Being animals who breathe oxygen – the technical term is “aerobic” – we quite understandably have an aero-centric view of life on this planet. We cannot envisage a world devoid of oxygen or oxygen-dependent organisms, even though the earth was just so for the first few hundred million years of life existing on it. It was only until photosynthesising bacteria evolved that oxygen became abundant. The question is often asked: “what would happen if all the oxygen were to run out?” One answer to that question is that, for many organisms, life would continue much as before. Why? Because a significant chunk of the microbial world has no need for oxygen, apart from that bound up in the water molecule.
First, let’s look at oxygen’s function in aerobic organisms.
You remember that famous equation from school?
C6H12O6+6O2→6CO2+6H2O+energy
One molecule of glucose + 6 molecules of oxygen give 6 molecules of carbon dioxide, 6 molecules of water and energy.
Put at its simplest, our cells take an energy-rich molecule such as glucose and combine it with oxygen to release the energy necessary for life, with carbon dioxide and water as by-products. This is called aerobic respiration. The process of breaking up, for example, glucose to release the energy contained within its bonds is akin to burning it. You can take some grains of sugar, hold a lit match to them and see this combustive release of energy. Our cells cannot simply burn glucose: the energy released by this would be far too high and lethally destructive for the components that make up the cell. Instead, our cells release the energy from glucose in tiny packages. In that way, the explosive release of energy we see during combustion is broken up into more manageable packages.
Thus, three stepwise processes, very gradually relieve glucose of its energy. These are:
glycolysis (the breakdown of the glucose molecule into two three-carbon molecules of pyruvate);
the tricarboxylic acid cycle (or Krebs cycle; the pyruvate is pulled apart bit by bit to release the energy stored in those covalent bonds and where most of the carbon dioxide is formed;
the electron transport chain and oxidative phosphorylation (the electrons harvested from the breakdown of glucose are mopped up by oxygen, producing water and a heck of a lot of energy in the form of ATP). Oxygen, acting is this role as chief-mopper-upper of electrons is called a terminal electron acceptor.
The difference between aerobic and anaerobic respiration is that in anaerobic bacteria the electron mopper-upper is something other than oxygen. Alternative electron acceptors include nitrate, sulphite, sulphate, carbon dioxide and sulphur itself. These compounds are less powerful electron mopper-uppers than oxygen – technically they are referred to as having smaller reduction potentials – and so generate less energy. Following on from this, anaerobes are less efficient growers than aerobes, who are essentially turbo-charged through their use of oxygen. Important groups of anaerobic bacteria are the methanogens, who produce methane from the reduction of carbon dioxide. Some of these guys live in the bellies of cattle and termites and are responsible for much of the methane behind the greenhouse effect causing global warming. Anaerobic decomposers, which produce eggy smelling hydrogen sulphide gas include members of the Desulfovibrio genus.
One very important genus of anaerobes from a medical and food safety perspective is Clostridium. Everyone knows about C. botulinum, the organism responsible for botulism and the producer of the botulinum toxin, BOTOX, used in cosmetic surgery. Foods from which oxygen is excluded such as tinned food and low- or no-oxygen modified-atmosphere-packaged products are the ones most at risk from growth of C. botulinum, which, like the rest of the genus, Clostridium, forms spores which can survive cooking. A swollen tin, or one that makes an audible hiss or pop upon opening could very well be contaminated with C. botulinum or one of its cousins, the gas producing the swelling or pop being as a result of the bacteria’s anaerobic respiration. C. tetani is the dude responsible for causing tetanus, against which, thankfully, we are all vaccinated. Spores of this critter find their way into the anaerobic or microaerobic environment of deep wounds and grow to dangerous levels, producing the tetanospasmin toxin responsible for the horrific symptoms of this lethal disease. C. perfringens causes a mild form of food poisoning, but is a scourge of certain industries which produce dried food such as the dairy and infant formula industries. Clostridiodes (formerly Clostridium) difficileis an organism responsible for nosocomial (i.e. hospital-based) infections. Targeting post-op patients, many of whom have weakened gut microflora as a result of antibiotic administration, this bug causes potentially lethal diarrhoea which can often escalate to full-scale colitis. Once it establishes itself in a hospital environment it can be very difficult to eliminate as it also forms resistant spores.
I have something grave and shameful for any worth-their-salt microbiologist to admit: I have only minimal experience in cultivating anaerobes. There was that second-year laboratory practical back in 1995 where we played around with anaerobic jars and a few dishes of C. butyricum, and I isolated a couple of plates of suspected anaerobes last year for the fun of it, like, but that’s about it. And now, I find myself, for the purposes of the project I am working on having to learn how to grow up and perform experiments on Clostridium. I’m a bit scared. These guys are supposed to be very fussy, not only in terms of an absolute need not to come into contact with oxygen (it is toxic for many anaerobes), but as regards diet. I’ve just spent a grand on specialised media, replete with electron soaker-uppers. Let’s hope these boyos are kind to me and don’t catch a draft or decide to go off their food!
The Bug Hunter 