Micro-organisms
Hunt the microbe
Micro-biological organisms (microbes for short) are all around you – and inside you. Some are very useful, such as those for making yogurt, cheese or beer, or the ones in your gut that help you to digest food. Some, like mould growing on a bathroom wall, are just a nuisance. Others, like pathogenic (disease-causing) bacteria, are positively dangerous.
The problem is, being microscopic, they are too small to see. So, how can we tell what’s lurking where? For instance, are there salmonella in the chicken? Is the milk or water safe to drink? Will the bacteria in the milk turn it into cheese while you’re trying to make yogurt? What’s causing this person’s illness?
To find out, microbiologists take samples. They dilute them, using serial dilution, to get only a few microbes in each drop. To count the microbes, they don’t search for them with a microscope, they culture them. That is, they encourage them to multiply into colonies, by providing suitable nutrients and warmth. These colonies are large enough to be seen, and counted, with the naked eye. The number of colonies indicates how many microbes were in the original sample.
The Standard Procedure below shows you how to culture and count colonies. Identifying what microbes the colonies contain is more difficult – a job for the experts.
Culturing needs great care a) to be successful, and b) to be healthy and safe. You are looking for microbes in your sample. So, you need to be sure that any you find have really come from the sample, and not from somewhere else. You must use aseptic techniques. That means making sure that everything you use, except your sample, is sterile – and that no microbes can get in from the air, or from yourself.
NOTE: You need to be familiar with aseptic techniques before you perform the procedures below.
The conditions that encourage useful or harmless microbes to multiply will also encourage any pathogens that happen to be in the culture dish. You never know what you might be culturing. So, you must never open a dish which has a culture in it. When you’ve finished with them, the cultures must be properly destroyed. Follow the instructions given by your teacher / supervisor.
Scientists hunt microbes for two main reasons. They may want to use them, or destroy them. Even harmless microbes may need to be got rid of, if they are in the wrong place at the wrong time. For instance, wild yeasts grow on the surface of grapes. However, they are unlikely to be the type of yeast that the winemaker needs to make good wine. So, he sterilises the grape juice to kill the wild yeasts, and then adds a culture of the yeast he wants to use.
But how can he kill the wild yeasts? And how do we destroy other unwanted microbes? High temperatures kill them, so cooking does the trick for foods. However, boiling the grape juice would alter the flavour of the wine – and you can’t cook a person to cure their disease! The answer, of course, is chemicals – sterilising agents, disinfectants, antiseptics and antibiotics. But how effective are they, and what is most suitable for a particular purpose. A disinfectant may ‘kill all known germs’, but you wouldn’t want to add it to food, nor drink it to cure your illness.
The second Standard Procedure below illustrates a way of testing whether a particular chemical prevents the growth of microbes. Although it allows comparisons of different concentrations or products, it does not actually measure how effective a product is at killing micro-organisms. British Standards such as BS EN 1040:1997 Chemical disinfectants and antiseptics – basic bactericidal activity use a more complex procedure. They measure the reduction in bacterial counts after particular contact times with the chemical.
- SP 0003:2005 A method for counting micro-organisms in dairy products
- SP 0004:2005 A method for evaluating the effectiveness of anti-microbial products