Microbes and Metabolism - Environmental Biotechnology: Theory and Application - page 30

and which is wholly dependent on substrate level phosphorylation. This is the
synthesis of ATP by transfer of a phosphate group directly from a high energy
compound and not involving an electron transport chain. Additionally, and a
source of great confusion, is that fermentation may refer simply to any microbial
growth in the absence of oxygen but equally may be used generally to mean
microbial growth such as food spoilage where the presence or absence of oxygen
is unspecified. The definition used throughout this topic, except with reference
to eutrophic fermentation discussed in Chapter 8, is that of growth dependent on
substrate level phosphorylation.
There are very many fermentation routes but all share two requirements, the
first being the regeneration of NAD + from NADH produced during glycolysis
which is essential to maintain the overall reduction: oxidation equilibrium, and
the second being that pyruvate, or a derivative thereof, is the electron acceptor
during the reoxidation of NADH. What this means is that all fermentation routes
start with pyruvate, the end point of glycolysis, and proceed along a variety of
pathways to an end product indicative, if not diagnostic, of the organism. Fermen-
tation is therefore an option under conditions where there is an active electron
transport chain as discussed in the following section, but becomes essential when
fermentation is the only method for regenerating NAD + .
As noted above, the end product of fermentation for any given carbon source
may be diagnostic of the identity of a specific organism. This is more relevant for
bacteria than for yeast or other eukaryotic cells and arises from the predisposition
of that organism, to use a particular fermentation pathway. These are described
in detail in Mandelstam and McQuillen (1973) and are summarised in Figure 2.8.
Identification by the product of carbohydrate catabolism is somewhat specialised
and is very thoroughly set out in Cowan and Steel's Manual for the Identification
of Medical Bacteria (Barrow and Feltham, 1993).
Electron transport chains: oxidative phosphorylation and methanogenesis
As described in the previous section, NADH and other reduced cofactors may
be reoxidised by the reduction of organic receptors such as pyruvate. This is the
fermentation route.
Alternatively, the reducing agent (or reductant) can transfer the electrons to an
electron transport chain which ultimately donates them to an inorganic receptor
(the oxidising agent or oxidant). In aerobic respiration, this receptor is oxygen.
However, some bacteria have electron transport chains which use other electron
sinks such as nitrate, sulphate, carbon dioxide and some metals, with respiration
being described as anaerobic in these cases. The use of nitrate in this role leads
to the process of denitrification, which plays an important part in many aspects
of the applications of environmental biotechnology.
A number of events occur during the flow of electrons along the chain
which have been observed and clearly described for a number of organisms
and organelles, most especially the mitochondria of eukaryotic cells. These are
fully discussed in many biochemistry textbooks, an excellent example being