Micro 101: What is film plating?

NeoFilm_Fan_onGray_wLogo_PPWithout microbes, there is no microbiology.

It seems pretty obvious. But like all things in life, it’s never just that easy. Microbes don’t separate according to genus and species on their own nor do the dangerous ones hold up little red flags warning of their pathogenicity. Rather, it falls to lab personnel, microbiologists and other science-y and technical type people to puzzle out what exactly is present in a given sample.

That’s where traditional microbiological methods such as plating come in. Plating is when a person takes a representative sample and places it on (or in, in some cases) a nutrient rich media to see what grows. Personnel can use a range of methods for plating to either see if any microbes will grow using a general growth medium, or target a specific microbial type using a selective growth medium. Methods for targeting specific microbial types include inoculating films (such as Neogen’s NeoFilm) that select for target organisms, plating on traditional selective agar media (such as products offered by Acumedia dehydrated culture media that have selective properties), or using high throughput methods (such as Soleris). For simplicity sake, this post is going to focus on films.

Why plate?

Each selective media film type has specific properties that encourage growth of a certain microbe of interest and inhibits or restricts the growth of other microbes. Moreover, there may not be enough of the target microbe in a sample to initially detect it by other diagnostic methods. Plating fixes this by giving target microbes a nutrient-rich environment in which to flourish and multiply, thereby increasing the number of microbes, which can be detected easier given the higher levels.

Plating also allows technicians to count the number of colonies that grow, giving them an idea of the amount of organisms in the sample.

In summary, plating not only helps grow the microbes of interest but it also can provide a simple way to differentiate between types (e.g., coliforms vs. yeast and mold) as well as figure out how many colonies are present.

How do the microbes grow on a film?

The process for growing microbes on a film is very similar to traditional pour plating methods – a technician obtains a representative sample of the item he or she wishes to test and dilutes it in buffer. A small sample (1 mL) then is pipetted onto the appropriate film’s media pad and the transparent film cover is sealed over the sample to protect it (NeoFilm has several color-coded types for different organisms). The films are then incubated at a temperature and time selected to favor the growth of the target microbe over competing microorganisms.

Once that incubation time is up, the technician removes the films from the incubator and looks for colonies, which appear differently depending on type.

OK, so now we know the number of colonies. What happens next?

There are a couple different paths lab personnel can take at this point. We’ll break them down separately.

Call it good: Remember, certain films are conducive to growing (or not growing) specific microbes. For example, a film might be selective for E. coli but not for yeast and mold. Likewise, the color and shape of the colony can indicate its type (for example, on NeoFilm for E. coli/Coliform, E. coli grows as blue to indigo colonies while coliforms grow as blue to green. Mold colonies on NeoFilm for Yeast and Mold appear as large red diffuse colonies). Often, this is enough as it allows the user to determine what is or isn’t present in a sample.

Complete additional studies: Sometimes it’s not enough to grow the microbes and identify them from the film or the plate. Many times, an additional protocol called confirmation is required. This involves lifting a colony from its cozy agar/film home and replating it elsewhere for further study. There are different confirmation procedures depending on the microorganism and the application (more information on food safety confirmation procedures can be found in the U.S. Food and Drug Administration’s Bacteriological Analytical Manual).

 

For more information on Neogen’s NeoFilm products, click here. For a nifty video, head over to our YouTube channel here.

Check out these links for more information on Neogen’s Acumedia products and Soleris rapid micro system.

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