phage Φ29

Lambda phage infecting E coli (copyright Russell Kightley)	Phage P22	T4 phage particles

Virus Entry Into Cells

BACTERIAL CELL ENTRY