Structure of Bacteria and Archaea (Chapters 3, 20 and 21)

Constraints on the shape and size of microbes.....

s/v ratio - most microbes must transport all of their nutrients across the plasma membrane one molecule at a time, thus s/v limits the size a cell can be. Surface area increases as the square of the radius and volume increases as the cube of radius..... (more on that in the physiology lectures)

Shapes of microbes:

cocci (sing. coccus)
rods = bacilli (sing. bacillus)
spirilla and spirochetes
pleomorphic cells

Structural similarities and differences among Bacteria and Archaea.

phylogenetic context........ Show Figure 19.3 ...

You should already know the difference between prokaryotic and eukaryotic cells - if not review pgs. Figures 4.3 and Table 4.1 (pg. 75)

Let's look a little more closely at some aspects of Bacterial and Archaeal cells.........

DNA.

Bacteria and Archaea have their DNA (double stranded) contained in a single circular chromosome and they are haploid. Most Bacteria (and many Archaea) that have been examined, also have smaller circular DNA fragments called plasmids. Plasmids occur in some eucaryotes but are thought to be rare (more about plasmids when we discuss genetics).

Cell membranes.

The Archaea have unique plasma membranes (Fig. 20.3 and 20.4).

Ether (Archaea) vs. Ester (Bacteria & Eucarya) linkages (Figure 20.3)

Many Archaeal membranes are cross-linked (Figure 20.3)

Figure 3.7

Cell Walls

osmosis - cell wall must be strong to protect cells from lysis in hypotonic solutions

All Bacteria that have cell walls have peptidoglycan in them. The thickness of the peptidoglycan layer is the main determinant of whether an organisms is gram + or gram -. Figure 3.19 shows a typical Gram + bacterium (Bacillus licheniformis) and a typical Gram - bacterium (Aquaspirillum serpens).

Peptidoglycan (Fig.) Note the polysaccharide backbone consisting of alternating amino sugars (N-acetylglucosamine (NAG) and N-acetyl muramic acid (NAM)) linked by Beta 1-4 bonds (remember those from cellulose?). The Beta 1-4 bond can be broken by an enzyme called lysozyme which is present in your tears, saliva, blood, and other bodily fluids. These polysaccharide chains are held together by peptide cross links (thus the peptido in peptidoglycan). These peptides are rather odd in that they can contain D-amino acids.

Antibiotics in the penicillin and cephalosporin groups inhibit the enzymes that are responsible for the formation of peptidoglycan. This is why these antibiotics do not affect members of the Archaea or Eucarya.

More detail on Gram + and Gram - bacterial cell walls......

Gram + cell walls... see figure 3.25 showing how teichoic acids help stabilize the peptidoglycan layer and connect it with the plasma membrane.

Gram - cell walls are quite different in that they have much less peptidoglycan and no teichoic acids. Plus they have an outer membrane. See Figure 3.27.

Note LPS which is antigenic and can function as an endotoxin in gram - pathogens such as Salmonella spp. andYersinia pestis (plague).

Also note the periplasmic space where many enzymes either float free and/or are attached to the cytoplasmic membrane.

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Cell Walls of the Archaea

(Fig. 20.1).

Do not contain peptidoglycan or D-amino acids. Some of the methanogens have pseudopeptidoglycan (see Fig. 20.2). Note that it contains NAG. Other Archaea have protein or glycoprotein cell walls unlike any other cell walls known in the universe.

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Fungal cell walls.

The main component of fungal cell walls is chitin which is a polymer consisting of N-acetylglucosamine (NAG) the monomer. Thus, NAG is present in all 3 Domains of life.

See Fig. 2-2 from Moore-Landecker (1996) for structure of a fungal cell wall.

Moore-Landecker, E. 1996. Fundamentals of the Fungi. Prentice Hall.

Layers external to the cell wall.

Many bacteria produce extracellular layers that serve different functions in different critters.

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Capsules are slimy layers that are made of polysaccharides (monomers include NAG and glucose (in dextran) or polypeptides (monomers of D-glutamic acid in some Bacillus spp.).

Capsules help bacteria attach to surfaces - e.g. Streptococcus mutans attaches to your teeth using a capsule. Capsules also function as a protective layer - as in S. pneumoniae which can avoid phagocytyosis in the blood only if it has a capsule. Figure 3.31 shows a negative stain of a Klebsiella pneumoniae

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Sheaths are rigid tubes made of polysaccharides and polypeptides and are commonly found on some filamentous bacteria such as Sphaerotilus (see Fig. 22.12, p. 469).

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S-layers or "protein jackets" are also found external to the cell wall on some members of the Bacteria and can be the only wall in some Archaea. The function of these jackets is not known at this time but they may function as deterrents to predation in some bacteria.....

Bacterial Appendages

Fimbriae (sing. fimbria) are appendages made of protein and are found on many types of bacteria. One type of Fimbria is the pilus (pl. = pili). We'll discuss the sex pilus in the genetics portion of the course (show Figure of mating E. coli). The main functions of fimbriae are attachment and transport.

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Flagella (sing. = flagellum) are proteinaceous appendages that are used in bacterial propulsion. Compared to eukaryotic flagella (remember the 9 + 2 arrangement of microtubules) bacterial flagella are quite simple semi-rigid helices. The site of attachment of the flagellum to the cell wall is more complex, however.
Describe rotary motor........... (Fig. 3.37)

This motor is under complex control in bacteria. Many bacteria exhibit chemotactic behavior in which they can move towards a source of food. Describe how this works........