addendum to lectures 1 and 2

Why small microbes do not sink in water.....

Stoke's Law

Vt = 2/9 r2 g(Dp - Df/N)

r = radius (ideal spherical particle)

g = acceleration due to gravity

Dp = density of particle (g/cc)

Df = density of fluid (g/cc)

N = viscosity (g/cm/sec)

Sizes of organisms:

Salmonella typhimurium, 1.5 x 1 µm
Saccharomyces cerevisae, approx. 4 µm in diameter
Neurospora crassa, approx. 12 µm in di.
Stoke's Law says that particles below about 4 µm in diameter will not sink in water. Brownian movement also tends to keep things from sinking.

Microbial Structure (cont.)

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Bacterial Appendages

Fimbriae (sing. fimbria)

are appendages made of protein and are found on many types of Gram - 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.

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)

Flagellar assembly (Fig. 3.38)

Flagellar attachment site varies among bacteria and can be used to help identify some bacteria...

polar
peritrichous
lophotrichous

To answer this question we need understand how microbes perceive the world......

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Life at Low Reynolds Number (Re).

Re = inertial forces / viscous forces

Re = L u / v

where L = diameter or length;

u = velocity of organism;

V = viscosity of fluid

dimensionless parameter (all units cancel)

If Re > 1 inertia is important

If Re < 1 viscous forces dominate

Compare a fish and a bacterium........

Fish (10 cm long, swim speed 100 cm / sec)
Re = 100,000.

Bacterium (1 µm long, swim speed 100 µm / sec)
Re = 0.00001

10 orders of magnitude difference!

Bacteria live in a very viscous world compared to the world of a fish!

How does a fish move?

wagging it's tail displacing water in one direction and it goes in the other. When fish stops wagging it glides due to it's inertia.

How do bacteria move?

Well it lives in a viscous world - can't generate enough power to wag it's tail.

1) rigid helical flagellum (See Fig. 16 in Purcell, 1977)

2) helical body

3) Gliding motility - cyanobacteria, Beggiatoa etc.

Inertia not important at Re < 1; a bacterium would glide 0.05 angstrom if motor turned off (less than 0.1 the diameter of a H atom).

Purcell, E.M. 1977. Life at Low Reynolds Number. Am. J. Physics 45: 3 - 11.

Chemotaxis

The rotary motor is under complex control in bacteria. Many bacteria exhibit chemotactic behavior (Fig. 3.40) in which they can move towards a source of food. Bacteria can measure change in conc. over time (Fig. 3.43)

Describe how this works........

1) conc. decreasing as they move => tumble more often (Fig. 3.42)

2) conc. increasing as they move => longer runs tumble less often

Does E. coli have a nose? See Science vol. 259: 1701-1702.

Other types of taxis:

1. negative taxis to repellents (Fig. 3.41)

2. aerotaxis (cover slip illustration)

3. Magnetotactic bacteria (Box 3.3)

4. Phototaxis (spectrum overhead)

Chemotaxis by fungi and other microbes: Apparently rare in fungi, algae and protozoa, but this may be due to lack of research. Many fungi rely on random branching (in 3D) to find new food sources and then branch more to stay in the area with more food.

Known exceptions: Zoospores of Oomycetes and Chytrids show chemotaxis to nutrients esp. a.a.s. Amoebae and slime molds are attracted to bacteria (which they eat) and cell-free extracts of bacteria. Many fungi grow away from each other to spread out and this may be a positive tropism to oxygen(?).

Microbial Structure (cont.)

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Spirochetes

read pages 450 - 453 - note unique structural elements

Gram -
many have never been cultured but they are found in many environments including our mouths

Genera:

Spirochaetanon-pathogens, anaerobic
Treponemasyphilis
Borrelia arthropod-borne diseases
Cristispira symbionts of molluscs, some very large

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Inclusion Bodies

usually for storage of fat (PHB), sugars (glycogen) or essential nutrients such as phosphorus (polyphosphate granules). Inclusion bodies are sometimes bound by single-layered membranes (protein or lipid) or are unbound and exist free in the cytoplasm.

Poly-beta-hydroxybutyrate (PHB) inclusion bodies can be quite large (0.2 - 0.7 µm, di., Fig. 3.11) and can constitute up to 50% of the cell dry weight in a fat (well fed) bacterium.

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The Bacterial Endospore

life cycle: see Figure 3.47 and 3.48

structure: Figure 3.45

Endospores survive for years in a dormant state. There are some recent claims that endospores can survive for millions of years - but these claims are controversial. Read this article on a Bacillus sp. that was(?) revived from 250 million year old salt formations....

For more information on bacteria isolated from 40 million year old amber see: Greenblatt, C.L. et al. 1999. Diversity of microorganisms isolated from amber. Microbial Ecol. 38: 58-68.