When looking at our dipslides, people often ask us which one they should get. “What agar should I use?” “What does each agar grow?” “What is best for my application?” Like many of our answers to your questions, it really depends on the application and the results you are trying to achieve. But to help you make a decision, we’re going to take a closer look at each of the agars we offer. Today, we’re looking at Malt Extract Agar!
A Brief History
Malt Extract Agar and Malt Agar are not exactly the same thing, although both contain Malt Extract. Malt Extract Agar, which is what we supply, is a clarified water soluble extract of malted barley. It is commonly used for detection of yeasts and molds. It is high in carbohydrates, and highly acidic, making it the perfect environment for yeasts and molds, while inhibiting bacteria.
Malt Agar, on the other hand, was developed in 1919 by Reddish, using malt extract to support the growth of yeasts. Then in 1926, Thom and Church used the Reddish medium to successfully grow and study Aspergillus spp.
Malt Agar is recommended for detection and isolation of yeasts and molds from dairy products and food. Again, it does still contain Malt Extract.
Growth
Malt extract agar is a light creamy colored medium. It has an acidic pH (pH 4.7 ± 0.2) which allows for optimal growth of yeasts and molds, while restricting bacterial growth. Malt Extract Agar contains maltose as an energy source for yeasts and molds. The formulation also includes Dextrin, which is a polysaccharide derived from high quality starch, and glycerol, which are both carbon sources. Additionally, peptone provides a nitrogen source.
We combine the Malt Extract agar on a dipslide paddle with Nutrient-TTC on the other side. These media are very complimentary to each other. While Malt Extract supports the growth of yeasts and molds and inhibits bacteria, Nutrient-TTC encourages bacterial growth.
This dipslide is often used for testing diesel fuel, cooling towers, cutting fluids, and other similar industrial applications. It is used to detect gross contamination, and the media can distinguish between bacteria and yeast and mold growth.
Incubation
Malt Extract Agar is best incubated at 25-30°C and examined for growth after 48 hours. It may need to incubate longer, but it is best to check at 48. If incubating at room temperature, it should be left for at least 5 days, up to 7 days.
Here’s where incubation can get tricky when using a dipslide that has two uniquely different media. The optimal incubation temperature for bacterial growth (on the Nutrient-TTC agar), is more like 35 ± 2°C for just 24-48 hours. Yeasts and molds, on the other hand, prefer lower incubation temperature for a slightly longer period. So how should you incubate?
Well, if you’re focusing on the molds, lean more towards the lower temperature and 48 hour incubation period. If you’re focusing on the bacteria, lean more towards the higher temperature and 24 hour incubation period. If you want to focus on both simultaneously, focus on the middle of the range, somewhere around an incubation temperature of 32-33°C with a first check at 36-48 hours.
Check out our technical document for more information on our Malt Extract dipslides, or to view a microbe identification chart with images of microbes grown on Malt Extract agar.
That’s it for today’s micro-lesson. Hopefully, you now have a better understanding of how Malt Extract agar works and what microbes it’s best used for.
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