MICROBIOLOGY (C11105) Lecture 20

Aim:   To describe the diversity in nutrition found in microorganisms.

1. To know the basic nutritional requirements of microorganisms for their growth.
2. To understand the basis of nutritional classification of microorganisms.
3. To know the different kinds of media and medium design strategies used in Microbiology

Nutritional Classification
Microorganisms are classified on the basis of their utilisation of nutrients as an energy source and as a carbon source. The major level of classification is into chemotrophs and phototrophs. The former derive energy from chemicals and the latter depend primarily on radiant energy.

These two major groups are further classified on the basis of the carbon sources utilised.

Chemoautotrophs (Chemolithotrophs) - use inorganic substrates as sources of energy and C0₂ as the main source of carbon.

Chemoheterotrophs (Chemorganotrophs) - utilise organic substrates for both needs.

Photoautotrophs (Photolithotrophs) - use light as energy source and C0₂ as carbon source.

Photoheterotrophs (photoorganotrophs) - use light as energy source and organic carbon sources.

Not all bacteria can be so precisely categorised, for example, some phototrophic bacteria can also grow as chemotrophs.

Rhodospirillum rubrum exists as a photoheterotroph in anaerobic conditions but when oxygen is available, and in the dark, it grows as a chemoheterotroph. This important metabolic switch is also reflected in changes in internal cell structure. The vesicles which contain the photosynthetic pigments become disorganised when the organism grows as a chemoheterotroph.

All fungi are all chemoheterotrophic microorganisms.

Chemoheterotrophs show a wide diversity with respect to the sources of organic nutrients that are essential for their growth. At the two extremes are the saprotrophs (saprophytes), which utilise nonliving organic matter, and the biotrophs (parasites) which may be totally dependent on another living organism for their nutrition and cause disease (pathogens). In between there are several types of association between this group of microorganisms and another organism. These include mutalistic or symbiotic associations whereby both partners gain benefit, organisms that live as commensals and under some circumstances cause disease, and finally obligate biotrophs that can live both a saprotrophic and biotrophic existence.

The source and supply and nutrients
The natural environments colonised by bacteria and fungi provide a wide variety of nutrients which will support growth of these organisms. In some organisms, described as fastidious, a specific nutritional requirement can be met from only one source. These organisms are generally pathogens and cannot be cultivated in the laboratory in the absence of their host or host tissues.

The mixtures of nutrients used in the Microbiology laboratory to cultivate bacteria and fungi are called media. These range from complex, undefined mixtures to simple defined mixtures where the nature and amount of a nutrient is known. The former type of medium is used normally for the routine and regular cultivation of an organism, or when cultures are required quickly for some particular use e.g. for genetic experiments. Conversely for biochemical and physiological experiments a defined medium is favoured e.g. where the regulation of synthesis of an enzyme is being investigated.

Media for the cultivation of bacteria
The nutritional diversity of bacteria requires the use of a range of different culture media. Thus autotrophs (chemo and photo) require a medium lacking organic carbon (Table 1). In the case chemoheterotrophs one nutrient is used as the energy source. A good example of such organisms are the nitrifying bacteria which derive energy from the oxidation of NH₄ ion (Nitrosomonas) and N0₂ ion (Nitrobacter). A defined medium for a chemoheterotroph has a very similar composition, the notable addition being the organic carbon source (Table 2).

A more complex medium is given in Table 3. This comprises extracts of natural materials, in this case an extract from animal tissues.

Media for the cultivation of fungi
Fungi are also cultivated on defined or complex media. Examples of these are given in Tables 4 and 5. Note the inclusion of organic carbon in both cases, reflecting the chemoheterotrophic nature of fungi, however, in fungal media extracts of plant materials are used in complex media. Comparison of bacterial and fungal media shows an important difference in the ratio of carbon to nitrogen; bacterial media have a low C:N ratio and fungi a high C:N.

Special Media
Different types of organisms require the use of particular media. For example anaerobic bacteria grow only on a medium lacking oxygen. Reducing agents, such as sodium thioglycollate, are commonly used, but most effective is a piece of apparatus called an anaerobe jar. This is a vessel in which the atmosphere can be made totally oxygen free.

Selective media are used to enhance growth of particular types of microorganisms. These are particularly useful for the isolation of particular types of organism(s) from a very mixed population e.g. the use of a medium containing brilliant green (a dye) which inhibits gram +ve bacteria, for the isolation of gram -ve types.

Differential media are used to distinguish particular types of organisms in a mixed population. MacConkey medium, which contains bile salts and crystal violet dye, is used to differentiate Enterobacteriaceae (Salmonella, Shigella and Escherichia) from other gram -ve bacteria.

An enrichment medium is used to selectively isolate organisms with a particular nutritional requirement of function. For example, if organisms that can hydrolyse chitin are required samples of natural material e.g. soil are spread on a medium containing chitin. Only chitin degrading organisms will grow.

Agar
All media can be used in liquid or solidified form. The latter include agar as an ingredient. This is a complex polysaccharide extracted from marine algae. It is normally added at a concentration of 1.5%. Agar has important properties being liquid at 100^oC but solidifying at 40^oC. This allows the incorporation of bacteria into the medium as an alternative to spreading on the medium surface.

Summary

1. Microorganisms have a requirement for the same basic nutrients as all life forms.
2. Microorganisms display diversity in nutrition with respect to the source of energy they utilise and the form in which they utilise carbon.
3. Microbiologists utilise a wide range of media. many for specific purposes.

J F Peberdy© 1995