Bacterial growth
Single-celled microorganisms, some of which are pathogenic in humans, animals and plants. Singular is bacterium. can replicate approximately every 20 minutes by Asexual reproduction of bacteria in which they copy their DNA and divide into two., which is a simple form of Process by which cells duplicate.. This level of replication will depend on the availability of nutrients and other suitable conditions, such as temperature.
There are many ways to grow, or Grown deliberately by humans., bacteria, eg:
- nutrient broth solution
- A cluster of bacteria visible to the naked eye. A single colony is assumed to have derived from a single bacteria. on an agar plate
Nutrient broth solution, or culture medium, allows a liquid or gel to provide all the nutrients needed for bacteria to grow successfully. These must include carbohydrates for energy, nitrogen for The production of proteins from amino acids, which happens in the ribosomes of the cell., plus other minerals.
A Petri dish that contains agar gel and usually some nutrients. Agar plates are used to culture (grow) bacteria and fungi in the lab. are created by pouring hot molten agar into sterile A clear glass or plastic dish, used to grow living cells from organisms so they can be studied., which is then allowed to set. Bacteria can be spread onto the plates, and allowed to form individual colonies of the specific bacterium.
Uncontaminated cultures
If a specific bacterium is going to be cultured, other contaminating bacteria would compete for nutrients in the broth or agar. Some bacteria could also be harmful, such as Microorganism that causes disease., and would complicate the results of experiments when testing the efficiency of antibiotics or other anti-microbial compounds.
It is therefore important that any petri dish or agar used is sterilised before use, and that aseptic techniques are used to inoculate the plates.
Use of aseptic techniques to avoid contamination
An inoculating loop can be used to transfer bacteria. It is sterilised by heating it to red hot in a Bunsen flame, before and after use.
To inoculate the agar, lift the lid of the Petri dish and tilt. Do not fully remove or place on the desk as the lid prevents micro-organisms from the air contaminating the culture, and vice versa.
Following inoculation, the lid of the Petri dish should be secured in place by strips of adhesive tape for safety reasons. The dish should be labelled and dated.
Inoculated agar plates are incubated at 25°C in school laboratories for no more than 24–48 hours. This encourages growth of the culture without growing human pathogens which thrive at body temperature (37°C).
For safety reasons, plates and equipment should be sterilised after use.
Colonies of bacteria
Bacteria are micro-organisms, and individual cells cannot be seen without a microscope. However, when grown on agar in a Petri dish, each individual cell divides multiple times to form a visible colony.
If we count the number of individual colonies of bacteria on the plates, it is possible to estimate the numbers of individual bacteria in the original sample.
In order to make an accurate calculation of the numbers of bacteria in a sample, the original sample will need to be diluted - this is known as a serial dilution. Only when the sample is sufficiently dilute that it produces clear individual colonies can a calculation be made.