Bacteriophages are duplodnaviridia viruses. These viruses infect only bacteria and archaea. Phages are the most abundant organic elements on earth. Phages can be virulent or temperate. Virulent phages can infection and lysis of the bacteria within one cycle. Hence their life-cycle is also called lytic cycle. On the other hand, the phages whose DNA becomes integrated into the bacterial DNA and at another time becomes spliced out and active are called temperate phages. The integrated DNA of a temperate phage into the bacterial DNA is called prophage. This bacteria is called lysogenic bacteria because it will lyse, but in the future. Let's learn more.

Bacteriophage, or sometimes just called a “phage”.  A phage is a virus that attacks bacteria.

There are species of phages that are specific to various bacteria.  This characteristic is used by public health organizations to detect what kind of bacteria are present in the community by looking at what kind of phages are present in wastewater.

Phages are the most abundant organic structure on earth. Numbering 10 to the power of 31. More than all other living things collectively, including bacteria.

Bacteriophage is derived from two words: bacterium  =  phage.  Phage means “to devour”.

It is a duplodnaviria type and infects only bacteria and not human beings. (But humans can be infected by the infected bacteria!  Herpes Simplex virus and Varicella Zoster are two examples).

( TC 2:14)

The structure  consists of:

• * a nucleocapsid head containing the genetic material (DNA/RNA).

• *Attached to the head is a long hollow tube.  When the phage is infecting a bacteria, the genetic material passes through this tube to be injected into the bacterium.

• *On the tube are a few more proteins: 

1. Collar, in the neck area just below the head

2. Below that is a sheath

3. At the bottom of the sheath is a baseplate near the end of the tube.

4. Finally, 6 legs, like a spider, and spikes attached to the baseplate.  The spikes are used to anchor the phage to the bacteria, a process called adsorption.

Bacteriophages are not motile on their own, in spite of their legs.  They move by “brownian motion” .

How do phages infect a Bacterium?

  TC 5:15. 

• * A phage lands on a bacterium;

• *The phage is  adsorbed to the surface by the spikes piercing the cell wall; 

• *As the adsorption takes place, the phage “squats” and the tube pierces the cell wall and the cytoplasmic membrane and penetrates all the way into the cytoplasm, a phase called “penetration”;

• *The genetic material then travels through the tube and enters the bacterium.

Then what happens?

  TC 6:11

   There are two outcomes:

1. If the phage is virulent, meaning its genetic material is such that it will start working with the ribosomes and start making new phages and will cause  bacterial lysis. This is the lytic cycle of the phage life cycle.

2. If the phage is temperate ( it has a better temperament), its DNA is integrated into the bacterial DNA  (TC 7:47).  In this case it is called a prophage.  It can make new phages, but it is in a dormant state, like a time bomb. When the bacterium replicates, the prophages replicate with it, creating lysogenic bacteria with the capability of being destroyed when the phage DNA becomes active.

   TC: 9:28   

At some point the phage DNA will be spliced off the bacterial DNA, and will be transcribed, and form mRNA which will work with the ribosome and translate and make the phage proteins and suddenly the time bomb has gone off and phages are being built.  This might happen a little later causing bacterial lysis.

Sometimes the temperate phages help produce enzymes in the bacteria which will prevent another infection from the same species, called lysogenic immunity.

 One more concept: 

Transduction:  The phage can introduce new genes into the bacteria and can transfer genes from one bacteria to another…that will be discussed in the next lecture.  See you there!