Saturday, February 25, 2006

The Bacterial Cell Interior.

Image from this site.

Chapter 3 of Microbe explains how the cell interior of bacteria is a very busy, crowded, and viscous place. A major component taking up space in this region is the nucleoid, and there is much more that can be said about it than there is space for in the chapter.

The DNA of the nucleoid is heavily folded and super-coiled. The DNA of Escherichia coli, if it were uncoiled, would be about 1000 times the length of the cell. Bacteria are thus very compact with genetic information storage.

An interesting point made by Schaechter in Chapter 3 that transcription of DNA into RNA only occurs at the nucleoid-cytosol interface, so organisation of the nucleoid's coiled domains is functionally important. Diagrams of this are presented in teaching sessions.

Recently, many dramatic advances have been made in scientific understanding of bacterial cytoplasmic filament proteins such as MreB and FtsZ , and these are already mentioned in chapter 2 of Schaechter.

Study Question:

How were MreB and FtsZ proteins first identified, and where do their names come from? Where are they located in the cell?

Friday, February 24, 2006

Prokaryotic cell envelopes and appendages.

(EM thin section image from here.)

Chapter 2 of Microbe deals with the outer layers of bacterial cells. It explains how these are complex in structure.
Important points are
The composition, content and function of the cell membrane.
Protection of the cell membrane in Gram positive and Gram negative bacteria.
The operation and assembly of the flagellum

Other links with images
Cells alive

Study question:

How are bacterial flagella and pili (=fimbrae) different from one-another in terms of structure, mode of assembly, and function?

Wednesday, February 22, 2006

The World of Microbes . Question 2.

In what ways have Bacteria and Archaea influenced the evolution of eukaryotes in the past?

How do they influence it in the present?

Discuss in the comments panel.


Microbe. The World of Microbes.

As a start to this discussion forum, it's good to go to the website devoted to book Microbe that inspired this weblog. This website is here.

We are starting discussions with chapter 1 of this book- The World of Microbes.

Let's get started by posting a study question for discussion in the comments thread.

Bacteria range in volume over a million-fold. Discuss some of the consequences of being larger or smaller than average?

Update of question and posting with more information to assist students. 4th March 2007
Here is some extra information that can assist students understand how size has implications for available membrane surface area to service the metabolic requirement of a unit cell volume.

To understand the point of the question about size, students need to think about have the ration of surface are:cell volume changes with increase in size. SA/Vol Ratio~diameter squared/diameter cubed~inversely proportional to diameter, with similar cell shapes.

To see some consequencences raised by the biology of large bacteria you need to read about observations that have been made on Epulopiscium bacteria, usually called epulos. The paper by K D Clements and S Bullivant (1991) An unusual symbiont from the gut of surgeonfishes may be the largest known prokaryote. J Bacteriol. 1991 September; 173(17): 5359–5362 provides some good interesting insights.

Figure 1 of this paper shows how large epulos are.
J Bacteriol. 1991 September; 173(17): 5359–5362. Figure 1. Light micrograph of an Epulo. The letter C indicates a smaller, eukaryotic ciliate.

J Bacteriol. 1991 September; 173(17): 5359–5362. Figure 3. Electron micrograph of a thin section of an Epulopiscium bacterium showing concolutions to cytoplasmic membrane. Such convolutions would increase the membrane area to cell volume ratio of these large bacteria

Thus epulos have a peripheral layer of highly convoluted cytoplasmic membrane - which has been interpreted as a mechanism increase in membrane area to compensate for some surface area to volume related challenges that they face because of their large size.

Extra reading for the high achieving student:
In the links at the Microbe Chapter 1 webpage, this is worth studying several times in the course.

Labels: , , ,