Madical brain Notes 10

  Microbial Nutrition and Cultivation We introduced the major groups of macromolecules found in living cells; the raw materials from which these are synthesised are ultimately derived from the organism’s environment in the form of nutrients (Table 4.1). These can be conveniently divided into those required in large quantities* (macronutrients) and those which are needed only in trace amounts (micronutrients or trace elements). You will recall that carbon forms the central component of proteins, carbohydrates, nucleic acids and lipids; indeed, the living world is based on carbon, so it should come as no surprise that this is the most abundant element in all living cells, microbial or other-wise. Of the other macronutrients, nitrogen, oxygen, hydrogen, sulphur and phosphorus are also constituents of biological macromolecules, while the remainder (magnesium, potassium, sodium, calcium and iron in their ionised forms) are required in lesser quan-tities for a range of functions that wil...

  Nutritional categories Microorganisms can be categorised according to how they obtain their carbon and energy. As we have seen, carbon is the most abundant component of the microbial cell, and most microorganisms obtain their carbon in the form of organic molecules, derived directly or indirectly from other organisms. This mode of nutrition is the one that is familiar to us as humans (and all other animals); all the food we eat is derived as complex organic molecules from plants and other animals (and even some representatives of the microbial world such as mushrooms!). Microorganisms which obtain their carbon inthis way are described as  heterotrophs , and include allthe fungi and protozoans as well as most types of bacteria. Microorganisms as a group are able to incorpo-rate the carbon from an incredibly wide range of or-ganic compounds into cellular material. In fact there is hardly any such compound occurring in nature that cannot be metabolised by some microorganism or ...

  How do nutrients get into the microbial cell? Having found a source of a given nutrient, a microorganism must: ·                have some means of taking it up from the environment ·                possess the appropriate enzyme systems to utilise it. The plasma membrane represents a selective barrier, allowing into the cell only those substances it is able to utilise. This selectivity is due in large part to the hydrophobic nature of the lipid bilayer. A substance can be transported across the cell membrane in one of three ways, known as simple diffusion, facilitated diffusion and active transport. In  simple  diffusion, small molecules move across the membrane in response to a concentration gradient (from high to low), until concentrations on either side of the membrane are in equilibrium. The ability to do this depends on being s...

  Laboratory cultivation of microorganisms Critical to the development of microbiology during its ‘golden age’ was the advance in culturing techniques, enabling the isolation and pure culture of specific microorganisms. The study of pure cultures made it possible to determine the properties of a specific organism such as its metabolic characteristics or its ability to cause a particular disease. It also opened up the possibility of classifying microorganisms, on the basis of the characteristics they display in pure culture. The artificial culture of any organism requires a supply of the necessary nutrients, together with the provision of appropriate conditions such as temperature, pH and oxygen concentration. The nutrients and conditions provided in the laboratory are usu-ally a reflection of those found in the organism’s natural habitat. It is also essential that appropriate steps are taken to avoid contamination. In the next section we shall describe the techniques used to isolat...