Microbiological analysis
Various aspects of food legislation require samples of food and food ingredients, as well as samples taken from surfaces in food processing and production areas, to be collected for microbiological analysis. These analyses provide information about the number and type of microorganisms present in the samples.
This section provides a brief introduction to the methods that are used to analyse microbiological samples. Once you have looked through the information, you should attempt the SAQs to satisfy yourself that you have understood the key points.
Read Section 5 of KT12 (pp. 115-31) for a brief account of microbiological analysis.
ACTIVITY 3.5
Watch the video sequence that illustrates some of the commonly used microbiological techniques. (Note that the term 'microbe' is used in this video and it is equivalent to the term 'microorganism' that we have used in this course.)
Using information from KT12 and the video sequence, you should be able to answer the following SAQs.
SAQ 3.6A
In what key respect does the polymerase chain reaction (PCR) extend the available techniques for identifying microorganisms?
PCR works by producing many copies - up to millions - of the DNA from a microorganism rather than multiplying the entire microorganism. The microorganism is then identified by presence of specific sequences of DNA. (See pp. 126-7 of KT12 for reference.)
SAQ 3.6B
Why do microorganisms need to be cultured in different media?
Different microorganisms have different nutritional requirements. In fact, microorganisms can sometimes be identified by the media they can, or cannot, grow on. (See the video for reference.)
SAQ 3.6C
At what temperature and for how long do media need to be steam-heated in order to become sterile?
Media must be sterilised before use. This is usually done in an autoclave - a glorified pressure cooker. Materials are sterilised by exposure to steam at a temperature of 121°C for fifteen minutes. (See the video for reference.)
SAQ 3.6D
What simple physical features characterise cocci and bacilli?
Cocci are round bacteria; bacilli are rod-shaped. (See the video for reference.)
SAQ 3.6E
If a food sample contained 100,000 bacteria per gram, how many serial dilutions would be required to give 100 colonies on an agar plate, starting with one gram of food?
Three serial dilution steps would be required. The first dilution of the food sample (1 in 10) would yield, in theory, 10,000 colonies per plate, the second 1000 and the third 100 colonies per plate. This number of colonies can be readily counted on a single plate. (See p. 199 of KT12 for reference.)