PRIMARY & SECONDARY METABOLITES
The metabolism can be defined as the sum of all the biochemical
reactions carried out by an organism. It involves two pathways: Primary
metabolic pathways (PMPs) produce too few end products while secondary metabolic
pathways (SMPs) produce too many products.
PMPs require the cell to use nutrients in its surroundings such as
low molecular weight compounds for cellular activity. There are three potential
pathways for primary metabolism: the Embden Meyerhof-Parnas Pathway (EMP), the
Entner-Dourdorof pathway and the hexose monophosphate (HMP) pathway. The EMP
pathway produces two molecules of pyruvate via triose phosphate intermediates.
This pathway occurs most widely in animal, plant, fungal, yeast and bacterial
cells. Many microorganisms however use this pathway solely for glucose
utilization. During primary metabolism hexoses such as glucose are converted to
single cell protein (SCP) by yeasts and fungi. Yeasts from the Sachcharomyces
species produce alcohol as cells grow during the log phase using an anaerobic
primary metabolic pathway. This account for most of the alcohol found in nature
and is widely used in the fermentation industry to produce beer, wine and
spirits.
For example in the citric acid fermentation process involving Aspergillus
Niger, hexoses are converted via the EMP pathway, to pyruvate and acetyl
Co-A which condenses with oxaloacetate to form citrate in the first step of the
TCA cycle. Ethanol, lactic acid and acetic acid were the first commercial
products of the fermentation industry. Several of these products have
applications as alternative energy sources, for example alcohol has been used
to produce a cheaper alternative to petrol in developing countries such as
Brazil and in Europe between World Wars I and II.
Secondary metabolism synthesises new compounds. Secondary
metabolites are not vital to the cells survival itself but are more so for that
of the entire organism. Relatively few microbial types produce the majority of
secondary metabolites. Secondary metabolites are produced when the cell is not
operating under optimum conditions e.g. when primary nutrient source is
depleted. Secondary metabolites are synthesized for a finite period by cells
that are no longer undergoing balanced growth. A single microbial type can
produce very different metabolites. Streptomyces griseus and Bacillus
Subtillus each produce more than fifty different antibiotics. Most
secondary metabolites are produced by families as closely related compounds.
The chemical structure and their activities cover a wide range of
possibilities, including antibiotics, ergot alkaloids, naphtalenes,
nucleosides, peptides, phenazines, quinolines, terpenoids and some complex
growth factors. The production of economically important metabolites such as
antibiotics by microbial fermentation is one of the major activities of the
bioprocess industry.
Secondary metabolites such as penicillin are produced during the
stationary phase (idiophase) of cell growth. Most of the knowledge concerning
secondary metabolism comes from the study of commercially important
microorganisms.
There are some similarities between the pathways that produce
primary and secondary metabolites, namely that the product of one reaction is
the substrate for the next and the first reaction in each case is the
rate-limiting step. Also the regulation of secondary metabolic pathways is
interrelated in complex ways to primary metabolic regulation.
Primary Metabolites
Fermentation products of primary metabolism such as ethanol,
acetic acid, and lactic acid were the first commercial products of the
fermentation industry. These industrial revelations were soon followed by
citric acid production along with other products of fungal origin. Due to the
high product yield and the low reproducibility costs, major interest has been
shown in the respective markets. Production of cell constituents i.e. lipids,
vitamins, polysaccharides as well as intermediates in the synthesis of cell constituents
such as amino acids and nucleotides are also of great economic importance in
present-day industry. The effectiveness of yeasts along with other
microorganisms as sources of the B-group vitamins has been recognized for more
than 50 years and like products of catabolic primary metabolism e.g. ethanol,
citric acid etc. are of great commercial importance.
Citric acid is an organic acid that is of major economic use in
today’s industry. It is a very important commercial product and is widely used
in the food and beverage industries as a food additive. In addition to the beverage and food industry,
citric acid is used in effervescent powders as well as being used in boiler and
metal cleaning. Factors effecting citric acid production vary considerable and
depend predominantly on the strain of A. niger used. Other factors that affect
citric acid production include the type of raw material fermented, the amount
of methyl alcohol present, the substrate’s initial moisture content as well as
the fermentation time and temperature. Much research has been conducted over
the years in order to increase the yield of citric acid production.
Nucleotides are used in the preparation of poly and
oligonucleotides as well as being of potential nutritional and medical
interest. However the greatest interest in nucleotides lies in the fact that
they have the ability to enhance the flavour of foods. Yeast extract is extensively
used as a flavouring agent in the food industry and is widely available either in
powder or paste form. After autolysis and partial hydrolysis of RNA,
ribonucleotides such as 5’-monophpsphate (GMP) and inosine 5’-monophosphate
(IMP) may be extracted from the biomass. Flavour enhancement is a property of
these purine ribonucleosides as well as the ribonucleoside, xanthylic acid
(XMP). These food enhancers are responsible for meaty flavours found in foods
and are available on the market worldwide. These products are of major
importance in the food industry and currently international trade surpasses US
$1.1 billion per year (23).
Secondary Metabolites
Antibiotics were first defined as a chemical compound produced by
a microorganism, which has the capacity to inhibit the growth of and even
destroy bacteria and microorganisms in dilute solutions. Sir Alexander Fleming
first discovered the antibiotic properties of the mould Penicillin notatum in
1929 at St. Mary’s hospital in London, when he noticed that Penicillin
notatum destroyed a
staphylococcus bacterium in culture. Penicillin is bactericidal to a number of
gram-positive bacteria and acts by inhibiting transpeptidation thus preventing new
cells from forming walls. It belongs to the beta-lactam family of antibiotics. During
World war two research was moved to the USA where large-scale growth of the
mould began. Firstly penicillin moulds were grown in small shallow containers
on nutrient broth. Methods of growth were improved by using deep fermentation
tanks with continuous sterile air supply and corn steep liquor as a source of
nutrients. In 1943 a cantaloupe mould, P. Chysogenum was found to
produce twice the amount of penicillin than P. notatum. Since then researchers
continued to find higher yielding penicillin moulds and have also improved yields
further by exposing moulds to x-rays and UV light. The first type of penicillin
produced was Penicillin G, which had to be administered to patients
parenterally because it is broken down by stomach acid. Penicillin V was later
formulated so that it could be taken orally; unfortunately it was less active
than Penicillin G.
The enhancement of antibiotic industrial yield has been achieved
through traditional strain improvement programs based on random mutation and
screening. Recombinant DNA techniques have existed since the 1970’s and involve
the introduction of DNA fragments into host cells using a vector (a plasmid or
phage) that contains a selection marker. The DNA fragments are integrated into
the host genome or autonomously replicated as a plasmid. Transformants are then
screened for improved characteristics. The pharmaceutical company Eli Lilly was
responsible for the first recombinant DNA improvement of an antibiotic
producing microorganism. Transformation of C. acremonium 394-4
caused an increase in the amount of antibiotic cephalosporin C excreted by the
organism. Cephalosporins are beta-lactam compounds that are structurally and
pharmacoligically related to penicillins. Cephalosporins resist hydrolysis by
enzymes referred to as penecillinases, which are secreted by a number of
bacteria. They are now one of the most widely prescribed antibiotics and are
very effective for the treatment of hospital-acquired infections.
Actinomycetes are aerobic spore forming bacteria that originate
from soil. A large number of antibiotics are produced by actinomycetes and in
particular Streptomyces. They resemble a fungal mycelium in form, but
have thinner filaments. These filaments are formed when cells divide to form
long chains of up to 50 cells. Actinomyces griseus was first isolated
from soil in the Andes, this bacterium produced a substance that killed many
bacteria unaffected by penicillin, including Tuburculosis bacillus. The
antibiotic was named streptomycin. However tubercle bacilli soon became resistant
to streptomycin and it has since been replaced by para-amino-salicylic acid (PAS).
Stretomycetes are still very important bacterial producers of antibiotics and cytostatics.
Due to the emerging resistance of bacteria to common antibiotics, new technologies
such as combitatorial biosynthesis are being used for the production of novel
metabolites using streptomycetes. This technology involves the use of a combination
of genes from different biosynthetic pathways to produce modified metabolites.
Ordinarily Actinomycetes use the EMP pathway to metabolise glucose
because this pathway is a more efficient one than the ED pathway. The secondary
metabolites of the fungi including Drechslera, Trichoderma, Aspergillus and
Curvularia have the ability to produce green dyes / anthraquinones.. These dyes
are from natural sources and do not cause the pollution to the environment
associated with chemical dyes.
So in nutshell Industrially important primary microorganisms are continually
being improved to optimize product yield and substrate utilization in order to
minimize the cost of production. Primary products of microbial metabolism have
made a significant contribution to the food and beverage industries. Primary
metabolites have been used to produce petroleum derived products as well as
ethanol for liquid fuels (gasohol). Considering the current trends in oil
prices, microorganisms that have the ability to produce such products will no
doubt be exploited to their full potential. In the not too distant future
biomass energy could become a major contributor to the Earth’s energy
requirements as petroleum resources run out. The introduction of antibiotics
revolutionised the treatment of infectious disease in humans. Antibiotics are
used in large quantities in animal farming to prevent infection as well as to
treat diseases. Smaller doses are added to animal feed to promote growth. Fruits
and vegetables are also treated for bacterial infections using antibiotics such
as streptomycin and oxytetracycline. Owing to this widespread use, antibiotics
have been found in liquid waste at animal feedlots and have spread into many
surface and groundwater supplies. Residues of antibiotics have also been
detected in sewage treatment plants and raw water resources in many European
countries. The prescence of antibiotics has upset the delicate balance of
microorganisms in the environment by depletion of microorganisms susceptible to
antibiotics and providing favourable environments for the proliferation of
resistant strains. Three EU projects were undertaken in 2003 to assess the
presence and effects of antibiotics in the aquatic environment and in soils:
the ERAVMIS, PEPHARMAWATER and POSEIDON projects. These projects also propose
solutions to this problem, such as the removal of antibiotics from wastewater
by ozonation and sunlight. Bioresearch Italia began a project to isolate
previously unexploited microorganisms from actinomycetes and uncommon
filamentous fungi in 2002. This project may provide some insight into methods
of combating the increasing number of antibiotic resistant strains of bacteria.
References:
Krishna,
C., 19xx, Solid state fermentation systems - an overview, Journal of
Applied Microbiology, Vol.
25 Issues 1-2, pp1-30
Learry, E; Keegan, J;
primary versus secondary metabolites
