Microbial metabolites

The growth of a microbial culture can be divided into a number of stages, as discussed in Chapter 2.

After the inoculation of a culture into a nutrient medium there is a period during which growth does not appear to occur; this period is referred to as the lag phase and may be considered as a time of adaptation. Following a period during which the growth rate of the cells gradually increases the cells grow at a constant, maximum rate and this period is known as the log, or exponential, phase. Eventually, growth ceases and the cells enter the so-called stationary phase. After a further period of time the viable cell number declines as the culture enters the death phase. As well as this kinetic description of growth, the behaviour of a culture may also be described according to the products which it produces during the various stages of the growth curve. During the log phase of growth the products produced are essential to the growth of the cells and include amino acids, nucleotides, proteins, nucleic acids, lipids, carbohydrates, etc. These products are referred to as the primary products of metabolism and the phase in which they are produced (equivalent to the log, or exponential phase) as the trophophase (Bu'Lock et al., 1965).

Many products of primary metabolism are of considerable economic importance and are being produced by fermentation, as illustrated in Table 1.2. The synthesis of primary metabolites by wild-type micro-organisms is such that their production is sufficient to meet the requirements of the organism. Thus, it is the task of the industrial microbiologist to modify the wild-type organism and to provide cultural conditions to improve the productivity of these compounds. This aspect is considered in Chapter 3.

During the deceleration and stationary phases some microbial cultures synthesize compounds which are not produced during the trophophase and which do not appear to have any obvious function in cell metabolism. These compounds are referred to as the secondary compounds of metabolism and the phase in which they are produced (equivalent to the stationary phase) as the idiophase (Bu'Lock et al, 1965). It is important to realize that secondary metabolism may occur in continuous cultures at low growth rates and is a property of slow-growing, as well as non-growing, cells. When it is appreciated that micro-organisms grow at relatively low growth rates in their natural environments, it is tempting to suggest that it is the idiophase state that prevails in nature rather than the trophophase, which may be more of a property of micro-organisms in culture. The

Table 1.2. Some primary products of microbial metabolism and their commercial significance

Primary metabolite

Commercial significance

Ethanol

Citric acid

Glutamic acid

Lysine

Nucleotides

Phenylalanine

Polysaccharides

Vitamins

'Active ingredient' in alcoholic beverages Used as a motor-car fuel when blended with petroleum Various uses in the food industry Flavour enhancer Feed supplement Flavour enhancers Precursor of aspartame, sweetener Applications in the food industry Enhanced oil recovery Feed supplements inter-relationships between primary and secondary metabolism are illustrated in Fig. 1.2, from which it may be seen that secondary metabolites tend to be elaborated from the intermediates and products of primary metabolism. Although the primary biosynthetic routes illustrated in Fig. 1.2 are common to the vast majority of micro-organisms, each secondary product would be synthesized by only a very few different microbial species. Thus, Fig. 1.2 is a representation of the secondary metabolism exhibited by a very wide range of different micro-organisms. Also, not all micro-organisms undergo secondary metabolism — it is common amongst the filamentous bacteria and fungi and the sporing bacteria but it is not found, for example, in the Enterobacteriaceae. Thus, the taxonomic distribution of secondary metabolism is quite different from that of primary metabolism. It is important to appreciate that the classification of microbial products into primary and secondary metabolites is a convenient, but in some cases, artificial system. To quote Bushell (1988), the classification "should not be allowed to act as a conceptual straitjacket, forcing the reader to consider all products as either primary or secondary metabolites". It is sometimes difficult to categorize a product as primary or secondary and the kinetics of synthesis of certain compounds may change depending on the cultural conditions.

The physiological role of secondaiy metabolism in the producer cells has been the subject of considerable debate, but the importance of these metabolites to the fermentation industry is the effects they have on organisms other than those that produce them. Many secondary metabolites have antimicrobial activity, others are specific enzyme inhibitors, some are growth promoters and many have pharmacological properties. Thus, the products of secondary metabolism have formed the basis of a number of fermentation processes. As is the case for primary metabolites, wild-type micro-organisms tend to produce only low concentrations of secondary metabolites, their synthesis being controlled by induction, catabolite repression and feedback systems. The techniques which have been developed to improve secondary metabolite production are considered in Chapters 3 and 4.

Recombinant products

The advent of recombinant DNA technology has extended the range of potential fermentation products. Genes from higher organisms may be introduced into

Primary And Secondary Metabolites

Glutamic acid (C5N}

Fig. 1.2. The interrelationships between primary and secondary metabolism. Primary catabolic routes are shown in heavy lines and secondary products are italicized (Turner, 1971).

Glutamic acid (C5N}

Fig. 1.2. The interrelationships between primary and secondary metabolism. Primary catabolic routes are shown in heavy lines and secondary products are italicized (Turner, 1971).

include dehydrogenation, oxidation, hydroxylation, dehydration and condensation, decarboxylation, animation, deamination and isomerization. Microbial processes have the additional advantage over chemical reagents of operating at relatively low temperatures and pressures without the requirement for potentially polluting heavy-metal catalysts. Although the production of vinegar is the most well-established microbial transformation process (conversion of ethanol to acetic acid) the majority of these processes involve the production of high-value compounds including steroids, antibiotics and prostaglandins.

The anomaly of the transformation fermentation process is that a large biomass has to be produced to catalyse a single reaction. Thus, many processes have been streamlined by immobilizing either the whole cells, or the isolated enzymes which catalyse the reactions, on an inert support. The immobilized cells or enzymes may then be considered as catalysts which may be reused many times.

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Responses

  • devon
    How are secondary metabolites are produced by primary metabolism?
    7 years ago
  • teija
    Are secondary metabolites produced by a microbial cell in stationery phase?
    7 years ago
  • nick
    What stage does secondary metabolites appear microbiology?
    7 years ago
  • Sabrina
    What metabolites slow the growth of other microbes?
    6 years ago
  • jackie
    What are the uses of microbial primary metabolites in industry?
    6 years ago
  • merico
    Which stage in the microbial curve can microorganisms produces secondary metabolites?
    9 months ago
  • campbell
    Is beer a microbial secondary metabolite?
    8 months ago
  • virgilio ferrari
    How is secondary metabolote and primary metabolite produced in the bacterial growth curve?
    4 months ago

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