Questions from Day 7 (1 week to complete):

1)     Describe the different shapes of bacteriophage.  Why do you think these different shapes exist?

Three main shapes of bacteriophages have been discovered: 1) Icosahedral tailless, 2) Icosahedral tailed and 3) filamentous (rod-shaped).  There is only one known mycobacteriophage named corndog that doesn’t fit any of the three groups because it has a long thin head (non-icosahedral) and a tail. Because it is a bacteriophages purpose to multiply its genes, it’s assumed that each structure is made to efficiently store and insert the genes into the bacteriophages host. Filamentous phages store a single line of material and therefore are able to store a lesser amount of genetic information compared to phages with icosahedral heads, which are able to store considerably longer strands of DNA, which are compacted into the head of the phage. The other major reason that each phage is different is because each has a different way of inject its genetic material into the bacteria. Due to this, the shape is most related to the type of bacterial receptor the phage has. Some phages can easily attach themselves to the bacteria, while others need a slightly more complex way of inserting it self, so it uses things like a contractile tail.

    --Lazlo and Stephen

 How are phages D29, BXZ1, TM4 and other mycobacterial phages unique with respect to their plaque sizes and morphologies? 

Mycobacteriophages consist of three seperate parts, the head or protein capsid which contains genetic material, the tail which is either contractile or not, and the base plate which may or may not have tail fibers attached to it.  Phages also differ in that they form different types of plaques.  A lysogenic phage will either form turbid or hazy plaques.  A turbid plaque appears to have a lysed ring around bacteria with a lysed plaque in the middle.  A lytic phage forms clear plaques with no bacteria left in the middle.  D29 is a lytic phage, it has an average sized head, tail and base plate.  It forms large clear plaques.   Bxz1 is a lysogenic phage that forms turbid plaques.  Bxz1 has a large head as it encodes a large amount of genetic data.  It's tail is short and contractile with a base plate at the end.  Bxb1 is a lysogenic phage that forms turbid plaques.  It's head and tail are of average size and it encodes a small amount of DNA.  TM4 is another lysogenic phage that also forms turbid plaques.  It's plaques are smaller than Bxz1's.  It has an average sized head that encodes an average amount of genes and it has a medium sized tail.   In general lysogenic phages appeared to contain more genetic material than lytic phages.  Two exceptions that we found to this rule were Bxz1 which had less genetic material than some lytic phages and Psp3, a lytic phage has more genetic material than psp2 a lysogenic phage.  Other than that we could draw no further conclusions from our data. 

    --Kristen and Amanda

What does a different plaque size and shape tell you about the bacteriophage?

            A lot can be determined about a bacteriophage from looking at its plaque size and shape.  There are two known forms of phage reproduction, lytic and lysogenic.  In a lytic cycle, a bacteriophage infects a host bacterium and converts it into a “phage factory”.  The bacteriophages inside eventually destroy or lyse the bacterium and burst out as many as fifty to two hundred new phages.  Larger plaques mean that closer to two hundred phages are produced every time the process occurs or that the phage has had more time on the plate to consume the bacteria.  Smaller plaques mean the opposite.  Fewer phages are produced or the phage has had less time on the plate.  A clear plaque is strong evidence of a lytic phage.  In the lytic cycle, the host bacterium is eventually destroyed, so there is nothing visible in the area.  In a lysogenic cycle, the infecting phage mixes some of its D.N.A. with the host bacterium D.N.A.  It then becomes immune to any other phages of the same type.  These are known as temperate phages, which use both a lytic and lysogenic life cycle.  They create turbid plaques because many of the bacteria are not destroyed, just genetically different.  If a plaque has both a lytic and lysogenic area, it means the phage has had both lytic and lysogenic life cycles.  It means the phage is temperate. In some cases, plaques do not form because of bacterial mutations. When the bacteria mutates, it may cease to produce the necessary markers on its surface that the phage uses to attach itself to the bacteria, thus the bacteria will not be killed and is considered phage resistant. One interesting type of plaque resembles a bull's eye; it has a clear center, a thick, turbid ring then a thin clear ring. It is still not known why this type of plaque forms and this is the topic of much discussion right now in the scientific community.

--Chiara and Chris

--Stephen.