Microbiology labs can use several methods for agar-based microbiological analysis in a Petri dish. These include the ‘Spread Plate Method’ and the ‘Pour Plate Method’.
The spread plate method consists of placing the sample on the surface of solid culture media. With this technique, colonies can be easily retrieved, for identification for instance. However, the size of the sample is limited to between 0.1 and 0.5 ml maximum. Beyond that, counting can become very difficult…
Advantages of the Pour Plate Method
The pour plate approach allows for more sensitivity, so is ideal for samples with lower bacterial numbers. It’s is also the best method for organisms requiring a microaerophilic environment. However, it can be a time consuming, labour intensive process, open to various errors.
Manual Steps Required for Pour Plates
STEP 1: Add 1 mL sample into an empty and sterile Petri dish.
STEP 2: Add the first layer of culture media, generally around 10 to 20 ml. Be careful: the media temperature must be under control. If it’s too high, your micro-organisms will not survive!
STEP 3: The stirring – movements must be performed following the shape of an ‘8’ in order to spread the sample homogeneously throughout the agar.
STEP 4: Add a second layer of media in order to make sure that no micro-organisms are present on the surface. Obviously, this concerns only the double-layer protocols.
This video from Alliance Bio Expertise demonstrates how using an automated approach – the DISTRIWEL combined with the MEDIAWEL, allows laboratories standardise:
- the temperature of the dispensed media
- the volume of media in each Petri dish
- the ‘8’ shape stirring
The DISTRIWEL is an automatic Petri dish filling and stacking machine that can prepare up to 440 plates within 30 minutes. It has a Plexiglas protective cover and a built-in UV lamp to help prevent contamination during the pouring process. It is also important to note that the compact base, the detachable carousel, and the removable transfer plate make the device extremely easy to handle and to clean.