Crowded plate techniques in microbiology

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Crowded plate techniques in microbiology
Crowded plate techniques can lead to new medicines, foods and other benefits. (Ryan McVay/Photodisc/Getty Images)

According to microbiologist K. R. Aneja, crowded plate techniques are used to isolate microorganisms. The most famous example of microorganism isolation is the discovery of penicillin by Alexander Fleming. Penicillin became the most significant medical discovery of the 20th century, although Aneja claims it was actually first discovered by Ernest Duchesne in 1896. The “crowded plate” refers to the Petri dish, or similar apparatus, on which the cultures are grown.


Microorganisms are widespread, diverse and potentially useful to humans in science, industry, medicine, food use and in other ways. However, they are too small to be seen by the naked eye in most cases and need to be isolated from other microorganisms so they can be identified and studied further. New microorganisms are constantly being discovered, sometimes in remote places, such as caves in New Mexico. The scientific community is keen to study them for their potential benefits to humankind.

Basic technique

Aneja documents the basic technique for obtaining soil-based microorganisms. He puts 0.1 g of fresh soil into 50 ml of tap water in a beaker. The dilution is therefore 1:500. He seeds another soil sample with Streptomyces griseus, an antibiotic producer, to serve as a positive control. Aneja heats the beaker and prepares several other soil suspensions at different dilution rates. He uses sterile pipettes to transfer samples of these onto Petri dishes of agar jelly and allows the plates to cool at room temperature.

Miles-Misra technique

According to food microbiologists Martin R. Adams and Maurice O. Moss, accurate results occur when a plate has between 30 and 300 colonies. The Miles-Misra technique, often used for analysis of food samples, allows for this. Using samples of smaller volume, the Miles-Misra technique allows food scientists to grow several different dilutions on a single plate. It is also less expensive than the traditional method in terms of Petri dishes and agar jelly.


Frances Pouch Downes and Keith Ito have gathered together a compendium of different techniques. Variations to the basic method include using different culture mediums, enriching the suspensions with chemical broths and very specific methods aimed at isolating particular species of microorganism. The latter are useful when confirmation of the presence of a particular microorganism is required. Techniques developed to isolate damaged or stressed microorganisms are available, as are rapid methods, where time is of the essence. Even counting colonies is a highly developed discipline and many different techniques are used.

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