Requirements for nitrogen and sulfur

Generally nitrogen and sulfur occur in the organic compounds within the cell in the reduced form as either amino and sulfhydryl groups, respectively.

An exception is that some microbes can utilise the most abundant natural nitrogen source, which is atmospheric N2. This process of nitrogen assimilation is termed nitrogen fixation and involves an initial reduction of nitrogen (N2) to ammonia (NH3) (more about this topic later in the metabolism section).

Growth factors
These are any organic compound that an organism requires as a precursor or constituent of its organic cellular material, but is unable to synthesize itself. Therefore, this must be provided as a nutrient within the growth medium.

These types of compounds generally fall into three groups, either:

1. amino acids - which are required as constituents of proteins

2. purines and pyrimidines - required as constituents of nucleic acids

3. vitamins - required so that enzymatic reactions can occur efficiently

Role of O2 in nutrition Many organisms require molecular oxygen to respire (i.e.it is needed to generate energy; more to come later in metabolism section). Those organisms that absolutely require O2 to grow are called obligate aerobes.However, some organisms don't care one way or the other whether they respire using O2 (i.e. they can substitute a different compound other than O2 as terminal electron receptor), and these organisms are called facultative aerobes.

At the other end of the physiological spectrum are those organisms where O2 is toxic (i.e. kills them) and these bugs are called obligate anaerobes. Likewise, some bugs that are generally anaerobes, but dont care either way are called facultative anaerobes.

An unusual class of bugs are called microaerophilic. These are organisms that are obligate aerobes but grow best at partial levels of O2 (i.e. they require O2 levels below those that are present in air).

Nutritional categories among microorganisms
Now to put it all together: biologists like to classify organisms with respect to two main parameters; the nature of the energy source (i.e whether they use light or a chemical compound for energy) and the nature of the carbon source (i.e. whether they use CO2 (autotrophs) or organic carbon (heterotrophs).

This has led to four general classifications:

1. Photoautotrophs - uses light as energy source and CO2 as carbon source - generally photosynthetic bacteria, higher plants etc.

2. Photoheterotrophs - uses light as energy source and organic compounds as carbon source - includes purple and green bacteria.

3. Chemoautotrophs - utilises a chemical energy source (generally from reduced compounds e.g. NH3, NO2- ions) and CO2 as carbon source - only certain weird types of bacteria belong to this class.

4. Chemoheterotrophs - utilises a chemical energy source and organic compounds as carbon source - the vast majority of bacteria fall into this class.

Note: these terms are somewhat arbritary as bacteria are so diverse that they can flip in and out of each of the above categories.

Finally, when the requirement for growth factors is taken into account, we come to perhaps the two most useful terms, which are prototrophy and auxotrophy.

Prototrophs are bacteria that do not require any external growth factors to be added to the external medium, i.e. they possess the genetic capacity to synthesize all necessary organic components from the primary carbon source.

Auxotrophs are bacteria that require growth factors to be added to the medium in addition to the primary carbon source for them to grow.

Note: in each of the above categories (i.e. photoheterotrophs etc), prototrophy and auxotrophy may also apply.

Therefore, when constructing a culture medium the primary goal is to provide a balanced mixture of the required nutrients to permit good growth. An overly rich medium may be toxic, or, alternatively, if growth does occur, microbial metabolism may eventually change the nature of the medium so that it becomes unfavorable for growth. Bacteriological media are generally classified as either, minimal or complex, with the former being highly defined, providing the specific growth factors (plus carbon source) for that particular organism, in contrast to the latter, where an all-purpose additive (e.g. yeast extract) is added to provide the necessary nutrients. Consequently, complex media may provide components that are not utilized by the cell.

4. Microscopy
As discussed previously, the light microscope was invented by Leeuwenhoek (see figure 3:1). The development of the compound microscope took a further two centuries to achieve.

Basic Principles of Microscopy
Microscopy is the technology of making small things visible. See figure 3.2 to see relative size of microbes and the instrument that can visualize them.

Properties of light
Wavelength (l) is defined as the distance between one crest to another, or, alternatively, one trough to another (see figure 3:3).

Resolution (or resolving power of a microscope) is a direct function of wavelength (see figure 3:4)

Resolving power (d) is defined by the equation:

What does resolution mean?

Definition: Resolution refers to the ability to see two items as separate objects (see figure 3:5).

We can magnify two objects, but unless they can be resolved no matter how much magnification is applied they will remain together. Light must be able to pass between the two objects.

Therefore, why is resolving power a function of wavelength? If the wavelength of light is too long to pass between the two objects they will remain as one. See analogy in Figure 3:6. Therefore, as the wavelength shortens from visible light (i.e. average 550 nm) to electromagnetic radiation in electron microscopy (i.e. 0.005 nm) resolution goes from approximately 220nm to 0.2nm.

 

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